1991 R100 high speed sputtering out...

[Gibson]

Fuel petcock should be checked. Take fuel lines off at carbs and make sure it flows freely with into a glass jar…not just a trickle…

[MotoD]

Updates. Took both carbs completely apart. Laid everything out and went through every aspect of the carbs again giving each another proper cleaning and adjustment. I did have the right choke dimple 180 out which honestly could’ve just been the issue but who knows.

Also, I reset the fuel mixture screw to 1.25-1.5 turns out from seated and that helped out a bunch too. I can now get the bike up over 90mph with ease and it just feels smoother as well. Plugs have a nice light chocolate brown to them. Starts immediately and purrs to life with a quick turn or the wrist.

Working on a R65 with a 860 big bore kit and just ordered a set of Mikuni TM34’s. Stoked on trying those babies out!

You guys rock, thanks for the tips and info. This community is of much help and hope to be able to contribute some day

[Rob Frankham]

I did have the right choke dimple 180 out which honestly could’ve just been the issue but who knows.

That'll do it for sure!

Rob

[melville]

I actually have been parking my bike with the choke on as a security device. Even from cold, it won't run more than 100 yds before it starts to sputter on full choke.

[gspd]

I actually have been parking my bike with the choke on as a security device. Even from cold, it won't run more than 100 yds before it starts to sputter on full choke.

If you're paranoid about theft, an old trick is to tighten down the adjuster so the rear brake is jammed solidly on when parking overnight in 'iffy' areas.
Also, leaving it in 5th gear makes it almost impossible to quickly ride away if it's hot-wired.
It's hard to believe that at one time all BMW's (and many other bikes) had the same key. What were they thinking?

[daz]

gspd, not as many thieves back then. People are more lawless now.

[melville]

I'm not paranoid. I just don't like to take the key out where it's buried deep inside the fairing.

In the odd case that someone looks inside the fairing and sees the key and somehow knows how to turn the fuel on and the other wee things he's gotta do to fire the thing up, I'm figuring they won't figure out the choke, which is unmarked on a /7.

If he knows about the choke, he's probably an Airheadist and not a thief.

[Rob Frankham]

What make you think that airhead riders aren't immune to the temptaion to be light fingered?

Rob

[jackonz]

Have read this post from end to end and I have seen this issue before but not on a BMW, it was on a Norton Commando that had been tuned up, the issue was the fuel lines and taps not giving enough fuel flow at high speed and load.

All they did was put bigger taps in so you would need to find taps with higher flow and T connectors with larger bore, have also seen it on a Kawasaki 400 twin here where I live not enough fuel flow but that was because of low fuel in the tank.

Rob is correct revving it with no load is no indication of how it will perform under load, you could seek out a shop with a Dyno to really see what is going on.

[gspd]

.... so you would need to find taps with higher flow and T connectors with larger bore...

The stock BMW system WHEN NEW, even with only one tap turned on, provides plenty of fuel flow to feed both cylinders under any conditions you will likely encounter. What happens to the screens and inside the hoses and connectors WITH AGE is another story and a build-up of crap in there can greatly reduce flow..
No need to go to a larger bore, just a cleaner one will suffice.

......you could seek out a shop with a Dyno to really see what is going on.

I agree with this 100%. A dyno IS the only way to extract max performance* .
Airheads hate dynos.
Unlike chain drive bikes, airhead rear (non-paralever) suspensions rise under load causing the u-joint to be at an extreme angle which will cause horrendous sounds and vibrations that you've never heard or felt under typical road conditions. You have to run 50 psi in the rear tire, set the rear suspension to minimum preload, and cinch the rear of the bike down to compress the rear suspension until the driveshaft is in line with the transmission output shaft to even attempt a smooth dyno pull that doesn't sound destructive. If you doubt this, run your airhead up to 80mph on its main stand and you'll sort of understand.

* ...or follow the advice of the pros that dyno'd their bikes to podium finishes (or to destruction)