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MG Midget and Sprite Technical - The LUCAS 'Hide-and-Seek' game

1275, Cooper S distributor, high comp ratio, Lucas gold coil, BP6ES plugs, 30 thou gap, Hypalon covered carbon leads, 45DCOE.

Now, it was running fine for ages but suddenly developed a misfire at rpms higher than 2500 at high throttle settings. Fine at idle. I've fixed it, so....who is gonna have the first guess?





rob multi-sheds thomas

High RPM and WOT may mean something must be wrong with the advance mechanism and associated electricals. Warm?

(Mind you, could be insufficient fuel from the pump...!)

A
anthony

Warm-ish.
Good pump, good pipework and 3 psi measured at the carb. Not fuel pump probs.
rob multi-sheds thomas

Obviously condenser.
G Lazarus

I could believe the main jets were being starved of fuel due to possible mis alignment but equally a faulty "king" lead from the coil would do the same.
Bob (robert) Midget Turbo

You set the "30 thou" plug gaps with a metric feeler gauge, didn't you?
Steve Clark

NGK BP6s are on the warm side but if they've coped before I'd wonder how old the coil is but if it's not old I'd be scratching my head and eliminating the cheapest things first - like the condensor.
Daniel Thirteen-Twelve

Hi Rob
Alittle bit more info on your missfire would help to try and diagnose it. Is it an intermittent little missfire or does it feel like it drops a cylinder completley or two cylinders or--? Gary
Gary

Condensor...rotor arm....cracked dizzy cap.

Did it get worse when hot ? if so then possibly coil just on its way out. When mine went it would work OK cold but not hot. but it was more of an all or nothing rather than misfire.
Dean Smith ('73 RWA)

dirty fuel filter, dirty air cleaner, dirty fuel tank, dirty carbs....are my guess, being that the others were not the culpret.

what about using diesil fuel???
forgot to release the choke???


prop
Prop

Misfires on a few cyl in a row, nothing to do with fuel (The LUCAS bit in the title is a dead giveaway!)
Not the condensor and the cap is not cracked.

Very warm, though!
rob multi-sheds thomas

rotor arm
David Smith(davidDOTsmithATstonesDOTcom

or the carbon brush
David Smith(davidDOTsmithATstonesDOTcom

Dingadingadinga-ding!
We have a winner!

Carbon brush in the cap had developed a resistance, due, I would guess, to an internal crack. When tested it gives 120,000 ohms from one end to the other whereas a new one has almost zero. Seems it was enough to dampen the ardour of the spark under any load. Fairly new cap but this engine puffs a bit of oil due to bed rings and I guess some got into the cap and gummed the carbon brush enough to get it snagged somehow.

Took EFFIN' ages to find that one out!

And your prize? A half-full jar of genuine Lucas Wiring Loom Replacement Smoke. Great for refilling your loom after your smaoke has escaped.
[Be sure your headlamp switch is set to either 'flicker' or 'dim' when refilling
rob multi-sheds thomas

Quite likely to have become resistant due to rust in the connecting spring (or oily slurry ditto), where the brush fits into the centre electrode
Bill

Brilliant Rob, and thanks, but I have my own rig for generating Lucas smoke, two of them actually ;-)
David Smith(davidDOTsmithATstonesDOTcom

This thread was discussed between 03/05/2008 and 04/05/2008

MG Midget and Sprite Technical index

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