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1979 BMW R65

Bike starts, idle, revs, no problem. Then i warm it up and head to a faster road. It then revs up to higher revs, then 'bogs down', and loses a lot of power. After this, it refuses to rev, and limps on. If I leave the bike to cool, we're back to the beginning....
Not even sure if it's an electrical or carb problem. Any ideas really appreciated

Could be anything.
This is impossible for anyone to diagnose with out a road test and a bit of poking around.
All suggestions would be purely speculative at this point.
The only thing I can say with confidence is that it IS fuel or ignition related.
Probably not a big deal.
Start with a basic check up/tune up and take it from there.

My shot in the dark guess, for what it's worth...
faulty condenser

As mentioned, check the basics.

Just a thought, if it has a fuel filter on the petcocks or elsewhere, it could be clogged. Had a bike that would cut out at freeway speeds after a bit. Add on VW type fuel filter looked clean, but was for whatever reason not allowing enough fuel to flow. If I stopped and shut it off for a minute or two, it would then start and run normal, then again after a bit start cutting out.

So, if it takes just a minute, maybe fuel filter issue. If it takes a half hour or the like, maybe heat related, like an electrical component. Coils can do funny things when they go bad or with heat.

Tom

This is classic for the 'chokes' being wholly or partially left on.. Are you sure they aren't sticking and not going off completely. Have you had the chokes apart and refitted them with the rotary slides backwards or swapped from side to side? There are other causes but this is the first thing I'd check...

Rob

Loosen the gas cap and try it.