Originally Posted by supershaft
I think the 32mm Bings work better too. I am not much of a fan of the 40mm Bings. I am a big fan of bigger than 32mm carbs, just not Bings.
Whatever the issue is with a square box/40mm Bing combo I don't think it is volume. If 40mm Bings pulled that much more air than 32mm Bings they would run that much stronger with 40mm Bings and they don't. Besides, I know from experience that the stock airbox can pull a lot more air than 32mm Bings on a stock engine for my bike making way more power with a stock square box and 38mm Dellorto's, a sport cam, mild port work, and a sport exhaust.
The standard PD engine has too low a compression ratio to make the most of the 40mm carbs at higher RPM and the standard airbox (tops and all) cause a depression midrange that takes the mixture way too rich effectively killing the power delivery. The 32mm Bings were great and kept the mixture within a very tight band making good power all the way through. I was lucky at the time in having a two GS's, both the same year, one with 32mm carbs and the other with 40mm that I could run back to back to see what the differences were. The 32mm equipped bike was so much better all the way through.
What I do find strange is that BMW chose to produce the R100R and 100GS in Europe with 40mm when the rest of the 1000cc bikes came with 32mm carbs. Makes no sense when the GS especially is not intended as a high speed machine.
The clamshell airbox doesn't cause such a midrange depression in the 40mm carbs mixture as the standard airbox allowing the bike to run cleaner but it cause the mixture to run very weak at the top end. I know Plaka lists a range of jets but I could only buy 160 and 170 in the larger sizes. 160 was too weak and 170 too rich so I ran with the 170 mains along with variations in airbox designs for a while before putting it all back to standard. What the clamshell did do was give a nice progressive power delivery, unfortunatly the GS doesn't have the compression as standard to make the most of it at the top end. At the end of the day there's little point trying to hotrod a standard GS when the chassis, weight, tyres and brakes are the biggest limiting factor. Personally I think the biggest single improvement to the 100GS if you don't want to be messing with high comp pistons, airboxes, exhausts etc is to fit a pair of 32mm Bings.