90/6 Bing Carb Discrepancy

[troypla]

I have scoured my available references on jet size and needle position for my R90/6 and found discrepancies that I wish to know if they are critical or not. Specifically, Haynes and Clymer state 150 main and jet needle in the first clip position. Bing manual says 150 and second position. Ian Falloon (who I know as a Ducati expert...not necessarily BMW but a good resource) has in his "Original BMW Boxer Twins" main jet 135 and 1st position.
Anybody know what should be?
I ask this not for trivial reasons. My concern is loss of power at 1/2-3/4 throttle. Also, the bike's earliest known owner had it up to 1000cc and may have changed main jet. I put it back to 900cc, rebuilt carbs, and left the 145 main jet I found and also left the jet needle where I found it in 2nd position. Today I will lower the needle and see what happens but would like to know what the consensus is. I am in Louisiana...hot and humid which may call for leaner jetting?

[88bmwJeff]

While I don't know what the specific set up would be for your bike, I would trust the Bing manual over Clymer and Haynes. I've heard from the "experts" that there are many mistakes in those manuals.

[20774]

My understanding is that 1/2-3/4 throttle is still running on the needle jet - jet needle combination. When you get above 3/4 to WOT, then you're on the main jet. The needle jet is for fine tuning; moving the needle up or down is gross adjustments. As mentioned, I'd start with the Bing numbers and work from there. Get the performance, gas mileage, and plug colors you want and that's the best you're going to do.

When's the last time the floats and jet needle were replaced? The needle wears and can cause a rich situation. The floats can begin to sink and also cause a rich condition. An "OK" float is one which has about 1/3 of the float sticking out of a container of gas...2/3 should be below the survace. How about the float position when the fuel shuts off? Should be parallel to the body of the carb. You should also have about 20-22mm of gas in the bowl, measured in the center, when you allow the bowl to fill then shut off the petcock, and then remove the bowl. If the fuel level is too low, it's harder to pull the fuel up into the throat of the carb, thus resulting in a lean condition.

Kurt in S.A.

[Polarbear]

My '78R100 had 150's from factory. I bored the cylinders a long time ago and put bigger mains in. Did not seem to hurt, but definately ran a tad richer, so I lowered the needle and all was well. I just put 146's in to try and improve my mileage. Bike still performs well under full powerband, with needle still lowerwed all the way. We'll see. 32mm carbs here. My mpg's run about 40-45, depending on throttle hand. The mains are about 5$ a piece, through Bing Agency in the midwest somewhere. Kansas?

[pmdave]

I'd suggest calling the Bing man and asking for advice. Be prepared to describe the symptoms accurately.

I had a '78 R100 with some carb issues, and a brief conversation had me changing the pilot jets, which immediately solved the problem.

It's easy to get one needle a notch lower or higher than the other side. Measure the length of both needles extending below the slide, and make sure they are the same.

pmdave