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BMW R100S 1977 Bike revs fine up to 3000rpm but reluctant to go higher, i think it is a carb problem, but any tips and suggestions would be gratefully received.

Blocked air filter? Torn diaphragms in the carbs?

Kurt

Hi, having had a similar issue on my 78 R100RS I managed to sort it after much (enjoyable) messing about involving:

Valve clearances.
Checking coils and leads.
New points and ignition timing reset..
New plugs and air filter.
Carb strip, clean and refurb, replacing gaskets, springs, main jets, needles, o rings and diaphragms.
New throttle cables.
Reset mixture and idle.
Balance throttles.

With hindsight assuming fuel is filling the float bowls I'd check the main jet diffuser is clear without disturbing anything else and then take it from there. This can be done with the carbs on the bike, assuming they're Bings which you may not have on your S. No idea for Dellortos...

Let us know how you get on.

Ade

Thanks for the info, will check it out.Ian

ithebruce77 wrote: ↑Fri Sep 19, 2025 10:20 am BMW R100S 1977 Bike revs fine up to 3000rpm but reluctant to go higher, i think it is a carb problem, but any tips and suggestions would be gratefully received.

Immediate thought was diaphragms. Works fine up to a point, then feels choked, no further rise in rpm...then back off slightly and it feels normal again. Ime though it happened at higher rpm.