MG Car Repair: Engine cuts out under 3000 RPM

Question
Hi Barrie,
I've got a situation with my 72 MGB that I don't even know how to search it.

Out of the blue, about 10 or 20 miles after filling up my tank with petro, I noticed the engine started to run a little rough.  Then, thankfully after I got home, it practically died in my driveway.

I am not completely convinced though, that it's bad fuel.

It is extremely hard to get running now.  It starts, kind of.  It sputters, spits, kicks, and I even got a backfire once.  Pumping the gas pedal seems to be the only thing that keeps it sputtering, spitting, etc... but if I do get lucky, sometimes I can get it revving above 4000 RPMs... which at that point I can keep it running rather smoothly.  I can feel an occasional skip, but I mean it is revving nice.  As soon as I drop it below 3000 RPMs, the engine cuts out as if I turned the key off, until it almost stops, when then the sputtering, spitting, etc kicks in.

If it were bad fuel, I wouldn't think it would rev.

Any thoughts?  I read somewhere that a leaky intake manifold would cause low RPM problems... but I am at a complete loss, and the poor dear has been sitting in my garage for almost 2 months.

Answer
Hi Joe.

My immediate thought is that it must be related to the gas that you bought.  In particular, if the gas was contaminated with water, then the water would drop to the bottom of the tank.  

The next time you went up or down a hill or round a corner some water was sucked into the fuel pump and deposited into the carburettor float chambers.    

Before you do anything else, you need to check if I am right.

If you have an inline fuel filter, take it off carefully and empty the contents into a glass jar.  Inspect it for water in the gas, which will separate into a liquid bubble at the bottom of the jar.  

You could also try removing the float chamber lids from the carbs and inspecting the fuel in the chambers for water.  Dry them out with a rag before reassembly.

I hope you didn't fill the tank, because if I am right you will have to syphon off most of the gas (for re-use).  Any water should separate out, so you can decant it and you shouldn't lose too much gas.

Finally, take the tank off the car, drain the last gallon or so out of it, and then leave the tank in the fresh air to dry out.