[DickDastardly]

Had a similar thing on a 1980 FXS this year!

When cold it was fine stop after a few miles, and re-start it would spit back though the carb, ‘phat and phit’ from the exhaust, thought it was running lean so adjusted the carb, it was worse, made it lean, it got worse, advanced the timing it was ok for a while then it got worse, retarded it, it was ok then got worse…..we were trying everything!

It was that bad we were about to rip the heads off and check the valves, we were that desperate!

The carb (original non-CV Keihin) must have been off and on a dozen times, cleaned, float heights checked and re-checked, new exhaust gaskets, new battery, plugs, new alternator (it was shagged anyway), battery earth-lead, points, condenser, etc etc.

(BTW, this was 'electronic' ignition converted to points)

In the end we found that the condensers we’d tried were crap, basically! Then bought a ‘Blue streak’ condenser, from a local Indie!

Sorted!

Might not be the answer to your problem but there is a lot of ‘custom crap’ out there that is not up to the job, you know what I mean!

Moral to this, check the simple things first and last!!

Good luck!