Airhead Lighter Carb Springs
Almost since I'd purchased the R100GS it was clear that the throttle pull was heavier than it needed to be, even after replacing the worn throttle cables with new BMW items. I had a drawer full of carb springs left over from my Norton days, but somehow the idea of converting something from a screen door to fit one of BMW's finest left me cold. The idea kept rattling around in my head, manifesting only as a line on my "todo" list until I stumbled across an ad at the IBMWR Motorcycle Marketplace for lighter airhead carb springs. I posted off an email, and was promptly put in touch with Tony, a middle age entrepreneur with a passion for older BMW twins. Tony was curious about the GS, as his test mule was a /7, and he wasn't sure his replacement springs would fit. In a "it's a small world" type coincidence, Tony lived just over the hill from Dublin in Castro Valley, so it wasn't long before we were rubbing elbows and comparing springs. Sure enough, the prototypes didn't have enough reach on one end, and measuring tension revealed that they were just a bit stiffer than the stockers off my GS. Tony was having the springs custom wound by an old tool'n'die man, and offered to come up with a new variant that would take care of the GS. I had an extra spring to use as a template, and a week later we had springs to test. The new samples were made from stainless steel wire and were easier to stretch between the fingers. When I got 'em home, I set up a test rig with an RCBS trigger scale to find out just how much lighter they were.
The rig was pretty simple: A nail driven into the workbench secured one end of the spring. The other end was held by the hook on the scale, and a clipboard provided a recording surface. Marks were made at the end of the spring with no tension, then again when 48 ounces (three pounds) of force was applied. The difference between the two marks recorded the deflection, and dividing the force by the deflection gave the spring rate. The measurements were made for all three springs: