Boxerworks.com

Asking a question on behalf of my son's friend who owns a R100. Both live in Denmark.

He reports that the bike will rev happily in neutral, 1st 2nd and 3rd but will not rev past 3000 rpm in 4th and fifth. There's no obvious reason.

Any bright ideas please?

glyn

If you mean won't rev past 3000 rpm on the road then perforated diaphragms are a possible cause as that would prevent the slides from rising.

That happened to me on the '84 RS while on a weekend trip. Engine would flutter at highway speed. It was a slight tear in one of the diaphragms. Quick fix was to stretch the tear just a bit and use a drop of super glue. The next day, it got me the 200 miles back home like nothing had happened.

dwerbil wrote: ↑Fri Jun 06, 2025 9:41 pm That happened to me on the '84 RS while on a weekend trip. Engine would flutter at highway speed. It was a slight tear in one of the diaphragms. Quick fix was to stretch the tear just a bit and use a drop of super glue. The next day, it got me the 200 miles back home like nothing had happened.

A snippet from the 'net.

BTW, a condom can be used to temporarily repair cracked CV diaphram!! A co-rider on trip last year did fix on his R100. Cheap truckstop brand OK. No need for anything exotic. Hide it somewhere on bike where your wife won't find it and come to incorrect assumption!!

Or just carry a spare diaphragm.

I'm still not seeing how to explain how the bike revs in the driveway but can't rev or accelerate when on the road in higher gears.

Kurt

Slide lift is not so much rev related as load related. The engine needs very little air to rev on the driveway. Think of how the rpm can rise quite high purely on the idle jet when not set up correctly or with stuck timing advance. But the engine needs a lot more air when it's under load and even then it will do 50mph before the piston starts to lift.

But the bike runs well it seems in 1, 2, and 3rd. Guess I'm not really getting all the dynamics.

Kurt

The only explanation can be that the load on the engine in 1st,2nd and 3rd was low enough not to need the slide to lift.

Due to the cut away on the bottom of the slide and the fact that even with the slide at it's lowest point, fuel or rather premix can flow between the parallel part of the needle and the needle jet. A 2.66 needle jet has an area of 5.56 mm2 while the parallel part of the needle has an area of 4.71 mm2 so there is still room for fuel to flow and space under the cut away for air to flow when the slide is fully closed.

The poster didn't really say what was meant by revving easily in the lower gears. But seems to me wide open throttle in 3rd gear would almost certainly have to lift the slide.

Kurt