Idle Jet change 32mm Bing

[Gibson]

I finally changed the pilot jets on my 94 r100r bike from 45 to 50. Before I made that change I changed the rubber mounting tubes to the head, but they were not leaking, just distorted. It runs noticeably better from 0-1/8th throttle. Off idle engine response is better too. I thought I would need to mess with the air screws, but they appear to like where they were with the 45's. I have heard from several sources that these bikes were jetted on the lean side.

[SteveD]

I suspect that jetting in the 90's was deliberately lean to get past emissions rules. Once sold though....

[Beemerboff]

If you didnt have to change the idle mixture screw with the new jet it was probably wrong in the first place!

[Wobbly]

I agree with Beemer. Stated another way... If you'll look at the jet tables for all Bings you'll see that #45 was always the idle jet size, even back in the early 70's well before EPA mandates. I have no doubt that your bike runs much better now, but I think the larger jets in this case are masking a tiny air leak possibly at the throttle pivot o-rings, intake rubber hose, or threaded intake spigot.

[Beemerboff]

A colortune plug is a great help when tuning CV carbs.
You can block the bike up on the center stand and run the motor against the back brake for a second or two so you can actually see what is happening when the twistgrip is turned and the butterfly rotates.
Without one all you are setting is the mixture when the butterfly is closed!

[Gibson]

Thanks. Yes I know that 45's are used on almost all BMW airhead bikes. I have electronic ignition advance and Kehin y pipe and sst head pipes without crossover. I think maybe a the combination of these could have some influence as well. Could be a leak in butterfly o-ring or maybe diaphragm. I just changed intake rubber pieces before I made jet change.

[Wobbly]

Thanks. Yes I know that 45's are used on almost all BMW airhead bikes. I have electronic ignition advance and Kehin y pipe and sst head pipes without crossover.

Agreed. Changing the pipe configuration from stock could be a big contributor. So few people change something like that it's going to be hard to find someone to compare notes with.