I have a B21A (carb) engine that has been acting up lately.

When cold it fires right up, but runs horribly. It will not idle, backfires, and just generally sounds and feels like it is running on 3 cylinders: really rough and noisy with no power. This usually goes away once it has warmed up, but it will occasionally stumble briefly while at highway speed. It is also brutal to start when warm.
Oddly, when I fire it up cold, run it for a little while, and shut it off, and restart it(which is difficult) it will then run just fine.

I've checked the timing, replaced the cap and rotor, set the dwell angle all to no effect.

Any ideas?

If you are sure the compression is good, and the ignition system is also good (wires and plugs too) then the problem must lie in the carburetor, which should be an SU HIF6. If you need further help with the carb, contact me through www.sucarburetors.com
Rhys

First I new that the 78's were made with carburetors. What country is this in?

It sounds like it is running rich and on fuel injected it would be the cold start injector and possible stuck on thermal time switch.

A carburetor could be a stuck choke or float level changing. Trash in the bowl or water sloshing around inside.