MG Car Repair: MGB suddenly runs on 2 cylinders, mg guy, brake master cylinder

Question
Hi,
My 1977 MGB roadster has an intermittent problem - always the worst kind - It will run fine for a while then will suddenly run on 2 cylinders. It is baffling the MG guy that I use. I drove it yesterday to work then back home - round trip of about 18 miles. I then drove to the other side of Basingstoke, stopped for about 10 mins and started back. It was then that it started. had to be recovered on low loader. When arrived back at my place it was fine and the guy drove it off the low loader. The same happened a few weeks ago coming home from the Windsor Great Park event. It went in my MG guy and we thought it was fixed. Now going back to him for a re-think !!!!
Recently fitted......
electronic ignition, new coil, leads, vacuum, new jets, floats and needle in the carbs.
Any suggestions gratefully received
regards
Jeff

Answer
Not an easy one, Jeff.  You have already done many of the things I would have suggested.  Seems to be fuel related, so I have a couple of thoughts:

1) Have you replaced the inline fuel filter?  This could be restricting fuel flow, so the car starts and runs, but when you put the engine under sustained load, the fuel is used quicker than it can be replenished, so eventually one carburetter empties.  Since the HIF4 SUs are in series, the front carb will be filled back up first, and the back one will only get supplied during those times when the front one is full up.  Next time it happens, pull plug lead number 3.  If it makes no difference, then the back carb has emptied ...

2) How about the fuel pump itself?  These can get lazy and give the same symptoms.  I would pop the lid off and inspect the points.  Accessible via the boot in a 77 B.

3) The wiring to the fuel pump is via two 4-way bullet connectors.  One is by the brake master cylinder and the other is under the dash near the ignition switch.  Look for a bunch of white wires.  The bullet connectors tend to corrode.  Then they get very hot.  Then they lose their tension.  The result is a weak power supply to the pump and the coil.

I would love to know what the problem was, so please let me know.