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MG MGB GT V8 Factory Originals Technical - Chronic misfire

Hi Folks

(Sorry this is so long.)

I'm not sure anyone will be able to answer this as it's a bit vague but everyone has been very helpful in the past, so I'll give it a go.

The car is '77 roadster. No idea when it was converted. The donor was probably a P6. The car retains HS6 carbs.

It sometimes has a bad misfire. It only happens when you initially start the car. It sometimes clears up after a few miles. On a couple of occasions it has continued to misfire for the entire journey - then run OK next time.

When it's misfiring the idle speed is up at 1200 (it's 600 normally). It does not depend on the temperature. I thought it would clear up once the car warmed up - it does not.

One obviously thinks of wet electrics causing a misfire - but that couldn't make it idle too fast (I think). I'm wondering if it could be something stuck in a carb making one of them chronically rich. But if it was that I'd expect it to clear up when I open the throttle - it doesn't it completely refuses to rev out.

I'm stumped.


Nick Wilson

Could be a sticking vacuum advance plate - sticking in the advanced position. Could also be similar with centrifugal advance. And again could be a carb piston sticking.

I cleared up several long-standing ignition problems on both my MGs within a few minutes of buying an adjustable timing light, which makes checking of both advances very easy, a good investment.

Paul Hunt

Thanks Paul,

BTW, what brand did you buy? The only ones I've seen for sale are gunson, but their construction doesn't inspire confidence.

(Err... I hope I've not upset thousands of owners of Gunson gear...)


Nick Wilson

It was from Halfords where I also bought my Gunsons Exhaust Analyser (!) but can't recall the name on the box. Seems sturdy contruction with chromed metal case and a big black dial on the back, substantial leads and clips for plug and power. When I get home I'll have a look at the box.

Paul Hunt

This thread was discussed between 05/09/2000 and 06/09/2000

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