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MG MGB Technical - Carb popping at 3k?

Once more I supplant myself before those who know far more than I in hopes that some knowledge may be passed on. My '73 B runs good, clutch works great, breaks are great, fuel pump is great...all because of what has been passed on to me on this BBS...thank you everyone. However, now my little car has a problem with the carbs popping at 3000 RPM. It seems to be when mostly when I'm at a cruising speed. If I zip past 3000 it does ok. Where do I start? Is it a mixture problem? I've run a couple tanks of fuel additive to clean the system. I've tried running 89 octane and 93 octane, no difference. My spark plugs all have the proper grey coloring. Boy is this car increasing my confidence in all things mechanical. Thank you for all previous help and hopefully someone can shed a bit of light on this one.
S J Kelly

Possibly a sticking inlet valve if it's popping back through the carb.
Stuart Robson

The mixture may be lean, which could be a fuel supply problem or a carb adjustment problem.
Jeff Schlemmer

Could be air getting into the inlet manifold somehow...
John Prewer

...or the timing is too far advanced, peaking at high vacuum with a light throttle.

That's quite a list of things to check now :).

Timing check is easy - disconnect and plug the manifold side of the vacuum advance pipe, if the popping goes away try a couple of degrees less advance with the pipe reconnected (a turn or 2 on the distributor adjustment if you have it).

If you suspect it's lean, turn the carbs a flat or two richer and try it. If the plugs have black specs after your run it's too rich. Up here grey would be much too lean, we see a tan colour on ours.

Manifold leaking can be checked with various sprays around the suspect areas, looking for changes in RPM.

Not sure how to check for sticking valves.
Mike Polan

Yes I'd look for an air leak - has it just started happening? Have you checked the dashpots for oil recently?


Thank you. I'll check all of the above. The dash pots are fine, I have been able to keep up on those atleast. Again, thank's for all of the advice.
S J Kelly

This thread was discussed between 21/06/2005 and 23/06/2005

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