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MG Midget and Sprite Technical - Annoying short-term breakdown

After a fantastic track day at Donington Park yesterday, I broke down about half a mile from home on the return journey. The car performed faultlessly all day and on most of the journey home but something started about a mile from home. The tickover became rough and every time I started from a standing start (e.g. traffic lights) it almost cut out and stalled when I opened the throttle (which I mostly managed to catch in time so as not to stall - thanks to man who hooted at me when I did stall!!!). I could get going by open the throttle wide and starting with a lot of revs after which it seemed to run OKish until the next stop and then a repeat. Finally just before arriving home it stopped completely and would not start (despite turning over vigorously). A passer-by helped me push the car to the kerb side and when I tried to start again after no more than 3-4 minutes it fired up and ran perfectly again.

I initially thought of fuel starvation but I have a half full tank and plenty of fuel comes out of the fuel line when disconnected from the carb. So the question is - what might the cause be? I'm thinking:
1) debris in the fuel tank (although nothing in the fuel filter at the carb end)
2) an electrical problem such as my Accuspark module failing. I'm not sure about this as I thought electronic ignition tended to work/not work so would not explain the running but poor performance stage before total stop.

Any suggestions gratefully received on what it might be. I don't want it to happen again - I found the lack of consideration by others for a fellow motorist who had broken down quite astounding!


Chris Hasluck

What was the tach doing ?

Did you feel the coil for temp

does it now restart and run fine as if no problem

might check the float needles for crude

check that the spark plugs are good and not fouled... esp if NGK

prop
Prop and the Blackhole Midget

Chris,

Try another rotor arm, mine played similar tricks, missing at low revs, nearly stalling and finally would not restart.

I had a spare dizzy and swapped that in and it ran fine.I didn't have a spare d45 rotor with me.

As it was a fairly new Aldon unit with Pertronics fitted I sent it back to them - they checked it and found a faulty rotor arm - a red one this time - think it was a Bosch one.

R.
richard boobier

And check the LT wire for continuity between the coil and the Accuspark unit. I have had the wire itself fracture, although the insulation remained intact. When the insulation was warm the cable sagged and the broken ends of the wire made an intermittent connection, which cured itself after a few minutes of cooling down again.
GuyW

What does the petrol smell like?

Swap to a known good rotor arm

Swap to a known good distributor cap.
Daniel Stapleton

All good suggestions - thanks.

The petrol smells like - er - petrol! A fresh tank full at Donington at £1.40 a litre!!!

I have a vague recollection of the tachometer being a bit erratic (but was busy trying to keep going). If erratic what does that indicate?

I'll definitely try a different rotor arm and dizzy cap if I've got one. I didn't have an opportunity to feel if the coil was warm but will try that next time I'm out.

Good point about checking electrical continuity for the Accuspark. Will do

I'm reluctant to open up the carb - it's a HIF44 so no separate float chamber. It should be well protected by the filters (before the pump and immediately before the carb).
Chris Hasluck

Erratic Tachy is a good check for low tension electrical problems. Its a good way of eliminating fuelling and other issues and giving one a diagnostic narrowing down of the possibilities before you even step out of the driver's seat.
GuyW

Another vote for rotor arm. If your current one has a rivet in the top, then it's the culprit. A red arm from somewhere reputable source and you're laughing.
Jeremy Tickle

We had a red rotor arm in an Accuspark distributor melt in a VW Bay window Kamper yesterday. The black stuff covering the brass melted and spread around the distributor cap....second time we have seen this! New rotor arm sorted it.

Apart from the rotor arm, if you have an SU, the piston can stick when open and cause the symptoms. The same can be said for a loose needle which is very intermittent. When cooled the piston/needle returns to normal.

Peter
Peter Burgess Tuning

Hi All
Tried most of the suggestions now but the problem is that nothing made any difference because it is running OK now. I've got to try and recreate the conditions when it broke down and so won't know if I've cured the problem or not until I break down again some place else! Dooooooohhhhhhhhhh. Frustrating.

I do have a red rotor arm on order from the Distributor Doctor (claimed to be very good - we'll see).
Chris Hasluck

Chris,
As you were returning from a track day, the car's clearly exhibiting the same symptoms as the dead parrot in the dead parrot. "its just shagged out after a particularly long squawk!"
Apologies not helpful!
Matt1275

Float valve sticking?
Coil beginning to break down?
Yes...crud in the carbs - which may have been flushed through.
Leak/split in a vac hose/gasket opening and closing up.

Intermittent problems are indeed a pain!


M Ogden

I feel a bit dumb but I think I've found the cause of my breakdown. Thanks for all the suggestions - so I replaced the rotor arm and coil with spares, checked all wiring for continuity and everything was OK. Then I thought of checking the petrol cap as suggested above. While trying to work out how to check the cap was venting OK I noticed imprinted in the plastic underneath the cap the words "Non-vented". Doooohhhhhh!!

I've had that cap for at least 10 years and never had a breakdown although I had noticed that the car sometimes ran rough after long journeys. Thinking about my pattern of behaviour at track days, I generally fill up the tank on the way to the track and top up during the day (I'm afraid of the embarrassment of running out on track - the petrol gauge does not work properly). I usually then fill up for the journey home. Thus the cap is off several time during the day. Needless to say I did not follow that pattern this time but did not fill/top up after the first early morning fill up. That must have been enough to create a partial vacuum in the tank and cause fuel starvation. When I broke down I topped up from a can (so cap off) and the car started straight away.

I've replaced the cap with a vented one and hope that has now done the trick. Good job I left my electronics and timing alone. Thanks to whoever it was suggested checking the cap.
Chris Hasluck

This thread was discussed between 15/03/2016 and 21/03/2016

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