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MG MGB GT V8 Factory Originals Technical - Acceleration problem!

Hi all - I have recently bought a '76 BGT V8 conversion and am experiencing a slight hesitation on acceleration. When I accelerate it seems to hesitate initially then picks up power when I have given it more throttle. It has a Holley carb, and the local MG mechanic thought it may be the accelerator pump on the carb. The engine is in good shape, there are no induction leaks, and the electrics are all spot on. It does however have a standard MGB fuel pump. Has anyone had a similar problem?


Mike - very common problem - many possible causes - your post eliminates all but two :-

It is most probably (but not definitely) not the fuel pump.(Filter OK ?)

I would start by turning up the idle speed to 1000 to check whether the problem is still there -- then

The linkage especially accelerator pedal/carb needs to be precise but not too tight -

I am not familliar with your carb but the accelerator pump linkage is critical (and there must be no splits in the pump diaphragm ) - the carb handbook is the key.

Without wishing to put your advisor's nose out of joint - the electrics may be set up correctly but still give a poor spark: check the length of the spark at the plugs, the vacuum advance is still working and comes off the inlet manifold, the leads are all new(ish).

The next area to explore may involve the previous owner - when was the chain last changed - is the timing off because the chain is slack.

My own experience is of three new vacuum advances in 200,000 miles - the diaphragm corrodes.

Good luck - keep us in the picture


Sounds like a flat spot because either the overall mixture is too weak or there isn't enough enrichment on acceleration. Because it does pick-up when it is given even more throttle I'd tend to discount fuel supply problems, also spark problems but they are easy to check (off load anyway) as RMW suggests. I have had to replace the vacuum advance capsule on my factory V8 twice in about 40k. The problem with those is the the take-off is on the bottom of the carb throat, hence raw fuel can and does run down to capsule and rot the rubber diaphragm. After the 2nd one I fabricated a small separation chamber which I mounted above the carb throat, so any fuel that gathers in it tends to get sucked back into the carb as the vacuum increases. Apart from this problem it doesn't matter whether the take-off is on the carb or the inlet manifold, and in fact the former gives better overall results. It was only moved as an emissions measure. Even without vacuuum advance, and a siezed centrifugal advance, and a sloppy chain, the effect on performance was undetectable, by me at any rate. The MGB fuel pump should be more than man enough for a V8, as long as it consistently delivers at least 1pt per minute at the carbs, I can't see anyone needing more than that for a road car at any rate.
Paul Hunt

I had the same problem when I first bolted a Holley onto my car. You can check a few things yourself, float level, idle adjustment, acc. pump lever clearance, but you should really take it to someone who knows Holleys. Someone with a Dyno.If it's an old carb. it might need rebushing where the throttle shafts enter the body. They're notorious for leaking air.It might need rejetting. Maybe the previous owner can shed some light on it's history. Barrie E
Barrie Egerton

You may very well have the wrong accelerator pump cam for the engine. They are cheap and easy to change. By the cam kit and try a few out - you'll know when you have the right one. Don't mess with the cams until you are certain that the jetting is correct for your engine. You'll want the fuel mixture around 12:1 for performance, so have it tested - throughout the rpm range. The primaries and secondaries will probably need different levels of change.
Jeff Schlemmer

This thread was discussed on 12/11/2004

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