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MG MGB Technical - SU HIF Carb Issues

Hi there,
I am new to this forum but have read a number of the threads relating to SU Carburettor problems to try to help me out.
Last year I bought a 1977 MGB Roadster, in pretty good condition, well tuned and set up engine from a reputable MGB and classic dealer. It ran well through the whole of last summer with the only issue being a split in the fuel connection line between the two carbs. This was replaced and everything continued as normal.
I garaged the car in November and then brought it out in April. It fired up after a couple of goes but on getting it out of the garage there was petrol pouring from both breather pipes.
I know there are probably a couple of mistakes I have made, not running the carbs dry before storage or starting the car a couple of times a week to keep fuel flow through the system. I have also read all about the possibility of unleaded fuel "gumming" up the needles etc.
In detail, the progress of the problem was this:

First off the carbs are twin SU HI and as reminder the car is a 1977 1.8 Roadster.

Car fired up ok, idled ok but fuel poured from both carb overflows.

I switched off and prior to doing anything I replaced both the short connector hoses from the carbs to the metallic breather pipe as they were worn. I know this won't solve any problem but they both needed replaced.

On trying to fire up again all I got was the engine wanting to start then immediately dying.

Plenty of fuel in the tank, fuel getting to both carbs as I have checked by removing the fuel line to the front carb.

Read through the Haynes workshop manual (not the clearest) and various threads on this forum.

From there I removed the carbs and checked the float chambers for damage to the float, all ok with them. On checking the "height" of the floats they both seemed to be sitting much higher than the recommended adjustment so I reset them with 1mm clearance from float centre to body casting.

Everything else seemed clean and well set up. I replaced the carbs but still the problem persists.

Last night I have removed the carbs again and stripped down cleaning everything with carb cleaner, blowing through fuel pick up jet, piston needle and main jet channel. Cleaner fluid coming freely through all orifices.

I have sprayed the nylon and metal tipped main needle, checked again the float for damage and petrol being contained, there is none, rechecked the float adjustment and rebuilt.

I have left the carbs overnight to let everything soak through and will replace today.

Given that everything ran perfectly last year but I made the mistake of leaving the car unstarted and carbs with fuel in over the winter I would reckon the problem will be gumming up with unleaded fuel and the needle sticking either down (explaining the starting ok but fuel overflow issue) and then maybe sticking up hence the fuel starvation to the carbs issue?? There is fuel in both the float chambers when the base plate is removed. Not sure though.

If when I replace today and still get no joy, given the above description has anyone got a "next step" for me to help me get the car on the road?

Thanks in advance for any pointers.

Andy Lamb

Are you using 'fresh' petrol? It does go off quite quickly, apparently.

If it ran well last year, I wouldn't have adjusted the floats.
Dave O'Neill 2

Fair comment regarding the float adjustment, made the adjustment after reading the workshop manual and reading some stuff on this forum. I did top tank up with some fresh petrol although the tank had a minimal amount of fuel left over from last year.
Also discussed the fact that unleaded is more hydroscopic (?) so absorbs water more so tank should be left full over winter to minimise this absorption.
Andy Lamb

Just noticed that I have mis-spelled the carb type, probably recognisable but anyway they are twin SU HIF4 carburettors.
With regard to re-setting the float height, this was done after the engine tried to start each time then died and was one of the potential resolutions to the diagnosed problem.
Easily enough changed back but if everything cleaned and clear I'm hoping that on re-assembly tonight I will get a result.
I have ensured that the metal tips of main jet needle are all clear and not sticking and all parts are well cleaned with carb cleaner. I am sure that it can't be anything other than a petrol issue after being left for five months (my fault) as all was running pretty well last year so in theory there shouldn't be anything wildly out with regard to adjustment. Just cannot get it firing after initially getting it going but having the overflow problem.
Andy Lamb

Flooding aside if that is sorted, the old adage that most SU problems are caused by Joseph Lucas maybe accurate!! Time to check the ignition system.
Allan Reeling

Two completely different issues here, one of the carbs overflowing and another of it not starting.

Firstly as it can be checked without actually cranking the engine to try and start it, are they still overflowing? If so this is the first thing to fix. having accessed the floats you should have checked the valves first, before doing anything with the floats really.

You also need to check the floats haven't punctured and taken in any fuel. Ordinarily I wouldn't expect both the fail, but these days ethanol in fuel has a lot of question marks hanging over it. I had overflowing on an HIF some years ago which turned out to be the float, but I couldn't see any fuel in it as it was brown and opaque, it was only by shaking it and hearing it.

If neither are overflowing now, but it is not starting, I'm wondering if having altered the floats you have got it wrong and they are keeping the valves closed even though the float chambers are empty.

As far as not starting goes, rubber bumper MGBs have a ballasted ignition system which means the power to the coil comes through a length of resistance wire in the harness. But that's only for running, for starting that is bypassed and full battery voltage is connected to the coil by the solenoid during cranking to boost the HT when the battery voltage is at its lowest. If the ballast circuit has failed then the engine will start, but cut out again as soon as you release the key. You can check that with a meter on the coil positive terminal, which should have two white/light-green wires on it. With the ignition on you should either see 12v or 6v on that terminal depending on whether the points are open (12v) or closed (6v).
Paul Hunt

Thanks for the input here.

Paul - I only altered the floats after the "non-starting" issue appeared. Not being able to start it then I'm not sure if both are still overflowing. You mention that this can be checked even if engine does not start. Not sure on how to do this. I have checked both floats and although it is difficult to see, having shaken them both there is no sound of liquid inside and under magnifying glass no sign of puncture or leaks at the plastic seams.
The float adjustment that I did actually lowered the float away from the valve so more chance of it staying open than closed. (If I'm getting this correct??)
With regard to suggestions on the ignition system it may be I need to look at that once I have put the carbs back on tonight and tried again but puzzling that after five months it fired up within a couple of goes and runs okay apart from the overflow through the breather pipes. Then all of a sudden develops this fault, although, yes, faults develop anytime.
I'm not the best with electrics but do have a voltmeter and can take my time finding my way around any checking suggestions made here and on other threads that I'm picking up on now.
Any other suggestions welcome and I will report back after re-fitting the carbs tonight.
Thanks again
Andy Lamb

Paul, on re-reading your email regarding the valves being closed even although the float chambers are empty, on removing float chamber base plate there is petrol in both chambers so fuel is getting that far.

Is this starting to point to an electrical/ignition issue? Hoping not, as a mechanical engineer I can understand mechanics and their principles. Electrics - I could live without, but will work my way through things.

Andy Lamb

Have you checked to see if the choke cable is retracting? Silly but very easy to overlook. Which type of fuel pump do you have? Lastly do you have a fuel pressure gauge installed?

Any of these three things not working properly could also cause too much fuel entering the carbs.

Worth checking. Good luck!
JS Ahlund

Your metal float needles may not be seating properly anymore. Viton tipped needles are the current standard and should be used in place of the original metal tipped needles. RAY
rjm RAY

Choke cable retracting ok, not sure on fuel pump and no fuel pressure gauge installed. Fuel wise all worked ok last year. Viton tipped needles may be the answer and I've seen other comments about these. Avoids gumming up from unleaded, cleaned metal tipped needles thoroughly and rebuilt but on re-attaching today am back to the car trying to fire up and then dying.
I do have an electronic ignition unit installed which was not a standard in a 77 B.
Hoping it's the needles and think this may be the way to go, even cleaning the metal tipped ones - will this not prevent them seating badly??
Andy Lamb

"Not being able to start it then I'm not sure if both are still overflowing. You mention that this can be checked even if engine does not start. Not sure on how to do this."

Just for info, the MGB has an electric pump that starts operating as soon as you turn on the ignition. It does not need the car to be cranked or running to see if you have leaks from the overflows, just turn on the ignition, listen to the pump and watch the overflow pipes.

It's not clear from your posts whether you still have overflowing or not. If you still have overflowing then that is the problem you must fix first, you shouldn't try starting it with fuel spilling out. It seems odd that both valves have failed at the same time, you do need to double-check that the floats aren't punctured I couldn't find any source of a leak despite squeezing it and heating it up in water looking for bubbles. Being tight I was hoping to repair it. Changing the valves for Viton-tipped if the floats are good at least, if not in any event.

Viton-tipped won't stop gumming any more than brass-tipped, it's just that they perform better over time. The original brass develop ridges which cause them to start seeping, gradually at first then more and more. For a long time you will not notice this as the seep will be less than the rate consumed at idle let along running. I changed mine over winter as I had the ignition on doing electrical tests (ignition coil disconnected) and it was only after many minutes of this that I noticed a very occasional drip from one of the overflows.

Electronic ignition usually works or it doesn't, under which circumstances the only option is substitution or replacement. You need to do that voltage check first if it still fires up but cuts out when you release the key ... but only after any leaks have been fixed.
Paul Hunt

After re-fitting the carbs yesterday and getting no joy, back to trying to fire but not getting there, I did check if there was any overflow from either carb and there does not appear to be. Based on what you say Paul then with ignition on then the fuel pump would start and I would see this overflow.
So absolutely nothing from either. Again, for the record, it was not a "drip" from the overflows, more "pouring" from each.
I am thinking this is beginning to look like an electrical fault, guessing the coil/electronic ignition - that has just decided to appear.
Really not too sure about electrics at all, I haven't yet had time today to check the voltages, any pointers on most accurate way to do this greatly received.
Single 12V battery, connected to unit in boot and from there to electronic ignition. Haynes manual not overly brilliant with respect to description which not being an electrical genius I need.
I will take things so far but if getting out of my depth with electrics will defer to auto-electrician, but as you say Paul, looking to save money where possible.
Thanks again for the help.
Andy Lamb

For clearer wiring diagrams in colour, try these There are several pages so scroll down to find the one that is right for your age of car. The model is shown at the bottom right of each diagram.

Is your fuel pump working? when you switch on the ignition you should hear it making a sort of ticking noise from near the fuel tank. It should stop ticking once the carbs are full, the back pressure stalling it. Or you can take the fuel feed pipe off the carb, point it into a jar and switch on. Fuel should pump steadily out of the pipe.

Experience generally shows that when cars won't start the carbs are often blamed, but more often it is an ignition problem.
Mike Howlett

"Single 12V battery, connected to unit in boot and from there to electronic ignition"

Explain? What part of the ignition is in the boot?

Further to Mike with the fuel delivery pipe directed into a container the fuel pump should deliver a minimum of one Imperial pint per minute, and in practice more than double that, in a continuous series of pulses with minimal bubbling.

The easiest way to check the ignition is to lay a plug on the block and watch for sparks while cranking.

If those are OK then if cranking for several seconds with the choke pulled has not started the engine check the plugs and they should have a strong fuel smell, if not be wet. If they are not then for what ever reason fuel isn't getting through the carbs.

Is it still firing up then dying? If so you need to check the voltage at the coil as described earlier.

If all OK so far then you need to check the timing even though you may well have a spark. Whether you can do static timing with your electronic ignition I don't know, if not then you will need a timing light. In my experience 12v types may not work when cranking so would need its own 12v supply from another battery. The old neon type with just two cables that go in series with the plug will work when cranking.
Paul Hunt

Thanks again all,
At work just now, but have quickly read through latest comments. I will check the plugs "smelling of fuel or wet", can also check the fuel pump is delivering well, what I do know is that the float chambers in the carbs have petrol in them when I remove base plate so petrol seems to be getting that far.
In line fuel filter, which is clear plastic, showing fuel passing through as well.
Unit in the boot I am taking to be the ballast resistor. I have technical data sheet and installation/wiring diagram for the electronic ignition system in the full history folder that came with the car so will look at doing the voltage checks and the other checks as described.
As an aside, I used to spend a load of time working on Mini's and helping building and stripping various cars back in the 70's and 80's. After that I could afford decent cars that never needed much done and as for modern cars, well, you just never touch them these days. Last year I decided to go back to basics with the MGB so took the plunge and bought one. So a bit "rusty" in getting my head back into actually doing stuff. So all the help/pointers and suggestions are greatly received.
Cheers and I'll keep updates coming.
Andy Lamb

I'd be surprised if a ballast resistor is in the boot!
Picture please and scan of the installation instruction will help.
Where are you in Strathclyde? Perhaps an onsite visit from some kind local member (Mike Howlett ;-) ) will get you going or you'll miss the summer!!! I would but I'm working away from home this summer.

Best of...

M McAndrew

I'll post some photos hopefully tonight. Strathclyde - Scotland's biggest Region taking in loads of places. I'm in Strathaven, so if someone was able to help out on site that would be great and much appreciated. Food provided of course.Yes keen not to miss the summer if we actually get one, not missed anything by it being off the road just now as the weather all year has predominantly been rubbish.
thanks again.

Andy Lamb

Ahead of potentially uploading photos of the ignition set up etc. tonight I tried to upload a photo of the car to ensure it all worked ok, seemed to work but now can't see the photo at all??
Andy Lamb

I'm in Troon, so only 45 minutes from you, but I leave for my holidays tomorrow. Sorry. I'll be glad to get away from the gales and torrential rain, and the 8 degree temperatures. They say it's due to global warming!
Mike Howlett

No worries Mike. Enjoy your holiday and I hope to have things sorted before your return but if not then can catch up on your return. Will now try uploading photos and see if I can suss out voltage readings.
Andy Lamb

The resistor is a length of resistor wire, white and pink, in the forward wiring loom. Sometimes this may have been removed, or simply by-passed, but you will know when you test the voltage, with ignition on, at the coil, or by measuring the resistance of the primary coil windings. 6 or so volts should match up with 1.5 ohms. 12V with 3 ohms. A miss match is bad news either way.
Allan Reeling

At one point you said that you had lowered the float from the neddle. This will cut off the fuel long before you have sufficioent for the carbs to work. I would recheck the float level on both carbs with them removed from car.

Took some photos of coil etc. last night for posting on here. What is best way to upload them so visible on the forum?

Sandy, when I first removed carbs, the float levels seemed very low in comparison to the specified height tolerance indicated in the workshop manual. In other words when turned upside down to remove base plate, the top edge of the float (as viewed when upside down) was higher than the float chamber casting. I adjusted, or "lowered" this to the 1mm clearance specified. This made no difference to the "non starting" so I removed and reset to where they originally were when the car started and ran okay. Still no difference. I will check by cranking and removing plugs to see if fuel getting to cylinders.

Will then need to check if decent spark getting to plugs. Will measure voltage at coil too.

If I can figure out how to upload photos then I'll get them on soon to maybe clarify the electrical side of things.

Andy Lamb

Andy, to upload your photo's do the following
1. Press the upload button
2. Press browse in the window that opens to find your photo - this may be on your camera (which must be connected to the PC) or in your picture / document files (if you have saved the photo to your PC)
3. Double click the photo you wish to upload. Then press submit in the window that opened for you to browse and find the photo.
4 Press submit in the main window where you have typed your message

I have tried a couple of times to upload a photo and got the error message that the file was too large. This may be caused by an IP address problem. Try closing your browser and re logging in to the BBS website - this just worked for me.

Hope this helps

Andy Robinson

What's the problem with pic's?
simple as click upload, browse to the file and insert!
there will be a file size limit and you may need to resize your pi's for web (anything around 750kb should be fine)

Best of...
M McAndrew

Morning Andy & Mike,
Re the photos, I did all as you outlined, even reduced the file size as first off I got the "file too large" message. I have uploaded three photos of the car just to try it out but I can't see the photos anywhere. Weird. I will try again just now to see where I get. Usually I have no bother doing stuff like this, between uploading photos and not being able to get the MG going this is making me look a complete fool instead of just the partial fool I normally look!!


Andy Lamb

Just tried again with apparently no joy.
When I upload the image I see it below the comment box then click submit and it seems to disappear.
Andy Lamb

No wait a minute, I see the "Image" link at the bottom of my script now.

Good, at least something has worked!!

Andy Lamb

Going to try to upload some photos of the coil etc which I hope might show the wiring and set up clearly enough for some pointers and suggestions with regard to checking voltages etc.

Thanks in advance.


Andy Lamb

Photo 2

Andy Lamb

Photo 3

Andy Lamb

Photo 4

Andy Lamb

Photo 5

Andy Lamb

Photo 6

Andy Lamb

Photo 7

Andy Lamb

Hi Andy in your pic 2 the vacuum advance pipe appears to not be connected to the advance bellows/ vacuum chamber on the distributor.
With this disconnected it can cause rough / not idling.
You seem to have sorted the leaking fuel issue so this could be worth a look.

Ray Bester

Your '77 was originally a ballast resistor ignition. In the pictures I am seeing what I assume is a separate ballast resistor.
I say again check the voltage at the live side of the ballast resistor and the resistance across the + and - terminals and refer to my previous note. If your circuit is twice resistored, you won't be getting much output from the coil.
Allan Reeling

Andy, I've studied the pics and can't understand how that ever worked! It looks like only one side of the ballest is connected and that to one side of the coil! Have you moved/disconnected any of the cables? Do you know which ingnition unit is fitted (luminition or other)?
Checking coil voltages won't help until the car is running as the ingnition unit will prevent any current flowing and therefore there will be no voltage drop over the ballast resistor.
If you confirm the unit fitted I'll have a look at the install instructions and advise what should be connected to where.
Email me if you want to do this off line.

Best of.....
M McAndrew

As Mike said, the ballast resistor doesn't appear to be connected at one end.

It's always more of a challenge when your wiring has been modified by a PO.

Also, you are able to see your photos in the thread, rather than the 'image' link. Click on the 'customise' button at the top of the page, scroll to the bottom of the customise page and tick the 'yes' box for 'View images within posts in-line'.
Dave O'Neill 2

Ray, Alan, Mike & Dave,
Thanks for the new input re the photos and I'll have a look at the settings to ensure that images can be seen in the thread too which will make things a little easier. Got some free time today to look at these issues so will check a few things. I will scan and copy the installation/wiring diagram and instructions for the electronic ignition Mike and yes, if you don't mind then I can email you directly with them and hopefully move things on from there.
I know these things happen, but just seems weird that the car fired first time and ran pretty much perfectly (apart from a slightly lumpy idle) right throughout last year. Anyway I will get to work on some of the stuff and report back.
Thanks again.

Andy Lamb

Ok, I admitted defeat and called the breakdown guys under my home start policy. Given that we are all here, into older cars, most of us will remember the days when we rented televisions, for younger members here, yes, really, that was the done thing for many of us through the '60's and '70's!
Do we also remember when the TV went on the blink, the vertical or horizontal hold went and the picture spun around the screen madly and no amount oif twiddling with the controls on the back made any difference? So then our parents called the TV repair man who turned up and as if by magic the TV started working before he had done anything?
You are all guessing where this is going now, aren't you? Breakdown guy turns up, jumps in and it fires up third time, runs very lumpy for a bit, drives it out the garage and within a minute it is running as sweetly as it was last summer. The carbs will need balanced so I'll get that done this week, but no fuel running out the carb overflows so that is sorted and everything working away fine!
So thanks for all the input here but all that was needed was the threat of being towed to a local garage to get her going. Summer here we come!!

Andy Lamb

A 'thing' in the boot near the fuel pump will almost certainly be a radio interference suppressor.

You have to measure the resistance of the coil primary (between the spades, wiring removed) and the voltage at the coil +ve to see if the two are matched. 12v coils measure about 3 ohms, with sport coils about 2.4 ohms, 6v coils measure about half that.

However the coil primary has to be carrying current for you to see the correct voltage. Easy with points, you simply turn the engine until they are closed, or just connect an earth to the -ve terminal of the coil then measure the voltage, and you should see 6v on a ballasted system, 12v on a non-ballasted system. If there is no current passing through the coil then you will see 12v on the coil +ve of both ballasted and non-ballasted systems.

Electronic ignition complicates matters as they may well only pass current through the coil when the engine is running. You can connect an earth to the coil -ve as with points, but must disconnect any other wiring from that terminal first to avoid damage to the electronics.

Some vendors of electronic ignition systems say you must fit their coil as well, and some say you need their ballast resistance as well. And as a pal found some places know nowt about electrics as they connected their ballast in series with the factory ballast, which reduces the current through the coil and hence the HT voltage by about a 1/3rd, and compounded that by fitting a 12v coil in any case which reduced the HT by about three-quarters!

This complicates matters still further as if you have a 12v coil in series with two ballast resistances you end up with 6v on the coil +ve, which seems right, but is very wrong.

In fact the easiest check is to measure the current instead of the voltage, and you should see about four amps, which means you do have the correct combination of coil and ballast under all circumstances. It will be 2.7 amps if you have a 12v coil in series with a ballast or a 6v coil in series with two ballasts, lower still if you have a 12v coil in series with two ballasts! If you have a 6v coil with no ballast you will see about 8 amps and the coil will get very hot.

Did I say all circumstances? Not quite. Some electronic ignition systems use a very low resistance coil as they only send very short pulses of high current through the coil. The high current has the effect of giving much higher HT voltages, but the short pulses means the coil stays cool. In that case you shouldn't connect an earth to the coil -ve until you know what resistance the coil is. If it's less than 1.2 ohms then you can only hope it is the correct one for the ignition system, and only the manufacturer can tell you that. Some coils can measure as low as 0.1 ohms i.e. one tenth of an ohm.
Paul Hunt

Crikey Paul, thanks for the input - V=IR and P=IV is about as much as I can remember from my school and university days. Exceptionally in depth details but to be honest I would defer to an auto electrician on any of the above as you obviously have to know so much more about electrics than I do. You obviously know your stuff there, but to avoid blowing myself or my car up I'd definitely use a professional with regard to the above.
Replaced the throttle cable towards the end of last week - the pedestal guide that is screwed to the top of the pedal box also disintegrated so renewing that and the cable was a tad fiddly and time consuming but managed it and throttle operating so much better now.
The car was out all weekend, no fuel overflow and running well. covered around 180 miles and loved every minute of it. Maybe some slight carb issues - needing balanced, but not much at all.
Thanks again all for your input on this subject. I will keep in touch through this forum.
Andy Lamb

"I'd definitely use a professional "

Select your professional with care! Even auto electricians may not know all the ins and outs of the various MGB systems, depending on how old they are.
Paul Hunt

"Select your professional with care! "

Point taken Paul, I have already made enquiries through local garage owner that I've known for years and he has pointed me in the direction of a couple of guys that have worked on MGB's for years, if he, himself felt it was beyond his capabilities.

Electrics obviously can be a minefield even if you know what you are doing, never mind for an owner like me who can change a bulb, re-wire a plug and recite Ohm's Law!
Andy Lamb

This thread was discussed between 29/05/2015 and 17/06/2015

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