MG Car Repair: acceleration problem in 3rd gear, fuel filler cap, fuel starvation

Question
My 1972 MGB (US) has a problem accelerating in 3rd gear.  No problem in 1st and 2nd, lots of power.  In 3rd  gear the car will seem to starve out under hard acceleration.  If you let up off of the accelerator, the engine will seem to "catch up" and the car will then accelerate, but slowly. The timing, distributor, plugs, and fuel pump all check out fine.  Any ideas??  Thanks.

Answer
Hi Marc.  This sounds like fuel starvation to me.  The reason you get it in 3rd gear is because (a) you don't stay in 1st or 2nd long enough for it to happen, and (b) after accelerating in 2nd, the float chambers have already partially emptied, so 3rd has less to start with and that's when one of the float chambers runs dry.  

You say that the fuel pump checks out fine.  Have you done a flow test?  Pull the fuel line from the carburetter and put the end into an empty jam jar.  Turn the ignition on for say 30 seconds, and see how much fuel it delivers in that time.  Imagine you are doing 60 mph and accelerating hard.  You are probably doing 10 miles per gallon at that point.  That means 6 gallons per hour, or one gallon every 10 minutes.  So in 30 seconds, you need 1/20th of a gallon or else you will run dry.  Did the pump deliver that much fuel?  If not, you need to trace the reason for the slow delivery.

Check out the in-line fuel filter if you have one.  If you have the earlier fuel pump, there is another filter built into it, which needs to be removed and cleaned from time to time.

Do you have the Evaporative Loss Control System fitted to your USA spec. car?  This causes all kinds of trouble.  If state laws permit, I would strip it out and throw it away.

Have you replaced the fuel filler cap?  If so, are you sure that it is a vented cap?  If not, you could be pulling a partial vacuum inside the fuel tank which makes it harder for the pump.

Please don't be offended by this question.  When in 3rd, you haven't left the overdrive switched on, have you?  This would give the symptoms of a loss of power.

You asked for a few ideas.  I hope this helps.  If not, get back to me.