Originally Posted by Rob Farmer
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.
I agree with you completely about the larger, non bing carbs, and intend to fit MkII Amals on my current project bike.
Which top did you have. It sounds like the one with both horns being the same size? My 8.5:1 Dell/sport cam setup had a big flat spot in the midrange with that top on the box. One large/one small horn fixed that perfectly with almost no penalty on top but the cap with two large horns killed midrange with absolutely stock setups as well. IMO, the GSPD types are too heavy but the G/S's are light. Tires and brakes issues are easily fixed. The single biggest thing I did to my bike which had the same engine as yours excepting lower 8.5:1 CR pistons (I think your came with 9.5:1 pistons?) and coming stock with 32mm Bings was putting a 336 cam in it. Huge step in the right direction! If I was going to go back to 32mm Bings I would rather instead go back to 36/38mm Dell's or Mikuni's. 32mm Bings are great at what they do. My only problem with that is that they don't do much at all to start with!
Bing makes mains for our CV's from 70' to 300's. I keep them at my shop from 120's to 170's. I keep needle jets from 260's to 274's. You can't get 260's and 262's through BMW.