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MG Midget and Sprite Technical - Low speed misfire

1275 MkIII standard with +30 over bore
Low speed lumpiness/misfire which clears when the taps are opened and she goes well.
Lumpiness less noticeable when the engine is fully warmed, when it goes like a rocket - suspect in part thanks to Nigel's Millers VSPe suggestion.
Only been able to check the plugs which are coffee coloured.
One thing nagging in the back of my mind, is that last year (not been used hardly at all) it was a bit tappety and that noise is is no longer evident.
Before I panic and think it's a sticking valve, what should I check first?
Compression, carb pistons, valve clearances or something else?
Thanks
Jeremy MkIII

Jeremy,
did you notice if the tacho twitches with the misfire?

Did you try pulling the choke out a bit at lumpiness?

Have fitted a new part or adjusted something?

If all the plugs are coffee coloured with using the Millers then you might be running a bit rich, or too much on choke and only short distance runs but that should blow out when you open the taps enough.

You could check things like the service set up, rotor, dissy cap, HT leads, the carb cables, linkages and choke, air filters clean and clear.

Spray around manifold for leaks.

If the car's not been used much then perhaps more than one thing has got a bit stuck or off tune.

If the tappets are worrying you then just check them but don't be worried about them being a little different from your settings, if they are no major differences you've checked them you can leave them as they are and move on.

If you have a compression tester then you might as well use it if only for reassurance.

For the first time ever I think I got or used some iffy petrol, that's when I also realised I should have been storing the Millers in a cool place and not in the boot where even in winter it can get warm from solar gain. I done nothing more than topping up to dilute the existing tank of petrol rather than my usual running the tank as low as possible before refilling to literally brim and the problem seems to have gone (famous last words).

How's that for scattergun, now how are your reversing lights?
Nigel Atkins

Cheers Nigel,

Red rotor arm & Distributor cap are from The Distributor Doctor. Ignition is by Piranha.
No new parts fitted recently (except clutch master and slave cylinders).
Probably need to spend a few hours methodically checking through each item you've suggested but was being lazy and hoping someone may have the answer already :)
It's just a bit puzzling that the lumpiness clears at speed.

Reversing lights are on the To Do List (new switch). Along with rattling steering column,(going to experiment with a plastic collar, rather than oiling the felt ones, again), weeping radiator neck, shock absorbers need topping up plus oil change and fitting DRLs to the headlights for a bit of fun.
Oh and rejoining AutoAid despite its continuous payment authority and automatic renewal policies.
Jeremy MkIII

Well tacho twitch and pull choke are quick, easy and free to do.

You've reminded me I've got a reversing lights switch to finish installing, I must get around to actually writing a to-do-list, if not actually doing the stuff.
Nigel Atkins

Fault found, I hope.
A quick squirt of carb cleaner at the rear part of the inlet manifold resulted in a noticeable increase in engine revs.

The dashpot dampers were also dirty and not descending equally so they're clean now.

Full marks to MGBHive. Ordered new gaskets at 3.15pm yesterday and they were delivered today at 10.00am. Good service.


Jeremy MkIII

Sorted.
Replacement manifold gasket did the trick. It took longer to scrape the remnants of gasket from the head and especially the heat shield (opted to replace all the gaskets), find a throttle spring which pinged off and was hidden away in the triangular cut out in the inner wing and was only discovered by fishing optimistically with a magnetic pen, than do the spannering.
The heat shield has cracked by the rear carb, possibly because I didn't fit the bracing pieces and will need replacing at some time. The throttle and choke springs were wrongly hooked up - I wondered why the choke was so difficult to operate, so they're now hopefully, correct.
Will take it out when it's cooler to check.
Jeremy MkIII

Well done and thanks for the truth of these simple jobs often turning into a PITA. Having the springs wrong and bracing bracket missing will get you a job at some of the specialist garages I know, consider yourself a (unpaid) professional. 😉
Nigel Atkins

Cheers Nigel, will amend the CV to include those 'skills'- funny what you put up with especially when it turns out to be such a simple solution.
Jeremy MkIII

This thread was discussed between 19/05/2021 and 30/05/2021

MG Midget and Sprite Technical index

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