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MG MGB Technical - Carbs, fuel pump or what?

At a steady 2000 rpm, my '67 GT hesitates and surges as if there isn't a constant flow of fuel. Under acceleration the problem goes away. It is especially noticeable in 2nd and third gear. All fuel hoses are tight.

Any ideas would be appreciated.
Bernie Lowe

It could be a timing issue.

Check the vacuum advance module on the distributor to see if it holds vacuum.
Steven 67GT

Is the tach jumping about? If so it is ignition, although that would likely be accompanied by backfiring in the exhaust. If it were a fuel supply problem it would be more likely to affect acceleration than steady running. Does pulling the choke have an effect either way? Could be vibration affecting the float valve, giving high or low fuel level at certain revs, although in my experience that has little effect on running.

If the vacuum module is punctured it will give a weak mixture, although that is more likely to cause a problem when accelerating not at steady speed. Whilst it does affect the timing of course, the timing will be constant and not altering as much as when the capsule is working correctly, and is the same as so-called 'performance' distributors with no capsule. Of course the timing could be off anyway, as could the mixture and carb balance, and any number of other things.
PaulH Solihull

90% of fuel problems are electrical!
My B had a similar problem and I chased the "fuel" problem for a week...new filter, cleaned the carbs, adjusted the float level, re-tuned the carbs...turns out my rotor arm was rubbing on part of the dizzy cap creating a black dust inside the cap...
Bob Dougherty

That is a behaviour known as "Lean surge", caused by weak mixture at small steady state throttle openings. On SU equipped cars, it is commonly caused by a dirt & oil film build up inside the vacuum chambers and outside diameter of the carb pistons. There will be a dark ring at the bottom end of the vac chamber. Cleaning these (soft rag and lacquer thinner or carb cleaner only) will frequently fix it with no other treatment needed. Part of a tuneup.

Especially on cars that have been sitting, fuel evaporation causes gum buildup at the level of the fuel in the jet and on the needles. This is exactly at the region of the needle that is operational at low speed/light throttle, and will cause the same lean condition. Put some fuel injection system cleaner in it and drive. See my reply to Wm Fox "No gas at carbs" nearby.

Any small vacuum leak, including but not limited to, a bad advance canister, can do it. The "small" leak is a significant factor at low throttle/high vac conditions, but is negligible at greater throttle/lower vac conditions.

If all that is in order, a flat or so richer may cure it. Timing changes generally move it up or down the speed range a bit, which may get it out of a range you normally drive in, but this is limited in use and is a pseudo cure, assuming the timing is set correctly to begin with.

FRM
FR Millmore

This thread was discussed between 16/05/2011 and 17/05/2011

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