Bo,
Interesting about the 39mm flatslides being used on the new Bonnevilles. My only real experience in the increase in carb size, is when I first took my roadrace B50 to Bonnevile. My first run, using a 36mm Amal Mk II (clone of a Mikuni round slide) I went 99mph. For my second run, I changed to a 40mm Amal Mk II, and went 104mph. Unfortunately, I also changed the rear sprocket from a 47 to a 49, so I'll never know which change had the biggest effect. I chose a 42mm Mikuni Flatslide for the short stroke motor, even though the guru of B50 flattrack builders (Ed Valiket) told me he uses 44mm roundslides (which are monsters!) It's just too time consuming to try all the possibilities when we only have a 4 day event with a couple of runs per day. In any case, the motor dynoed at 42hp in Longmont, Colorado (elevation 5200 feet), so it is not doing too bad for a 40 year old 30 cubic inch pushrod 2 valve motor on gas.
Jon,
Thanks for the link on the shift interrupter. I won't get a chance to try one this year, but it makes sense. I did source a close ratio gearbox for this year, so that should help if I can get off the line (1st gear is 1.94 to 1, and I added a tooth, or maybe 2 to the front sprocket). I think Bo is thinking about an air shifter as well as a spark interrupter, but that would of course be more $ and time. At least in my class, I only need about 1/2 mile to get up to speed, so a few burps along the way are no big deal. Don't know about your build though, Jon!
Tom