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MG MGA - Carburettion Issues - Still an issue

I'm getting really frustrated with this.

I have had 2 attempts at trying to get the mixture right (WSM & Barney's casual way) but I am not able to get past certain steps;

- WSM - I don't manage to get the RPM profile by lifting the lifting pin.
- Barney's casual way - I can't get the engine to gallop (i.e too rich).

I have checked many things and what you guys said;

- Check the float levels - correct
- Taking the plug lead off on cylinders 1 or 2 cause the engine to missfire really badly and eventually stall. Whereas taking of plug leads of 3 or 4 the engine will carry on running quite happily.
- After a run the plugs were not sooty but dry and rosy. I could be too lean as the engine shakes a lot particularly at around 1500rpm.
- Plugs seem to have a good spark.

To add insult to injury today I think I was also getting vapour lock due to the extreme heat here in Miami, FL. Again if I am running lean then this would aggravate the problem.

- I need to check the rubber seals of the float bowl since they may be blocking off the passage of fuel. I get some intermittent RPM when I move the bowl side to side.

- I am also thinking that my timing could be very advanced inducing some of the problems if the tuning is not correct. I do get engine run-on when I turn it off for a couple of cycles indicating to me that he engine is running hot.

- The carburetor and coil are too hot too touch after 10minutes of running.

- I even thought it may not be getting enough fuel from the pump? I am going test this today.

Bottom line, is that my car will misfire constantly, I am not sure if I am too lean or too rich. Sorry for my ignorance.

Any suggestions?
Gonzalo Ramos

Gonzalo, make sure that the timing is correct before adjusting the carbs, in fact, make sure that everything is correct before adjusting the carbs! Wrong type of coil (ballast type) will cause trouble in the way of arcing inside the distributor cap and rotor as well as melting the plastic heel of the points. Try running without the air filters so as to rule them out as being the problem. I had some thick felt filters on my car and it wouldn't run at all right until I changed them for K&Ns. Get the carb synchonization right. If one is more open than the other, you will find that altering the mixture and lifting the pin on one carb makes a difference while the other one makes very little difference.
Lindsay Sampford

Gonzalo, make sure the choke is not stuck. With the choke knob pushed all the way in, reach underneath each carb and pull the bottom of the jet down. If it comes at all, the choke is not releasing properly and will cause the car to run too rich.
Andy Bounsall

Are you sure you have plug leads in right firing order?
Mike Ellsmore

Now that you mentioned it Mike.

I just went to check the ignition stuff. This is what I found;

- My points are barely opening. When open I can't even push through a 0.05" gauge. I need to adjust them.

- Also looking at the distributor cap and tracing the cables in the direction of how the rotor turns, I have the impression the plugs could be in the wrong order.
I understand the cylinder towards the front of the car is no.1 .

The manual it specifies; 1,3,4,2 right?
I also see MGAguru has a section so I will follow it.

Also I will check the coil as per the guru.

Will report the findings,
Gonzalo Ramos

Sounds like you are on the right track - as they say "95% of carburettor problems are electrical"!
Mike Ellsmore


Get a tach/dell meter if you don't have one - cheap and good to have. After setting your points, check the dwell to see if it's in spec, and then readjust the points accordingly. Non strictly necessary, but a good peace of mind "yes, I did it right" feeling.

Check your valves first, then your points/dwell, then set timing statically to 10* BTDC. Dwell effects timing, but timing doesn't effect dwell. Then check your plug gaps and ignition wire connections.

The dwell meter is also a very accurate tachometer, which is useful for troubleshooting your carb issues as well since it's easy to tell very small changes in rpm as you adjust them.

I just went through my carbs for the third time, and am getting pretty good at it. Barney's "casual way" actually works very well for setting sync, so I don't think that is your problem. The big difference between 1-2 and 3-4, if not electrical, sounds like you need to pull the carbs off and check the fit of your butterfly valves. Back off the idle screw so that the yoke arm can go to a dead stop - loosen the two screws holding the butterfly in place, then re-tighten carefully after ensuring that they are well seated and holding the throttle shaft tightly closed.

Firing order is 1-3-4-2.

I had an air leak in my rear carbs (from a cracked vacuum line) so that was the source of a high idle conition on cyl 3-4. The little pin to test the mixure is hard to reach on the rear carb, so I had only been testing with the front one and missed this.

Set the mixture on your carbs to 12 flats backed off from home as a starting point. Mine are set now at 13 flats.

AJ Mail

I don't like the fact that your carburettor is too hot to touch.

Have you got the thick spacers in place between the carbs and the manifold as these act as heat insulators.
Malcolm Asquith

SU carburettors are almost the simplest known to man but if they are not in good (ie excellent) order your car will not answer to tuning and will not run properly. Apart from butterfly spindles, worn needles and jets are common after many years of use!
Barry Bahnisch

Yes there is a thick black spacer on each carb but they still get pretty hot.
Gonzalo Ramos

Gonzalo, if your points have mysteriously closed up you have the classic symptom of wrong coil type. If you have the type of coil that should operate with a ballast resistor fitted, it will have too much current passing through it. This will result in the CB points getting too hot and the plastic carrying the moving contact will melt allowing it too alter its position and hence the gap. See the picture.

Lindsay Sampford

I went down today to do the inspections and the distributor seemed to be in poor condition so I took it off and have now rebuilt it. Put in new points, rotor + cap.
What as interesting is that is was so turned anti-clockwise (i guess advanced) that it was touching the starter and also the vacuum line was on the other side of the oil pipe so I wouldn't have been able to turn it.

Will continue tomorrow...

Plug cables seems to be in the right place.
Gonzalo Ramos

Before your electrics became the focus of attention I was about to sound off again about the rear filter canister being upside down. It would cause similar symptoms to your running problems.

That said, I agree with comments above that electrics are the cause of many issues. Lyndsay's suggestion about removing the filters for a trial run would also highlight any issue with an upside down filter box.

Steve Gyles

Gonzalo, did the points you took off look 'cooked'?
Lindsay Sampford

The points I took off were not 'cooked' as in your image but they were very worn. The plastic leg was worn and the actual contacts too.
When I took the points of, the points were barely opening for a couple of degrees due to the wear.
Partially my mistake as I have never had a close look at them since I purchased the car 18months ago.

I have been reading a bit on this, hence I conclude that the dwell angle was indeed to too long leading to coil overheat (this was also the case as my coil was always too hot to touch).
Also this would explain the random hesitation and malfunctions when hot (identified by me as vapour lock), as when the points get hot and expand they may tend to remain closed?

Anyhow, before the rebuild the distributor was 'sticky' and did not turn freely. After rebuild & slight oiling of mechanical advance it turns very smoothly.

I checked the resistance of the primary windings of the coil and they read ~3ohms. So I believe it is the right type of coil.

Steve, also for peace of mind I have taken off the carburetor and I am going to check the filter box mounting and also the butterflies.
During my last carb tune-up I felt that even with both carbs CLOSED (idle screws backed off) the engine would still run happily at around 700rpm, which to me indicates that either one of the butterflies is open of there is an air leak somewhere.

Also inspecting the plug cables I found that they are carbon core. I guess this should not affect anything, right.

Next step is to put it all back together and set the static timing. But to be honest I am scared I will not be able to achieve it.... ;(
I will try using the WSM & Guru's instructions.

Gonzalo Ramos

Butterflies are beveled on edge, and if installed backwards or upside down, you'll never get them to seal. They are easy to swap or reverse when rebuilding a set of carbs.

I had an air leak because the distributor advance vacuum tube was cracked where it entered the carb body...made it hard to reduce the idle far enough.

For good measure, I replaced all the carb gaskets and added new Moss spacers as well.

Sounds like you've been running around with an engine that has not been set up correctly for a long time. The good news is that once you get the dizzy, points, dwell, timing and carbs all done - you'll be thrilled at the improvement in the performance!

AJ Mail

Well, I may say that Steve could be right althgh only partially.
The filter boxes are not mounted upside down, but Indeed one of the paper gaskests of the rear carb covers patialy one of the holes (like 33%). this may not be thé sole culprid but a contributor.

I iwill also check the vacuum line.

Btw i also ordered a strobe timing light to make sure I get things right on the timing. $18 on amazon! I guess it is always good to have.

Let's see how it runs when I put it all together.
Gonzalo Ramos

Gonzalo, -- A few points in order of importance:
1.) Before you endure more pains, reset the distributor drive gear and angle of the distributor to the correct orientation. See here:
2.) Pulling #3 and #4 plug wires makes no difference in running means there is no fire in those cylinders. First suspect is the wires being switched so it sparks at the wrong time. Firing order is 1-3-4-2. Hold the spark wire terminal near a cylinder had bolt to verify there is actually spark present.
3.) If the spark wires are in correct order and do provide spark, the next suspect is no fuel getting through the rear carburetor. Remove top of the rear float chamber. If no fuel inside, then the inlet float valve is stuck shut, or the inlet port is clogged. If there is fuel in the float chamber, then suspect that the rubber grommets between float chamber and carb body are deteriorated and swelling, and the rubber is clogging the side holes in the banjo bolt.
4.) I personally do not like carbon core spark wires, although when in good condition they should not affect running. Use an ohm meter to check resistance of the HT wires. They should have about 5000 ohms resistance for each foot of length. Common problem here is a bad termination connection at one end (no connection at all). Where spark has to jump a gap between end terminal and core it will burn out the carbon core for up to 1/2 inch from end of wire. Solution is the cut 1/2 inch off of the wire and reattach the terminal. Push a solid wire into the end of the carbon core to make a solid electrical connection. Bend the wire over onto the outside of the spark wire and crimp the terminal end over the solid wire.
5.) I also do not like a distributor cap with aluminum terminal posts. I wish the things were declared illegal, and only copper or brass terminals were used. Spark is constantly jumping the gap between rotor and post. This results in corrosion of the surface of the aluminum post, turning the surface into aluminum oxide. Aluminum oxide is a wonderfully hard grinding abrasive. It is also a very good electrical insulator. When the inside face of the post is covered with aluminum oxide, the spark has to jump the the side of the post. When the sides of the post are coated with the hard stuff, the spark has to jump around to the back of the post to find electrical path. When the spark path (length of spark gap) gets too long the resistance will be too high, and you get misfire. Aluminum terminal posts guarantee repeat sales of distributor caps, perhaps 24,000 mile intervals. My Mallory distributor caps with brass posts run over 100,000 miles with no such issues.
6.) Driving around town in hot weather, everything in the engine compartment will be too hot to touch. Hot coil is not necessarily any fault in the ignition system. If the coil gets scorching hot when idling with the bonnet open, that's a problem. Carburetor too hot to touch is a constant problem when we are forced to use fuel with 10% alcohol content. This should be okay driving at normal speeds, but stuck in stop and stop traffic can cause the fuel to boil. The more vapor bubbles the leaner the mixture. If pulling the choke out helps, then it is too lean. This problem should go away within a minute or so once the car gets back up to road speed.
7.) The engine is very tolerant of a large range of dwell setting (points gap). Lucas 4-cylinder dwell is around 60 degrees. Mallory dual points dwell is about 32 degrees for single points, and about 40 degrees for dual points (and will be much less for 6 or 8-cylinders). As long as the points actually open and close you get spark. Change of points gap does affect timing along with dwell, so resetting the points gab should restore both correct dwell and correct timing.
8.) Engine still running with all three idle screws backed off indicates at least one of the throttle plates is not closing all the way. The easy check first. Loosen one of the accordion camps on the shaft between the carbs, synch the air flow, then tighten the clamp. If it still runs with all idle screws backed off, then most likely a throttle plate is not centered and is sticking partly open. This happens when a throttle shaft gets worn and allows the shaft and plate to run off center. A worn shaft should be replaced. A temporary "fix" is to loosen the two screws securing the plate in the shaft, realign the plate to touch the bore all around, and tighten the screws.
Barney Gaylord

I have two timing lights but neither is an "advance" or "set back" light.

In order to set timing dynamically with a strobe, you need to either have a mark to shoot at at the correct place on the crank pully, a mark on the engine somewhere that will line up with the TDC mark at the correct number of degrees of advance, or use the setback type to use the TDC mark only - correct?

I am worried that the $18 light from eBay will not help you here...

I would like to set my timing dynamically at some point (to ensure my dizzy advance is working) but have decided I should go to Sears to buy an adjustable (not digital, though) meter.


AJ Mail

Barney, thank for the instructions! very detailed indeed.
I will follow them to make sure I eliminate all the variables out of the equation.
By the way congratulations on your award in Reno! You really deserve it and we all know why!.

Jim, Regarding the strobe, my understanding is that using the dimple (shine the light on it) of the crankshaft pulley you can use the strobe light to set both the static AND dynamic timing.
The pistol uses an inductive pick-up on plug wire no. 1.

Static timing would have to be set a very low RPM (1000rpm or less) to avoid the mechanical advance from kicking in. Even if it does, from what I have read you can just set it to 12deg BTDC at 1000rpm and should be fine.
Then see if the max dynamic advance can be 34deg with mechanical + vacuum advance and around 3000rpm.

I guess what is difficult is 'measuring' 34degs as the only marks are 5deg & 10degs. I suppose you would have to 'guestimate' it by geometry...

Gonzalo Ramos


Whilst agreeing with Barney's comment about the type of HT leads he prefers, I found that the dynamic timing light did not work with copper leads and I had to use the carbon fibre type. I got multiple timing light marks and was unable to work out which one to use. I seem to remember someone else on this BBS having a similar problem.

Steve Gyles

Steve, my Snap-on tachostrobe works OK with copper leads.
Lindsay Sampford

Yesterday I put everything back together.

- Reset the carburetor butterflies and indeed there was one which was not seating perfectly.

- All other things in the carbs. seemed fine, but I also changed the seals around the float bowl as I believe that they could also be restricting the passage of the fuel and we getting quite squashed even if they are only 1 year old.

- Checked the plug wires. I found 3 of them were around 4000-5000ohms except one which was 13000ohms so I cut 1cm off it and re-installed. This seemed to cure the problem.

- The dizzy cap I had was the original brass type, but the new one I bought was Aluminium, so I cleaned up the old one and re-used it.

- I set the Point gap to 0.015".

- Next I tried to identify Cylinder no.1 TDC for the timing but I need to read some more documentation in order to do it well.
Unfortunately my crankshaft pulley does not have a mark for the TDC. I suppose this was substituted by the DPO and now I have no timing mark.
I took of the tappet cover to try to look at the tappets. I understand I need to look for when BOTH of the tappet on cylinder 4 are 'rocking'. This indicates when cylinder 1 is at the top of the compression stroke.
My question is, is there only ONE position when this would be the case or do I need to check something else to make sure I am not 180deg out?

- Also another thing I will probably need to do is reset the dizzy gear but I am a bit scared about that one for the moment. I need to read carefully barney's notes.

Gonzalo Ramos

Be careful if you reposition the distributor drive gear, it can drop into the oil pan if you don't get a good hold on it. I think it takes a 5/16-24 screw threaded into the center, but you still have to be careful.
G Goeppner

Yes, be careful. I once dropped my drive when working on an 850cc mini engine. I spent an impossible effort of trying to fish it out with a magnet before taking the engine out & strip down. This was all because I wasn’t happy with the position of the dizzy.

R A Evans

It's still lying in the sump of my spare 1500. At least it won't rust!
Steve Gyles

The pulley has only one mark on it - a small notch. Put some white paint or white-out on the notch so you can see it better when you point the timing light at it. The timing chain cover should have three pointers on it at the bottom - a long one for TDC, and two shorter ones for 5 and 10 degrees BTDC. (The longer one is quite often broken off but the remnants should still be visible on the piece that was welded to the cover.)
The static timing for 1500 and 1600cc engines is 7 degrees BTDC, so the pulley notch should be between the two short pointers when the number one piston is on its compression stroke and approaching TDC. The pulley turns clockwise looking at it from the front.
An easy way to set the static timing is to turn the engine slowly until the number 1 piston is approaching TDC on compression and the notch comes to between the two short pointers. Then loosen the distributor clamp and turn the distributor until the points are just opening - you can put a light across the points, or put a thin piece of paper between the points, and when it comes out the points are just opening. Then clamp the distributor without moving it.
The 1622 engines are static timed at either 5 or 10 degrees BTDC, depending on engine number and low or high compression.
P. Tilbury

Unfortunately my pulley does not have a notch so I will need to identify the TDC for Piston 1 manually using the literature on Barney's site.
The timing marks on the cover are all there.

Seems quite straighforward, but how do you make sure that the the piston is on TDC on the Compression stroke and not on the exhaust stroke. I understand that cylinders 1 and 4 are mirrors of each other so they both would have both valves closed.

I think after this exercise I will wonder how my car was actually running up to now.
Gonzalo Ramos


It does not matter. As long as the piston is at the top. The pulley is bolted directly to the crankshaft, so the mark will always indicate No.1 piston position regardless of whether it is firing or exhausting.

Steve Gyles

Gonzalo, are you sure your pulley hasn't got a notch? It's not easy to see tucked away in the dark and dismal underbelly of your car. It is on the rim of the inside (or back) flange of the pulley and is not really that obvious, so worth having another look with a good light. Set #1&4 pistons at the top of their stroke (doesn't matter which one is on compression) by using a long screwdriver down a plug hole while you slowly turn the engine by hand, you should then find the notch somwhere near the pointers.
Lindsay Sampford

The piston on compression will have clearance on both valves, which you can feel by hand; the one on exhaust stroke will have no clearance on either at TDC, but the valves will be very nearly completely closed. So, turn the engine until #4 ex is open, then turn a bit more until it is almost closed. #4 Inlet will still have clearance until you get very close to TDC When the clearance on #4 inlet disappears, you are just before TDC, when #4In just starts to open and #4Ex is still closing you are on TDC within a couple of degrees; when clearance appears on #4Ex you are past TDC.

FR Millmore

Thanks for the clarification regarding the TDC. I think I would have missed this.

The good news is that I found the notch on the pulley. It was covered in dirt but I have now found it and marked with white paint.

I tried to reset the dizzy drive gear, but failed miserably. I took the clamping plate off but I was not able to remove the base casting. I took the screw out but it is frozen solid in the block.
I am going to make a second check before I try again.

I am hoping that by this weekend I will have everything back together and working.

Thank you all for your help.
Gonzalo Ramos

Before fighting the distributor drive, how are your plug wires located in the cap?

It is entirely possible that somebody just relocated the wires because they put the distributor in in the wrong position. From what you said it sounds as though your dist is 90degrees anticlockwise from where it should be. Which way does the rotor point when #1 is on compression?
Vac advance should be near straight up to a bit forward, and if not you might not be able to time it correctly.

FR Millmore

Plug wires were in the correct position.
Distributor was rotated 45deg anti clockwise from vertical.

BTW how does the advance wheel need to be set when you first re-install the dizzy at 7deg TDC?

Gonzalo Ramos

Gonzalo, set the A/R micrometer adjuster at the mid-point of its travel so that you have the option of fine adjustment in both the advance and retard directions.
Lindsay Sampford

The position of the knurled nut is irrelevant when setting the static timing. However, if you preset it to the middle position, ie, equal thread on the adjustment bolt, it will give you equal distance to manually "click" it to advance or retard the timing in the future.
Note that 10 clicks on the nut is equal to about 1 degree of timing. Also, turning clockwise on the nut retards the timing.
Another way to check the number one piston is on its compression stroke is to put your thumb, or finger, over the no. 1 spark plug hole while turning the engine clockwise, and you will feel the air being forced past your finger (both 1 and 2 valves are closed). If you are doing this by yourself, try putting the car in 4th gear and rock the car backwards and forwards.
P. Tilbury

OK, sounds like the dist drive is one tooth too far CCW.
Normally you can free up the casting by removing the screw and tapping the casting on the ears so as to turn it one way then the other. Penetrating oil helps.

The vernier timing adjuster gets set about in the middle of the range to start. There are a series of marks on the neck of the advance capsule that register with the edge of the dist body, right above where it is marked "A-R".

FR Millmore

This might help.

My dizzy is rotated too far forward, so I will need to do this as well.

(one tooth off, I think).

AJ Mail

My frustration keeps increasing. I spend 2h last night and 2h this morning trying to figure it out but new things keep coming into the equation! ;O

This is what I have done;

- Found TDC at #1 on compression & painted notch in white.

- Checked the position of the dizzy drive gear. And seems close to 1.30 o'clock (maybe slightly more towards 1 o'clock) so I did not change it since the rotor was pointing to Plug wire #1 (even though I finally managed to take off the base casting).

- Put in the distributor and set the static timing to 7degs. Actually at this position the vacuum line cannot be connected as it inteferes with the oil pipe going into the oil filter (this makes me think that the distributor gear may be 1 tooth out). Looking at Barney's notes he suggests 20deg dynamic timing at 1000rpms, so I set the timing to that using the timing light (actually the timing light for $18 from amazon works great).

- I start the car to tune the carburetors although they should be close to right as they are at 13flats from completely closed. The engine runs but shakes are lot at around 1300rpm.

- The engine RPMs don't seem to be stable and cycle permanently up and down over a period of 5 seconds. Is this what you call galloping? I tried to go poorer on the mixture right to the top but this effect does not go away. This is the first time I come across this phenomenon.
This makes it very difficult to set the idle as once set the engine will progressively accelerate on its own. If you turn the idle down it will die.

- I tried tuning the other carburetor with the same results.

- I tried playing with the timing while the engine is running but this effect seems to stay whether advanced or retarded.

- By the way in my old setup I realised that the timing was set up close to static 0deg tdc or a couple of degrees less! (all the way CCW)

- I also noticed that the position of the floatbowls (you know they can be moved slightly forward or back) had a big effect on engine RPM. So I wonder whether the float valves are working well (there again there is no overflow from the bowl).

- Also I notice some different noises in the engine depending on the timing and rpms but I am not sure if that is knocking / pinking or what.

- Again what is very abnormal for my engine is to shake so much particularly at around 1000rpm.

- This may be completely unrelated but; Is it normal to have some smoke coming out from the tappet cover (screw holes and oil filler cap)? It does smell like exhaust gas, but I am not sure this was occurring before. Can it be something to do with the timing, or just due to engine wear?

I realise this is a lot of information, but if you have any ideas I would be very grateful as I am not sure what to do next.

Gonzalo Ramos

Running too rich and timing too advanced, sounds like.

For the float bowl issue, one easy thing to check is the float bowl heights...7/16" drill bit should just fit under the fork when you flip the cover upside down.

If the rubber is in good condition and the washers are installed correctly, then moving the bowls a bit back and forth shouldn't have an impact on running speed, but it sounds like the rubber may be having an impact on fuel flow into the jets.

Go back and set the static timing at 10* BTDC and then do the rest of your checks and tuning. When you are all done, and everything else is sorted, you can go back to playing with timing to your hearts content.

At 1300 rpm, or even 1000 rpm, your vacuum advance should be kicking in a bit, so if you have it disconnected, that could be creating some of your galloping issues.

As far as smoke from the cover - I haven't experienced that yet, so don't know. But it doesn't seem right! Are you sure that you checked your valve clearances and set them correctly? John Twist has another video for that where he does it in 5 minutes - so probably 1/2 hour for you and me!

Take it one little bite at a time!


AJ Mail

The jet nut has 6 flats, so 1.5 turns for initial setting, as suggested in the owner's manual, would be 9 flats down from the top most position.
This is why you are running way too rich. (As per Jim in NH above.)
Check that there is no fuel leaking from the float bowl grommets when the ignition is on. Grommets get hard (old ones), or get soft and disintegrate (new ones that are not VITON material!) and can leak. This is dangerous, obviously.
P. Tilbury

Hmmm...I thought 12 flats was the starting position - but you say 9?? Now I question why I thought it was 12.

Please someone confirm!

AJ Mail

This is eerily like a problem that our friends had with their MGA 1500 on the way back home from MG2011 in Reno. We managed to limp to an exit on I-80 to look at it. We started pulling the spark plug wires and as with yours 1 & 2 made it run worse and 3 & 4 made no difference. Well we checked end cleaned the spark plugs and changed the points condenser etc. with no change. There was spark at 3 & 4. I had a spare MGB distributor cap with ignition wires on it in the boot of my MGB. We installed it and there was still no change. We should have done that first and saved ourselves a lot of time. It was none of the ignition parts. We pulled the dashpot off the rear carb and discovered that one of the large felt washers that seals the air filter in the Vokes can had got sucked into the throat of the rear carb. We removed the felt washer and it ran perfectly again. I should have known. Points, timing etc. would make it run bad in general, but not just 2 cylinders bad, and the other 2 fine. Way back at the beginning you mentioned that same effect.
The distributor cap or spark plug wires could be bad on both 3 and 4 but that seems unlikely. If the wires on the spark plugs were out of order you would likely be getting backfiring and terrible running. (Been there, done that :>)

1) Is there compression in #3 and 4?
(Blown head gasket?) Might explain smoke
coming out of the valve cover.

2) If the copression on all cylinders is
OK make sure the rear carb is
syncronized with the front. If you
don't have a syncronizer you can
temporarily loosen the damper caps
and run the engine, watching the caps
rise and fall when you raise and lower
the RPM. That would indicate even
airflow. Barring that there could be no
fuel grtting to the back carb, (Float
or inlet needle stuck)

3) As Andy mentioned above the jet may have
failed to return fully on the rear carb
after using the choke, except you push
upward firmly on the bottom of the jet.
You will feel it move if it was stuck.

4) If you want to continue on the ignition
route, If you have a spare distributor
cap and new wires to try that would
pretty well eliminate ignition if
there was no difference.

Good Luck

Well, this subject has been kind of quiet.
I went in the archives and looked at your first post. According to that the problem
all started after you removed the carbs, and heat shield etc. to improve, update the heat shield, and re installed them. If that is indeed the case, and the engine was running fine before you started, that is where you should be looking. Since adjusting the mixture nuts seems to make no diference, I am going to suggest that you have an air leak behind the carburretors. The leak is large enough that adjusting the mixture nuts can not overcome it. If it were me I would remove the entire assembly again. If you removed the intake take that off also. Possibly one of the gaskets is missing, out of place, damaged etc. Could your modified heat shield be preventing the gaskets from seating, creating a gap etc?
I don't know as much as some of the experts on this BBS, but I have been driving and 100% maintaining my MGB since new in 1973 after the first 12 months of warranty, and restored my MGA and driving it also since 2003. In 38 years with both cars I have never had to walk or call a flatbed once. We limped home a few times but always made our destination. My point is, if you have a workshop manual, it is always better to take a little time out and think things through to solve problems. You don't want to start jumping around or owning one of these, or I suppose any old car could become a frustrating experience. Are you a member of a local MG club? In our Windsor-Detroit MG Club there are always members who will come out and give you a hand if you ask. Sometimes all it takes is a second set of eyes.


Jim, you are right. according to the official S.U. instructions "Screw the jet adjusting nut untill the jet is flush with the bridge of the carburetter or fully up if this position cannot be obtained." "Turn down the jet adjusting nut two complete turns". That's 12 flats in my book.
Lindsay Sampford

You are right - 2 turns is the suggested starting point. I don't why I had 1.5 complete turns in my mind. Obviously 12 flats is two turns.
Apologies, Peter.
P. Tilbury

No apologies necessary Peter, I thought one and a half turns sounded right till, prompted by Jim, I looked in the book!
Lindsay Sampford

Whew...thanks. You know what a PITA it is to turn those adjusting nuts with the carbs on the car!

I was installing them late at night, though, and now I can't remember if I actually remembered to turn the second (front) carb out 13 flats like the rear, or if it is at 12...dangit. Now I have to run them all the way in and count going back out again.

Gonzo - how are you doing? I agree that you're problems could be caused by an air leak! I replaced my carb to manifold gaskets and used a bit of Indian Head sealant (gasket shellac) on the ones for the manifold-to-heatshield and the heatshield-to-carb spacer joins, just for good measure. One of my carb spacers was cracked (original fibre kind) and I installed the new plastic ones from Moss, which are thicker as well. Make sure that the vacuum line to the rear carb is installed correctly. I had a crack in the line near the end which was hard to find, but caused a lean running condition until I fixed it.

AJ Mail

Yesterday I spent a couple of hours on the car and I think I fixed most of the issues. I was going to say that the car runs perfect until I took it for a drive.

- I fixed the timing. 10degs static.

- I tuned each carb. individually and even managed to set them correctly even with the lifting pin!!
BTW; The SU tune-up manual says to start with 1.5turns.

- I realised I had an air-leak on the rear carb. as each time I touched the jet screw it would stumble. The nuts were not done up tight enough.
BTW; how the hell do you tighten up the bottom nuts on the carbs properly? Its impossible to put in a spanner properly, and a socket does not fit!

- Then I syncd. the carbs as per Barney's method and took the car for a drive.

- Immediately I realised the engine runs much better with possibly a bit more power and more smooth in general EXCEPT at very low RPM.

- After a good run when I come to a stoplight, the RPMs will go way down to 500rpm then come back up to 1300rpm and then permanently oscillate between 700-1300 at idle. The engine will shake too at this rpm level.

These are the problems I suspect;
- I am running with the vacuum disconnected so I suspect this could be an issue (but the vacuum tap is before the butterfly). I can't connect it until I reset the dizzy timing gear and I didn't want to do this until I had the car running properly first.

- The carbs may not be properly synchronised. I will do this again. This time with the tool (which I was took lazy to go and get).

- Could be the fuel delivery? When I open the hood often I see the fuel filter is partially empty (it is one of those clear glass ones). Is this just an air bubble or something more serious?
The car will accelerate fine and maintain high speed without a problem so the is enough fuel when the carbs suck, but maybe not enough at iddle?

- Tonite or tomorrow I will have another look, but I can see light at the end of the tunnel now!

Gonzalo Ramos

Ralph, that was a great call on Gonzalo's carb problem, it looks like you hit the nail right on the head with that one!

Glad you are getting somewhere at last Gonzalo.
Colyn Firth

I think you will find that the SU advice of 12 flats pertains to later carburetors. The H4s used on the MGA will not allow the jet to come up level with the bridge. It is recessed about 1/16 inch. I have found that 4 to 6 flats is about right.

John DeWolf

To tighten the bottom flange nuts on the carbs buy the cheapest 9/16" combination spanner you can find. Heat it red hot about 3" from the open end and bend at 45 degrees. This modified spanner will do the job.

The easiest way to work on the carbs and get everything nice and tight is to remove them assembled to the manifold with heat shield in place. You need an old fashion 1/2" offset ring spanner for this job.
Mike Ellsmore

Good going Gonzalo,
The fuel filter being only part way full with gas is not usually a problem. My MGB has done that for years. It used to worry me but it has never caused a problem. We should just buy metal ones, then we wouldn't know what was going on inside! You will see postings for that phenomena on internet bulletin boards for all kinds of vehicles.
The vacuum line disconnected could be causing your idle issues, especially if you still have a small air leak if you had trouble tightening the carb nuts. I have never run one with the vacuum line disconnected from the vac. advance. I know it will not run right, but I am unsure of the exact effects. I suppose you could plug the end of the vacuum line, but you still would have no vacuum advance.
It sounds like you are getting close. Once you get the carb nuts tightened well, and the vacuum line connected, hopefully you will be all set for a summer of MGA fun.


Whenever I disturb the jets to replace the seals etc, I first measure the gap between the bridge and top of the jet (red lines shown in John DeWolf's diagram). This can be done with an external caliper. I reassemble to the same measurements and find that this takes a lot of the pain out of having to set 4, 6, 9 or 12 flats. Perhaps a minor tweak may be necessary, but usually I find the mixture is spot on.

Steve Gyles

Ok, just to put in my .02c worth. Have you checked you have not let a carb spring (or any other small item) fall into the intake of the engine and is intermittently hold the valve open.

I did this once and could not get the B to run smoothly. Once found, I needed to have the valve re-seated.
David Lake


Vac line disconnected with no plug will mess up rear carb adjustment, and cause high idle.

Also, no vacuum advance will cause the oscillations, I you're on the right track!

A stubby 9/16" wrench works for the rear carb. I install the rear carb, tighten, then the front carb as that is the only way I can get a wrench in there. In a pinch, I have used, er, asked my daughter to help as she can get her whole arm under the carbs with no trouble at all...

If you chase the threads on the nut and bolt for the lower carb nuts, you can spin them on with a finger tip until seated and then only need to do a bit of tightening with the wrench.

I used some Indian Head shellac gasket sealer on the new gaskets between the manifold and heat sheild, and the heat shield and the carb spacers for a bit of insurance since I knew I had leaks somewhere. It does't take much, but you won't be reusing the gaskets either.

You and I are both going to try moving the distributor gears - this weekend? - on our cars! You first! (then tell me what you found...)

I would jump in and try the 9 vs. 12 flats, but I think that will be way too lean - and I have original H4's now set at 13 flats. I am going to have to research this more.

AJ Mail

I think Steve Gyles is correct, the important dimension is the distance of the top of jet below the bridge. The adjusting nut appears to be 24 tpi, so two full turns below the bridge, as suggested by SU would make that dimension 1/12 or .083 inches. The maximum amount the jet can be raised may vary between carburetors so it would be best to do the initial setting to this dimension rather than number of turns.
John DeWolf

I have also have set the top of the jets to the measurement below the bridge with the tip of a caliper. It is simple and works really well.


I think I found the source of my issues.

And the winner is; Jim from NH!
There is a crack in the vacuum line.

I checked the vacuum advance after resetting the dizzy gear and found the line was cracked just below the fuel separator.
It seems like someone tried to weld it back together before but was badly done.
I used some Epoxy glue to seal it. I will test it next week as this weekend I am traveling.

I also tried drawing a vacuum at the distributor with a MityVac gun and the spring moved ever so slightly.
The diaphragm does not hold much of the vacuum but I guest this is by design so the vacuum advance gradually drops.

I did the distributor gear reset as suggested and managed without much problem, but I actually dropped into the same position. So I don't think the gear is miss-fitted.
Moving it one tooth either was would make it completely OUT!.
Jim, it is really simple an should not take more than 15mins to do. Just make sure you screw in properly the screw into the drive gear so you don't drop it into the oil pan! ; ) I managed to not do it.

Now that I have the mixture right, I am hoping that the repaired vacuum line will resolve this idling oscillations.

Thank you for all of your help!

Gonzalo Ramos

Well, I am just reading that I probably have to replace the vacuum unit as it should not leak. The diaphragm must be damaged.
Any of you have a spare they want to get rid of? :)

Gonzalo Ramos

If you can't repair it you might try cutting it and installing a section of hard-walled plastic tubing like you would use to plumb an ice-maker. They make 1/8" tubing, and you can probably get it at Home Depot - provided there is enough good tubing to get a bite on under the oil bulb.

I tried refitting my gear tonight - and broke the cast boss on the sleeve! Dammit! One step forward, two back...

Moreover, my gear looked like it might be "right" also - but my vacuum advance pointed straight forward, so what gives?

I took the time (not in a hurry now!) to rebuild my distributor and it looks pretty good, though the replacement rotor cap I ordered doesn't fit (and is aluminum also). Some repro parts are horrible.

I am having trouble figuring out how to set the valves - I had trouble before I remember now. Will get help tomorrow from my neighbor if I can.

Good luck!

AJ Mail

If the gear is installed right, and the distributor is 90 degrees out of place, then fix is real simple. Pull all the wires off of the spark plugs, rotate the distributor body 90 degrees up to where it belongs, and then put the plug wires back on in the right sequence (starting with #1 wire being closest to #1 spark plug).
Barney Gaylord

So I needn't have broken anything! Dammit!

AJ Mail

In my case the position of the distributor is about 30deg CCW from vertical but I think this has to do with the vacuum advance being broken so it mechanism got stuck too advanced.

I think all my problems will soon be resolved.
Is it possible to repair the vacuum diaphragm?
I suspect you would have to pry it open and it could get messy...
Gonzalo Ramos

Gonzalo, I wouldn't think it likely that your vacuum unit is causing your distributor position problem, sounds more likely that you are still one tooth out as suggested earlier. If your vacuum unit is leaking it might be that the hex nut seal is loose or faulty, but if not, these people can supply the correct unit for your car
Lindsay Sampford

Dear Gonzalo, I see one of the posters(American) on this very interesting thread( we've all been there) addressed you in the diminutive.So I hope you won't be offended when I tell you that I once went into a Gonzo look alike contest(from the Muppets- I know class when I see it) but got rejected for being too ugly. Had to get an MGA to attract even the grimmest of birds.

Good luck with the problem.
M Blencowe

I used "Gonzo" affectionately, not pejoratively. (I have another friend whom we call Gonzo instead of it's seems natural). Gonzo is one of my favorite Muppets as well, though. I apologize if Mr. Ramos takes any offense, and I have no idea if he is diminutive or not!

AJ Mail

Looking at the vacuum unit link, mine is missing the spring that is next to the knurled nut on the end of the shaft - is it supposed to be there?

JIM (the diminutive form of James) in NH
AJ Mail

Yes. Otherwise the vac unit and the breaker plate vibrate like crazy, wearing stuff out and giving spark scatter.

FR Millmore

Don't worry, I don't get offended about calling me Gonzo. I don't have a long nose either!

Lu say I had a look at the site you suggest. $90 for a vacuum unit seems expensive for what it is. I am hoping to find one cheaper somewhere.

My impression is that as the diaphragm on mine degraded it got stuck half way on the advance hence the spring is in the middle of the travel. Not sure if this is right but that is my theory.

If someone has a spare they want to sell, let me know.
Gonzalo Ramos

Gonzalo, I had to pay $90 plus shipping across the Atlantic, plus import duty, plus post office handling fee!
Lindsay Sampford

For that money I would suggest one of the new chinese-made distributors that apparently cost around the same (or less) and have an excellent reputation. Or contact Jeff Schlemmer.
dominic clancy

Moss also have the vacuum advance units - featured on the back page of latest MGA catalogue
Mike Ellsmore

Found a spring in my magic junk drawer and installed it one the dizzy - thanks for the advice.

Still waiting for my inside pipe wrench to arrive.

How are you running now, Gonzo?

AJ Mail

My work on the car is stopped this week as I am traveling so will look into it this weekend.
I will have a investigate where I can get a bargain vacuum advance.
There are some guys on the internet that repair the back to original spec for $30. In thin its call of
Gonzalo Ramos

Gonzalo, I think you mean
Andy Bounsall

Gonzalo, get in touch with Jeff Schlemmer at www. Jeff is a very very helpful guy, he rebuilds Lucas dizzies very professionally, he can supply you with parts and most of all his prices are very reasonable. I highly recommend him.

F. Camilleri

Got my busted sleeve out (drilled holes and took it out in pieces) and got it all back together. I found another break in my vac line at the engine bracket and soldered it - reoriented my distributor, set the valves and went for a drive!

AJ Mail

I took the vacuum and inspected it. I have the impression it may be in working order but I realised that the internal spring keeping the membrane down is a very strong one. Is this normal? It must take a hell of a vacuum to move that!

After repairing the vacuum line and resetting the timing to around 12deg TDC at ~1100rpm the engine runs pretty good an does not have any more those oscillation at idle.

However what I notice is that after running the engine at high rpm for a bit (~4000rpm) when it goes back to idle it will stumble and die.
This happens typically when I drive in 4th gear and then stop at a traffic light. I have to keep opening the throttle to avoid it cutting out.
Do you have any idea what this could be a symptom of? Carburetors too lean? or timing?

I think I am getting there!
Gonzalo Ramos

Sounds like a fuel mixture problem. Likely one carb running rich and the other carb running lean.
Barney Gaylord

Sounds like running too rich, or float bowl valves leaking by (which also would cause flooding).

Check your plugs - smell gassy?

Check float height, sunk float, float valve (for grit or gunk)...

If Barney's right about one rich and one lean, the 1 and 4 plugs will look different, which tells you which is the problem.

I checked my plugs and 3 and 4 were blacker and sootier than one and two. I measured the jet height as suggested earlier by using a caliper to measure the height below the carb body - about 1/16" or so. I set the rear to match the front - about a flat and a half higher. This seems to be more accurate than just counting 12 or 13 flats. I think mine are now at about 12 and 11, front and rear, respectively.

Good luck!

AJ Mail

Thanks! I will check it tonight and let you know the outcome.
I think I am nearly there!
Gonzalo Ramos

Gonzalo, why do you not have an overflow on thr carb bowls?
Art Pearse

I think I got it right now!

I synched the carbs with the Unisys tool and started to run much smoother.
I also turned the mixture one/two flats up to leaner and now the engine does not die on idle after a big acceleration.
Also the engine does not run-one any more.

I did however check for vacuum leaks around the throttle shaft by spaying brake part cleaner. If found that as soon as I sprayed it the RPM would increase very rapidly so there must me some wear and air entering that way. But difficult to gauge what is acceptable.

I will not be fixing this immediately but something I need to think about for the next little project!
Now I am just going to drive the hell out of it.

Thanks for all your help.

Gonzalo Ramos

By the way this is what spark plugs 1 and 4 look like.
The whiter one is no.1.

What do you think?

Gonzalo Ramos

#1 looks about right, #4 is rich. Screw the jet up 2 flats on the back carb and check the plugs again after a run.
Lindsay Sampford

Agree - one looks great.

Two flats might be too much in the lean direction - no. 4 doesn't look THAT rich...but I guess I would prefer to sneak up on it.

Try the measuring the jet height thing - if it holds true, then your rear carb jet should be a bit lower than the the front carb jet right now.

Congrats on working through everything - it's been fun!

AJ Mail

THanks. Will do!
I think it has been more fun for you Jim and than me with your distributor plate incident!

Enjoy the summer rides.
Gonzalo Ramos

This thread was discussed between 18/06/2011 and 13/07/2011

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