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 Midget and Sprite Technical - Midget ignition problem??

Hello Guys,

Been having a problem with my midget ('72) recently. I've had what I thought was an issue with the ignition bits and bobs. I'd drive the car a couple of miles and then it'd cough and miss slightly and this would build up over a couple of miles to a wonderful crescendo of farting and popping before the car died completely.

I've now changed the points, condenser, rotor arm, LT feed wire, coil, dizzy cap, HT leads and plugs - all to no avail.

I've also changed the fuel pump (as I had a spare) and cleared the fuel lines, carbs etc.

However the problem is still there and I'm getting a stuck and pretty fed up now and I am considering some sort of sacrifce to make peace with the angry MG gods! So, any suggestions are very welcome!

Cheers,
Paul.
Paul Schofield

Paul. I am not as familiar with the Midget as I am with the B, but may be able to give you a few ideas. Others will be able to correct anything which does not apply to a Midget.

First, how does the tachometer act when this is happening? I think the Midget had an electronic tach by your year. If so, watch it and see what is happening as the problem manifests itself. If the tach drops to zero, it indicates a problem in the low tension (LT) ignition circuit. If it winds down evenly as the engine dies, it indicates a problem in the fuel system or the high tension (HT) circuit.

You note that you have replaced the distributor internals. Did you check the state of the ground wire between the points plate and body of the distributor? This needs to be in good condition for the LT circuit to function well. If not, it will fail to provide a good ground and you will have an LT fault. The vacuum advance moves the points plate in relation to the points cam. Hence, the vacuum advance, as it comes on and goes off, moves the points plate and a bad ground wire can fault on an intermittent basis.

Rotors have been known to go bad. But, this does not correct itself after sitting for a period, it is an all of the time thing. So, the rotor is probably good.

Are the spark plug wires firmly attached to the distributor cap and spark plugs? I have seen them come loose and cause problems.

Is there any sign of moisture in the distributor cap immediately after the engine dies? Water leaks have been known to spray onto the distributor and cause it to short out. When the engine is hot, the water evaporates and the engine can be started again and run until it is again shorted out. This would be consistent with your description.

The carbs need to be set up properly, the jets need to come up fully when the choke is off, and they need to be firmly attached to the intake manifold. If not, an air leak can develop. Take a look at the carbs and the intake manifold to make sure there is nothing that can cause a leak as the engine warms up. Pay attention to the two plugs, at the front and rear of the intake manifold, as they have been known to loosen and cause an air leak which is difficult to find and diagnose. Some carb cleaner sprayed around the carb/intake manifold, as the engine is trying to die, might tell you if there is an air leak.

More information about exactly what you are seeing when the engine starts to die, then dies, would be of use in trying to define where the problem might be.

Les
Les Bengtson

Also, 1. have a look at the diaphragm of the oil breather valve, to make sure there are no tears in it.
2. Check the inlet manifold for cracks and air in-leakages.
M J Chapman

Hi Guys,

Thanks for getting back to me and I will go and have a fiddle with the patient now.

Les, in answer to your question - as I'm driving along the car will run fine for around 2 miles or so, then it will just hesitate very slightly. This hesistation will eventually become full on jerking and coughing until the car wants to die completely. When this is happening the tacho does drop, but not to zero - probably more like the speed to which the engine is being turned by the momentum of the car.

The car will return to normal after a day in the garage, but the problem returns in the same way. I've had a good look at the fuel system and all appears to be well there.

I'm gonna have a good look for air leaks - I guess it's not beyond the realms of possibility that a leak will open up when hot and close up when cold?

Anyway, I'll let you know what occurs with it!
P.S. Schofield

...not sure why the board thinks I'm in Bristol - I'm still in Cambridge; I haven't moved house overnight!
P.S. Schofield

An odd one that can cause similar problems is moisture in the distributor. When it warms up, moisture from low in the body of the distributor evaporates and then condenses again on the cap, causing mayhem.

It's unusual, probably not the cause, but worth checking for - simply remove the cap when the problem occurs and check for damp on the inside.
Paul Walbran

have you changed the fuel filler cap lately? could it be caused by lack of tank venting?
David Smith(davidDOTsmithATstonesDOTcom

mmm ... another thought: litterature is a bit conflicting, but yours could be fitted with an in-line ballast resistor in the wiring harness. If so, these can fail. Test by borrowing (& fitting) a non-ballasted coil and running a wire direct from the fuse box (terminal with white wires) to the coil + and the white/black feed to the distributor to the coil -
Paul Walbran

Hi Paul,

I had almost the same problem. It was caused by a lack of tank venting through the petrol cap.
I boiled the petrol cap for a few minutes to clear all the muck, no problem since!
Regards,

Andrew
aj robinson

This thread was discussed between 26/05/2008 and 28/05/2008

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.