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MG Midget and Sprite Technical - Curious Misfire - Bet You can't Figure it Out!

Hi everyone,

Apologies if this is a bit long-winded! Got a '67 midget (MK3) and am having trouble with an intermittent misfire:
- Starts fine
- Runs a few minutes OK
- Develops a partial misfire, worsens over about a couple of minutes
- Eventually misfires so badly it stalls (from running on 4 cylinders to 3 or 2)
- Revs drop gradually, ignition light comes on originally, stays on during the stalling and after the engine's died
- Leave for 30 seconds or so (maybe even less) and restarts every time
- Perhaps slightly worse (immediate misfire rather than developing) upon driving after restart

Checks we've done:
- Pulled the plugs. First time were white-tipped (running hot?). Second time (nothing changed in-between) were sooty/black, most at the back (cylinder4) and best at the front (cylinder1). Now cleaned, re-gapped, tightened.

What we've changed:
- Fuel supply system: New SU standard fuel pump, new fuel tank, new filter, new carb lines and split carb fuel lines to deliver equal pressure to each. I know we've got fuel in the carbs as they overflowed a couple of times before we adjusted the floats.
- New jets in the float chambers of the carbs (for the 5th time!)
- Carbs taken off, dismantled, blown through and cleaned. Needles in good order, as are floats, pistons and jets). jets centred properly, and pistons not sticking.
- Replaced the points and condenser, adjusted.
- Replaced distributor cap and rotor arm (new distributor ordered) and brand new HT leads (nice silicone covered proper ones)
- Disconnected oil breather pipe from side of front carb

Any suggestions would be so greatly appreciated I can't tell you - my car's been off the road for about a month now and it's making me really sad (!)
Thanks very much in advance, and sorry for the dump of information!

Josh Spooner

I've had something similar. The cause was a poor float valve (Grose Jet) allowing the float chamber to gradually overfill at idle until the engine flooded and stalled.

Hope you get it sorted,

Gryf Ketcherside

two threads same subject

there's video (possibly J twist but not sure) about getting points fittings in right order, sorry I can't look for it as for the first time ever I'm almost on my monthly download limit (if I've not already passed it)
Nigel Atkins


Are the overflow pipes open?

happy new year
Flip Brühl 948 frog 59

Of all the ignition changes you haven't mentioned the coil. Does it get really hot? The wrong coil or one with an internal short will quickly heat up and break down. See what happens to the rev counter when it starts to die.
Allan Reeling

The new plug leads, are they pushed firmly onto the coil and dizzy connections? Some new ones are so tight a fit that when pushed in place they develop a movement away under "spring pressure" effect from the springy rubber squashing then unsquashing thus separating them from the sparky bit and giving the same symptoms.

A squidge of Vaseline (or WD40) inside each rubber cap can help prevent this effect by letting the rubber end go fully home without resistance

(no, not the electrical kind of resistance ;) )

Flip asked:

>>> Are the overflow pipes open? <<<

Yes, the overflow pipe runs from the top of the Zenith float chamber down to the forward charcoal canister in my '78 US-spec, and the canister was filling with fuel.

Anyway, that was quite a few years ago, and it hasn't done it since.

Gryf Ketcherside

This thread was discussed between 31/12/2011 and 01/01/2012

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