I run a cr500 powered snowbike and it doesn't use a lot more fuel than my buddies KTM450, maybe a little, but not a lot. I weigh 270 so I think I would eat up more regardless but it pulls well down low and opens up killer at the top. I am on A PWK carb, but I am a jetting nut and have been riding 500's since 1985 so you get pretty good at dealing with them. On my PWK it was pretty easy to get the pilot nd main right but took a couple needles to get the middle correct. I think this BRC thing sounds like it would be pretty cool. I need a 2 stroke 500 just to keep up with my light weight friends on 450's. If you ride a well tuned 500 2 stroke it just pulls harder and I don't care what the dyno says. It took me a few years to get it all sorted out and I am drama free now.
What needles were you using on the cr500?
I have a kx500 and am a bit of a jetting freak as well. Need a smaller diameter needle, I have a bit of a lean spot from 1/4 to 1/3 throttle. Since there is really nothing available anymore from Kawasaki anymore was thinking of trying the Suzuki L and K needles since I am currently on the kawi stock M needle. I also have a 6.5 slide kicking around I might try, stock is 7.