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MG MGB Technical - Stumble on take off when hot

Hi

I've a 1977 UK roadster. When the engine is hot (which it often is!) I am finding it stumbles / stutters on take off from a stop. It happens when I put my foot on the accelerator as I go to take my foot off the clutch.

At a stop the car idles smoothly at around 600rpm (relatively low I know), but on putting my foot down to pull away (e.g at junction or at lights) it behaves like it's going to stall - I have to slow the clutch release and wait for the revs to pick up, which is becoming a right pain and I seem to be forever needing to over-rev to pull away with confidence.

It's not me having poor clutch control as it also happens if you prod at the throttle in neutral. It acts like it's going to die for a second or so (probably nearer to half-a-second or so) and then picks up again. It never actually stalls.

The car otherwise runs as smooth as it ever has. I have topped up the carb piston oil.

any idea?

regards
Darren
D Lewis

Is the heatshield in good condition? you may be getting fuel evaporation
Bob
R Etches

Darren

Have you tuned it recently? could be something as simple as the timing.

Graham
GLG Lavis

Fuel evaporation is pretty-much a myth, especially in HIFs which this car should have, MGBs run just fine in Arizona even with HSs. The only place it can happen to affect running is in the exposed jet pipe on HSs, and then on in extremely hot conditions where the heat shield is faulty. If any evaporation should occur in a feed pipe then as soon as the float valve opens the vapourised fuel will be pushed out of the vent to be replaced by liquid fuel. In order for a continuing problem to occur the fuel would have to vapourised as fast as the pump could pump it, which is a *minimum* of a pint per minute.

Sounds like weak mixture to me, which could be setting-up or a problem with one or both carb dampers. The proper way to test the dampers is to remove them, invert them, then time how long the piston takes to 'fall'. But a quick and dirty way is to remove the air filters, open the throttle smarty, and see if they rise in unison, which should be well damped of course. They should fall smartly.

If that's OK, and it hasn't been tuned recently (or if it *has* been retuned recently and the problem only started then), is to set up valve clearances, plug gaps, points gap/dwell, timing, and then setup the carbs from scratch. If it's a long term problem then maybe there are 'weak' needles installed.

Weak mixture can be a problem with post-74 cars that have to pass an emissions limit test, if they are maladjusted to pass the test rather than setup for correct running. There is a way round that ...
Paul Hunt 2

This thread was discussed between 03/06/2007 and 07/06/2007

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