I have a '79 B converted to twin SU HS4s, manual choke and a points type distributor. The vacuum advance is not hooked up. It starts and runs well until the motor really warms up. After 20 - 30 minutes, it starts to misfire badly. It is noticeable first under acceleration. The longer it is driven the worse it gets, eventually to the point where it no longer even will idle on its own. I thought it was stripped threads in the head causing a manifold leak. The threads have been heli-coiled and the manifold has been reinstalled with a new gasket, high temperature sealant and nuts torqued to specification, but the misfire continues. Now, I suspect the ignition system. But, before I start spending more $, I thought I would ask the MGB swamis, what might cause a car that runs fine cold to run like crap warm?
Tom, how hot is "hot"? I drove the car for about 30 minutes this evening until it started to misfire. The coil was warm, but it was not uncomfortable to touch it. Based on that, I pulled the dizzy. It turns out to be a 43D4 which the Moss catalogue describes as being basically the same as a 45D4. The points looked fairly new but were pitted. The condenser and low tension lead look much older than the points. I may just replace everything: points, condenser, cap, rotor, wires and coil. What do you think?
I would check the timing, maybe it's too advanced, or the springs in the dizzy might be the wrong ones. Also, I'd check for proper mixture. Pull the choke when hot and see what happens. if it helps, then you must be running lean. good luck.