MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Misfire

The saga of getting my car to run properly continues to frustrate me

I have just reinstalled the SUs to try to get to the bottom of the running problems. The car is running unevenly, and I think it is an electrical problem.

I have tried alternative components so far without any change in the running: Coil, Plug leads, distributor cap, plugs, carb (from Judson to SUs) and still have not been able to get away from the stumbling in the running and tickover. I am now certain that this is the reason that the car is not developing any power when the Judson is installed.

I plan to bridge the fuse block and the ignition switch to check I don't have a loose contact there, but would be grateful for other suggestions as to where I could look for banishing the prince of darkness


dominic clancy

Dominic, with car at idle or fast tickover, pull each plug lead one at a time. Engine should stumble or stop. If it doesn't, that's the dud cylinder.
Art
Art Pearse

Dominic,
I have been caught twice (!) on MGA and MGB, by both the wretched condenser and the rotor arm, and have replaced all the other components without results until trying these. Shane
Shanerj

Dominic,
Check your distributor. I have seen bad distributor shafts cause an uneven rpm which does look like a misfire.
Mike Parker

I can remove each plug lead in turn wiithout getting any change in the running at all. And the timing light will fire only intermittently or not at all. I have a Pertronix installed, so it either works or it doesn't, and there's no condensor with the system. The distributor is from Jeff, freshly done last year, and worked fine until the misfire started, so I can only suspect a loose contact somewhere.

I will bridge from the starter switch directly to the white lead at the coil, which will identify or rule out any wiring issues, then I will check the distributor by swapping it out for one with points.

I know the firing order is correct.
dominic clancy

Well after wiping the dust of the engine analyser, I have a steady dwell angle of 55, but a very low KV reading on the HT (1KV or not even registering). The spark voltage appears to be so low that the timing strobe is not being triggered. I know the strobe is working as I have checked it on another car.

I have swapped out the coil, cap, rotor and HT leads as a set again, same result.

Anyone got any theories as to why this should be happening.

Primary voltage is a steady 13.4 volts (system charging as a result of repeated starts and short engine runs to identify the problems)
dominic clancy

Poor earth from your newly painted engine?
Steve Gyles

Do yo uwant to sell your Judson?
Mike
Mike Ellsmore

How about the distance between the pertronix rotor and the trigger? I recall having to elongate the trigger attachment holes to get the correct gap.

Steve

Steve Gyles

Poor earth I hadn't thought of. Can check easily too. On the way to do it now.

Mike, if you are after a Judson, two available here: Mine is not one of them :-)

http://clancy.ch/Judson_MGA_for_sale_1.html
dominic clancy

Bridged between head stud and chassis with a jumper cable, no improvementt. Tested the resistance between block and ground, zero resistance.
dominic clancy

Dominic, I was just kidding - checking your frustration level.
Mike
Mike Ellsmore

Output voltage to coil?
Steve Gyles

Dominic, -- Very low voltage spark = bad condenser. You might think you don't have one, but there is a condenser inside the Pertronix unit. I'm betting the Pertronix unit is bad. If this turns out to be true (after all this bother), would you buy another one? Still got your old points and condenser?
Barney Gaylord

Put the other cap and leads back on, and can measure 16KV on 1 &4, almost nothing on 2&3.

dominic clancy

Can you measure the coil primary current? (engine stopped, coil charging)
Art Pearse

I have a Snap-On MT1665 analyzer. IAs far as I know it can't measure current with the engine switched off, just voltage

12.7 running, 12.64 switched off.

I have refitted a set of points (had to make up a cable from the points spring to the coil, and hunted for ages to find a plastic insulator that goes to the coil casing, and eventually just robbed the rubber one from the Pertronix.

The car *is* running better with the same timing, and a quick tour round the block had a bit of a cough at the beginning, but as the plugs cleared, it got noticeably better.

I had immediately thought of the condensor at the start, but didn't realise that there is one built into the Pertronix. Maybe I should dissect it if it becomes very clear that that is what the issue is. I had always thought of the things as being digital - either working or not, and had never heard of a gradual failing mode on these. I will not be buying anioher on, as this is the second one to fail.

I'll go to work in it tomorow (working in a different office, so it will get a good run), and if all is fine, it will be back on with the Judson tomorrow night.

Only one thing is still troubling me, I still am measuring only 1KV on the HT leads
dominic clancy

No more misfire. It's on with the Judson and hoping for the best!
dominic clancy

I spoke too soon. Misfire is back.

I'm going to buy a new coil, cap, leads, distributor and points tomorrow and fit them - I can't think of anything else to do. The voltage is so low that the strobe is being triggered only intermittently. It's fine on the flat or downhill, but as soon as it is under load, it all goes to pieces.

dominic clancy

Dominic

What type of leads are you using? My strobe, for instance, will not work with copper leads, only modern suppressed leads. Does not help with the misfire though.

Steve
Steve Gyles

Steve

It worked with these leads until the misfire started. It also worked with the other leads I had. Now neither of the test devices (Snap-on Analyser or Snap-on Strobe) do anything. I could believe that one had died, but not both, and as the strobe is working intermittently, and the analyser also shows intermittent bursts of life, it has to be the ignition at fault.

Just ******extremely ******** frustrating.
dominic clancy

Dominic,
Have you changed the rotor arm. I had what I thought was a new one, and had the same rough running you describe when I first fired up the Coupe last week. Put in a new rotor arm, and BINGO, immediately ran great.
Peter.
P. Tilbury

Hi Peter

I have tried 4 different rotors, two black, two red, one of each being brand new. Makes no difference at all.

I just want to get back to driving after a year of having the car not running at all or running badly.

I'd rather just spend the cash and get everything new than continue spending my evenings trying to get it running right

dominic clancy

Dominic-
Developed HT voltage is determined by what it takes to cause the spark across the plug - or to ground somewhere else. Given that you have replaced everything, about the only thing left is the plugs shorting across the insulator, either internally or externally. That's what lead fouling used to do, as it made the insulator conductive, but I doubt you have any leaded fuel now. (I used to clean the unobtainable plugs for my AC Bristol with nitric acid, because it removes lead deposits) Possible that any fuel additives etc could do it. A few years ago there was a lot of trouble with fuel contaminated with hazardous waste of unknown composition here, did all sorts of strange things, depending on what the criminals had blended.

Pull the coil wire and hook up your strobe to it, trigger it by opening the points. By changing the distance to ground, you should be able to get a reading on coil output. Should be able to get to 30KV at a very wide distance, maybe 3/4". If that is OK, hook up the plug wires with the plugs out but grounded to the engine. Again hook up your strobe to the plug wire and crank it over. Holding the plug off the engine by a similar amount to the coil check should get you similar readings. If not, it is going to ground in the cap, rotor, or wires. If these tests show good, check HT output with the plug grounded to the engine, probably 3-5KV at normal gap, assuming non resistor plugs. Then try cranking engine in the dark while watching the plug gaps. See if it is jumping the gap or running down the insulator inside or out.

FRM
FR Millmore

FR, thanks for the thoughts, but I have a box sitting downstairs, and will be fitting the new parts in a few minutes. It's blazing afternoon, and I want the car running.

I'll be back if the coil, points, cap, rotor, leads don't fix the issue. I've even made a cable to go from the starter switch hot terminal to the coil to rule out any other loose connections
dominic clancy

Didn't fix the voltage problem with all the new parts, will now talk to Carl Heidemann and do what Fletcher suggests.

Car idles beautifully
dominic clancy

No sign of any arcing in the relative dark of the closed garage with the lights off (bit only for a short time because of the fumes - I'm not that desperate.....yet.)

I can get a spark to jump maybe 1/4 inch, no more. Plugs are clean and also brand new.

Maximum I can get on the 1&4 cylinders is 2KV with the timing really advanced - at a normal 10 BTDC it drops to non-measurable range like the others

I have also run a wire from the starter solenoid to the coil + to isolate any chance of dodgy connections in the car wiring - no beneficial effect.

Talked to John Twist, he too is at a loss for suggestions.

So to summarise:
3 caps, 3 sets of HT leads, Pertronix and two diffferent points and two different condensors, two distributors, 3 rotors, one each of evrything except the pertronix is new, and I finished with new everything. Still no power under load. Timing between 15 and 20 BTDC static (can't use the strobe because of low KV on HT).

There's now a reward for the solution!
dominic clancy

Dom,
Did you change the coil? (It is not on your summary list.)
Static timing should be about 7 degrees BTDC.
How much is the reward?
Peter.
P. Tilbury

Next guess is one of the flex wires inside the distributor may be broken. When the vacuum advance moves with change of throttle setting, the flex wire may break or make the circuit. First flex wire from side terminal to contact points would kill ignition completely. Second flex wire from moving plate to distributor housing can be broken, and you get intermittent ground through the center bushing of the moving plate. This would cause intermittent misfire, not just a general reduction of power.

The next suspect is a little more off the wall. Did this engine run okay before you started fiddling with the supercharger? If the engine was overhauled, and not run seriously since, I'd guess the cam timing is off, but that would be pretty radical. Remove valve cover. Rotate engine until the first two rocker arms are moving at the same time and both valves at the same height. Then check position of the timing mark on the crank pulley, should be very close to zero. If the timing chain is off by one tooth the crankshaft will be 18 degrees out of place.
Barney Gaylord

Dominic

Silly thought perhaps, but are you running the correct rotor for the distributor cap? Have you done any measurements?

Steve
Steve Gyles

Following on from Barney's suggestion regarding the flex wire inside the cap; There have been several reports on a Morris Minor forum with similar symptoms as you have caused by the wire not sitting flat on the distributer base plate:

"In despiration I took the cap off, and had another look. . . . The braded earth lead (replaced last year) that goes from the terminal to the points is slightly longer than the original and the loop of cable slightly sticks up. I re routed the wire, tucking it behind the condenser cable which I avoided innitally as I wanted to keep all the wires separated and not risk chafing the cloth brade on the advance mechnism' which was what happened to the original......Hey presto problem solved!!!! The spark for cylinder 1 was earthing to the cloth brade of the cable when under the load of accelleration! Hence loosing a cylinder. Not sure why it wasn't doing it at tick over???"

Also, is it possible that you have the wrong distributer cap and the pick up points for the spark are 45 degrees out and so "missing" the rotor arm at higher revs?

Worth a check!
N McGurk

Peter
Coil has been changed twice, current one is brand new

Barney
Yes the engine is a new one, and to be honest has never yet run properly with any carburettor, but better on the SUs tahn the Holley. Flex wires are perfect inside the dizzy - On the Pertronix setup it was Jeff Schlemmer's perfect work, on the other it is a carefully setup and tested dizzy with all new ignition parts (twice in case one set was faulty). The car will run OK downhill and on the flat, will idle with the occasional miss, but immediately falters and develops no power when put under even a small load, with or without advance attached - it's as if the timing is a mile off - but only under load. Advancing or retarding the timing makes no difference at all to the uphill performance.

Neil
Jeff's cap was matched with Jeff's rotor. New cap matched with new rotor again today.

dominic clancy

Dominic
I think Barney is dead on, your latest description exactly matches very late valve timing. I forgot that this was a new engine. This would also give very low running cylinder pressures, which might account for the low HT readings.
I learned this on an AH 100-6 that had a sheared crank key due to a loose center bolt that had been welded to keep it on! Valve timing was about 90 degrees crankshaft late. It started and idled perfectly, revved up fine under no load, but would not pull your hat off. If you got it going downhill it still ran fine at any RPM, but you couldn't get up the other side.

FRM
FR Millmore

> If you got it going downhill it still ran fine at any RPM, but you couldn't get up the other side.

That's what my dad used to call a Rolls-Canardley.
Andy Bounsall

How does cam timing affect the spark voltage?
Art Pearse

Max voltage is what is required to jump to ground, through the gap or elsewhere. The voltage across the plug gap in the cylinder varies by pressure and temp and mixture density (in non-intuitive ways!). An old time tester has an air gap in parallel with the plug, which you can dial up to match the voltage required to fire the plug. You can actually get it set to where the plug and the air gap fire alternately. I've seen them set at nearly an inch air gap, which indicates that V goes up a lot with cylinder conditions. So low pressure, temperature, and weak mixture from bad valve timing will drop the required and developed voltage.

FRM
FR Millmore

Ah - the old Rolls-Canhardly joke - i remeber it well - was in juniour school when we used to tell that one back in the 50s. Good luck with sorting the misfire Dominic - let us know when you are visiting Lancashire again - cheers Cam
Cam Cunningham

Looks like Barney gets the chocolate. Measured as he suggests and it's about 15 degrees ATDC on the marks.

Barney, name your flavour - almost any form is available here, and it's real chocolate not Candy

dominic clancy

This thread was discussed between 20/05/2011 and 26/05/2011

MG MGA index

This thread is from the archives. Join the live MG MGA BBS now