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MG MGB Technical - 3rd gear acceleration problem

My 1972 MGB has an acceleration problem in 3rd gear. In 1st and 2nd gear it has plenty of power. But under hard acceleration in 3rd gear it will seem to starve out. If you let up off of the accelerator, the engine will "catch up" and the car will accelerate, although slowly. The timing, plugs, wires, and fuel system all check out. Any ideas??

sounds like an accelerator pump gone bad, a choke issue or ... but you didn't tell us what type of carbs you are running so...I don't know.....
Peter Murray

I'm guessing fuel starvation, but you don't mention 4th gear. I'll assume the problem is worse yet in 4th. Maybe vacuum or centrifugal advance problem.

Accelerator pump? What accelerator pump?

Acceleration enrichment, either the accelerator pump of fixed jet carbs or the damper of variable jet like SU and Zenith has a bigger impact on *initial* acceleration. If it picks up OK when you first prod the pedal then it is unlikely to be an enrichment problem.

The centrifugal and vacuum advance will be the same in any gear, as should the fuel consumption in gallons per hour for a given rpm.

The problem could be that because one tends to hang onto each successive gear longer than the previous one, because it takes longer to go through the rev range in 2nd than 1st, and 3rd than 2nd etc., that you are getting fuel starvation at higher rpms i.e. higher demand, but it is only noticeable in the higher gears because you are holding on to them longer and are running the level in the float chamber down. The first thing I'd do is check fuel delivery by disconnecting the supply pipe from the carb(s) and directing it into a container, then switching on the ignition. You should get at least one Imperial pint per minute, and in practice closer to two, delivered in a steady stream of pulses with minimal bubbles.

If you are accelerating harder in 3rd than 2nd then it can also be HT components breaking down.
Paul Hunt

The 1972 MGB is running stock twin SU carbs.

Sounds like're gulping fuel while running hard from 1st thru
2nd gears - but by the time you're in 3rd the fuel flow rate has not been able to keep up with the demand and replenish the level
of fuel in the fuel bowls.

Fuel filter? fuel pump?
Daniel Wong

The fuel pump is a new electronic one from Moss. Fuel filter looks clean and is full of gasoline.

Fuel filter contents mean nothing, they can be anything from almost empty to full, and change from one to the other from time to time, without affecting running. Also the fact that it is a new pump means nothing, it could be faulty. You need to do the delivery check as I said, it is simple enough.
Paul Hunt

I was having low fuel delivery on my '67 and blew out the line from the fuel pump inlet to the tank with compressed air. The volume doubled. RAY

This thread was discussed between 14/08/2009 and 15/08/2009

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