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MG Midget and Sprite Technical - Misfire below 2000rpm

1098 A series, Simon BBS electronic ignition (so no condenser speculation!), new plugs.

Engine mis-firing on no.3 below 2000rpm, fine from 2000 up. Car still pulls hard above 2000rpm, no noticible drop in power. Swapped distributor cap, swapped plugs, fault stays with no.3. Spark at end of plug lead 3 below 2000rpm. Can't find compression tester but feels like there's plenty of compression resistance with plugs 1,2&4 removed (not very scientific I know). No white smoke or water in oil so suspect it's not head gasket nor blow between cylinders as I'd expect to see problem with 2 and/or 4.

Burnt valve perhaps? Had this before (burnt valve that is not misfire - car is supercharged). Anything I've missed? Ordered new compression tester, but thought I'd see if anyone had any other ideas.

Simon
SA Wood

hi simon,
have you tried putting points system back, i had one of them bbs ign play up on my 1098 so took it out and put points back and all ok.
just a thought bob.
trebor

If you've changed the distributor cap and swapped the leads about, and the misfire stays with #3, then a burnt valve/compression problem would be the most likely.

(thought: You haven't an optical pickup on your electronic ignition? The sort of thing that might pick up crud in the slot that triggers spark for #3?)
Growler

Magnetic pick-up on rotor. Made me think though. Will rotate it 90 degrees to see whether misfire moves.
Will also have a go with original points system.
Thanks chaps.

Simon
SA Wood

Be sure to let us know how you get on! I do love a good mystery...
Growler

Got there eventually. Took most of the weekend.
Compression OK, down slightly on 3&4, not by much though.
Replaced accuspark unit with points/condenser - no effect. Went through all the electrics again, plugs, rotor arm, cap, leads, timing all to no avail.

Put in colortune plug which revealed that no3 was firing intermittantly. Starting to question my sanity around now.

Eventually removed the head - to the naked eye I could see no apparent problems. Removed valves from no 3, noticed that oil seal on inlet valve had broken and was lying in the well of the spring. When valve was removed, there was a lot of burned-on-crud on the upper surface of the valve, extending up the stem a little way. Cleaned all this off, re-seated valves, reassembled with new seal and all is now OK.

Suggests that perhaps inlet on no3 wasn't closing properly though compression only seemed to be about 5-10psi down.

You live and learn.

Simon
SA Wood

Glad you got it sorted, and glad it wasn't something more dire. (I'm also glad I decided to pay for the Viton valve stem seals rather than the generics.)
Growler

This thread was discussed between 27/08/2012 and 04/09/2012

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