MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

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 MGB Technical - Misfire then broke down

Went out this morning for a drive.

Started fine; ran for about half a mile then started to misfire and after another half a mile conked out. Managed to restart and chug to a nearby friends house and rolled to a stop outside her house.

Not sure where to go;

Pulling the choke out when the misfire started made no difference.

Car will fire but not rev - just stutters and dies.

Checked fuel pump - good flow and ticking present.
Took off distributer cap all ok there.
Took out a plug and spark present.

( In the last 5 years fitted:
new distributer, coil, plugs and leads. Rebuilt carbs )

Until now car running very well although I only put on a few miles a year.

will have to wait for a tow home before I can investigate further - any thoughts as to what may have happened? I'm sure its something obvious but sometimes need pointing in the right direction before I start changing bits!

Initial thought was the pump but squirted fuel out when disconnected from the carbs and turned engine over. All connections checked and appear ok - next thing I probably need to check is the coil.

Any other help appreciated!
AdrianS

It could still be a fuel flow issue Adrian.
If you have checked the points, swapped the condenser and there us a spark at the plugs, then the fuel is the next thing to look at.

If your engine began with a misfire which became steadily worse until the engine stopped, then this could well have been caused by fuel starvation.
This happens if there is a partial blockage (maybe something floating around in a fuel pipe) that is pushed by the fuel flow over the end of a pipe and almost stops the flow causing the misfire.
Then when you switch off the ignition, the debris floats down out of the way and fuel flows again and the engine runs again.

It was a problem that plagued my car when I first got it, to the point that I almost sold it. Was some silicone half dissolved in the fuel and almost invisible.

You can check for this the next time you have this problem, take off the top of the top of the front float chamber as this is the one that first runs out of fuel.
Cheers
Colyn Firth

I'm going to say electrical.
In today's heat I'd put money on it being the condenser or the coil.

Best of...
MGmike
M McAndrew

Did you observe what the tachometer was doing throughout the stuttering/stalling? If it was dropping to zero during this process it suggests an ignition problem. Has the car run properly since the SU re-build? Check that the pistons rise and fall freely. When you look at the plugs, what do they show? "Took off distributer cap all ok there". Faulty rotor arms, condensers and caps are not that easy to see with a casual inspection. Points will show blueing if it's the condenser. Good flow from the pump? At least a pint a minute?
Allan Reeling

Difficult to tell what tacho was doing as it was all over the place when misfiring.

Car been running very well after SU rebuilds and fuel pump shooting fuel out at a rate when testing.

Did bypass fuel filter and took carbs off this morning to double check again. Cr started well, and then within 5 seconds started to miss again and using the accelerator did nothing to help.

Took plugs out and all looked healthy.

Ran out of time.

Still think it is ignition fault:

Tomorrow, will change capacitor and see if I can find a coil to substitute the one on the car. Not sure how the Ballast resistor works - its a 1978 car. Can I wire direct from the battery to the coil terminal for a short time to check there is no breakdown in the ignition circuit?

Spoke to a mechanic friend this morning who said as the bits were cheap, he would replace most of the ignition components to save time and if that didn't cure it run a compression test.
AdrianS

Adrian,
The ballast is a length of resistance wire wrapped in the front, slam plate, loom. If it hasn't been by-passed by a PO you should have a 6volt coil. Approx 1.5 ohms primary winding resistance. Check that you have the right coil, a 12 volt coil will have double the resistance. The fact that it starts OK when given full battery voltage and then stumbles might indicate a 12V coil fitted, trying to run off a 6 volt ballasted supply.
Allan Reeling

or a duff ballast wire. Yes run a wire direct to 12v for a short test before going to the bother of changing anything. Should be okay for a few mins whilst it's heating up!

best of....
MGmike
M McAndrew

x2 electrical
x2 condenser

Try changing the condenser first. Just going one by one in elimination.

Moss has a video on testing your coil and if you have a ballast resistor in circuit. Go methodically.
Max71

Adrian
Is your rotor arm good quality? Do you have a red rotor arm without the rivet? A cheap,poor quality rotor arm can give you problems.
C Harvey

If the tacho "was all over the place" it is likely ignition LT.

Your new capacitor can be mounted externally as a test (in parallel)

Points are simple things-a quick clean and re-set and they are good to go -or so manky it is obvious on removal.

Check the earth wire in the dizzy (braided) these flex and can eventually break. This is a vital bit as it is the earth. A hit /miss with this will cause the car to do as you describe.

Is the dizzy cap cracked or is a lead breaking down. If you can get it to run in the dark it will show.

Rotor arms -especially ones with riveted top contact do fail. Distributor Doctor goes quality ignition bits. Whilst your mechanic friend might suggest changing everything, I would advise against it. New items can fail and if you mis-fit an item you will be fighting on two fronts. Change one thing at a time.

Top up with new fuel but it is unlikley that this is the cause.

Fuel pump should be 1-2 pints per min with no bubbles or hesitation. n (Not just as you turn ignition on but constant.

Coils are pretty robust but get blamed for many "faults". The main culprit is heat which you will not have suffered on a short drive.

Oh and use it more when it's fixed!!The more you use it you better you become at "hearing" impending problems.
Michael Beswick

Thanks for all the comments.

So far ( today )this is what I've done:

1. fitted an inline electric fuel pump to see if this cures the problem - no.

2. hard wired the coil to make sure there wasn't a LT breakdown in the wiring - no effect.

3. Fitted a new Lucas capacitor externally
- no improvement.

4. Had got the old distributor which I replace 4 years ago. Fitted that - no change.

I hadn't got time to get a new coil today. Will get one next week and also possibly some new leads ( these were also changed 4 years ago).

The car if left starts fine and runs smoothly for about 5 seconds then misfires, backfires and splutters and stops.

I'll report back after I've changed the coil!
AdrianS

Thanks for all the comments.

So far ( today )this is what I've done:

1. fitted an inline electric fuel pump to see if this cures the problem - no.

2. hard wired the coil to make sure there wasn't a LT breakdown in the wiring - no effect.

3. Fitted a new Lucas capacitor externally
- no improvement.

4. Had got the old distributor which I replace 4 years ago. Fitted that - no change.

I hadn't got time to get a new coil today.I tested the primary windings before starting and the resistance was around 2 ohms. Will get one next week and also possibly some new leads ( these were also changed 4 years ago).

The car if left starts fine and runs smoothly for about 5 seconds then misfires, backfires and splutters and stops.

I'll report back after I've changed the coil!
AdrianS

"the resistance was around 2 ohms". If that is a reasonably accurate reading it would suggest a faulty coil, but not whether you still have a ballasted supply. Just to re-cap, a ballast system has a six volt coil which actually has two supplies. One is available battery voltage taken from the starter solenoid but only "active" when you are cranking the engine. The other supply is from the fuse box, via the resistance wire, which supplies the coil with 6 volts when the engine has fired and is running. The idea is to compensate for voltage drop when cranking with a cold battery (can be as low as 7 or 8 volts), with cold (more viscous) oil, the drain of the high demand starter and also the because of the losses caused by the long feed from battery to starter. If you have two connections to the + side of the coil, white and light green, you probably still have a ballasted system.
Allan Reeling

Even though you have changed the fuel pump, be sure to disconnect the line at the carburetors to see if it is flowing freely.

I had an obstructed fuel line at the tank once. After changing fuel pumps, I ran a wire through the line and broke up the obstruction.
Glenn Mallory

X 2 what Glenn said Adrian.

The only way to be certain that it is not a fuel starvation problem is:- The next time this misfire happens and the engine stops, turn off the ignition and take the top off the front float chamber to see if it is full.
With reduced fuel flow the front carb runs short of fuel first and causes a misfire. Usually, with a fuel starvation problem, the engine goes onto 3 cylinders, then 2, then 1 until the fuel runs out.

The misfire that plagued my car for 6 frustrating months was caused by a piece of clear silicone floating around inside a glass in-line fuel filter. This was invisible inside the filter but would be pushed over the filters outlet pipe by the fuel flow and partially cut off the flow.

Contrary to most owners, all misfire problems I have ever had have been caused by fuel starvation.

Colyn
Colyn Firth

Off out to get a coil today.

With regard to fueling, I bypassed the inline filter and disconnected the hose from the carburettor. Turning on the ignition produced an impressive spray of fuel from the pipe.

Also the misfiring and backfiring starts just a few seconds after starting smoothly - I would have thought a full carburettor would fuel the engine for a bit longer than 5 seconds!
AdrianS

Right,an update.

Fitted a new coil today and also bypassed the ballast resistor at the suggestion of Paul Depper where I got the bits from.

No difference _ still banging and spluttering..

Running out of ideas. Probably get the compression tested first.
AdrianS

When you change the distributor are you keeping with the same cap? Tracking, due to cracks, can cause cross firing and lots of spitting and back firing.
Allan Reeling

"Difficult to tell what tacho was doing as it was all over the place when misfiring."

If the tach was acting like that then it's not fuel but ignition LT as mentioned before. This could be any loose connection or faulty component from the ignition switch, through the coil, points, and distributor earth wire.

As far as the condenser goes then connecting another one in parallel will only highlight an open-circuit condenser, which wouldn't show up on the tach anyway. An intermittently short-circuit condenser WOULD show up on the tach, but it would have to be disconnected and another one fitted before that cause could be eliminated.

Likewise HT problems have much less of an effect on the tach than LT problems.

If the tach is still jumping around when the problem starts, then given everything that has been done I'd be suspecting the earth wire. This is a very flexible cloth-insulated wire running from the points plate to the distributor body. It's continually flexed as the points plate twists back and fore under changing vacuum, which is why it is not a standard plastic insulated wire, but does eventually fail. Originals maybe available from Dist Doc, Moss Europe have an alternative.
paulh4

This thread was discussed between 25/05/2017 and 08/06/2017

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.