'77 R100/7 high speed stumble

We’re trying to get a ’77 R100/7 with 18 K miles sorted and are running out of ideas. The owner has had the bike for several years and in the past it had run well. Motor was reconditioned about 10K miles ago. The current issue is a high speed stumble. Accelerates through the gears fine, and at a steady speed of 65-70 mph 4500 RPM’s, while rolling on the throttle it stumbles, and recovers in a few seconds. The carburetors have been rebuilt a while back and appear to be clean and OK. During the investigation of the carbs they have been disassembled inspected, cleaned and appear fine. Over three years the stumbling has become slightly worse, and gas milage is in the 30 mpg range, despite the efforts to resolve this. (or is it because of the efforts?)

The following is what has been done in response to the issue

Fuel flow at the petcock 700ml/minute
Fuel to the bottom of the bowl 350ml/minute
Fuel in the bowl is at 32 mm
Air leak test showed no leak. Ran bike and sprayed starter fluid on the outside with no increase in RPMs
New gas cap that was drilled for venting purposes
Diaphragms look fine
Tank and petcocks are clean
150 main jet (stock) have tried 160 and 145 no help
Stock needle jet
Stock needle
Needle clip is in position 3 (1 up from bottom) stock position
Valves adjusted and are dead on
Does not smoke at all
Boyer electronic ignition
Timing is a degree off as measured by a strobe
Starts right up from cold
Carbs 64/32 19-20

Rolling-on at that speed would suggest that you're still operating on the taper of the jet needle/valve and that changing the main jets would have little effect as you noted.

What about the float height? Could it be that you have a low fuel level in the bowl? When you ask for more throttle, it takes a bit of time for the diaphragm to respond, pulling the slide/needle up and getting the air through the throat to suck more gas up from the bowl. If the level is low, could take a split second for the gas to be drawn up.

Allow the bowls to fill with gas. Shut off the petcocks and drop the bowls. The height measured in the center of the bowl should be about 24mm for 32mm carbs.

Agree with bowl gas levels/float adjustment. Once I took the time to do a little fine tuning with my levels, many issues disappeared. Should only take 15 minutes and some patience re-bending the tang on the float. Easy first step to eliminate, but it really sounds like a fuel supply issue.

"The height measured in the center of the bowl should be about 24mm for 32mm carbs."

Kurt's advice is golden as usual. Measure from the bottom of the well in the centre of the bowl. I would expect your fuel economy to improve a lot. My '76 R90/6 running 150 main jets would regularly yield 52 MPG US. Do your plugs indicate rich running ie dark?
Russ

Roll on stumble at higher speed and then finally catching up is classic diaphragm problems. Don't just look at those ancient, thin, worn through, pieces of rubber skin...............REPLACE THEM..........

Thank you for all the information. We will look at the float height tomorrow. I hear you diaphragms, they were changed out two years ago and after inspection they looked good.

Remeasuring the floatbowl height did the trick. Thanks for the helpful suggestions!

Any feedback on what you found? What were the levels when you first measured?