ORIGINAL: earlwb
Measure your inside venturi bore area of the carb. That would tell you what your stock carb is in relation to one of the Perry carbs
But yes, a smaller bore improves fuel draw and a larger bore reduces fuel draw, abeit a smaller bore might reduce top end power some and a larger bore might improve the top end some.
So you measure the bottom outside base of the carb to match your engine's carb hole and then the inside bore of the carb is next to determine which one you want to get.
Yeah, I've measured the K&B carb awhile back and the numbers on perry's website match up fine. I guess what I meant to say was I found it odd that a .90 that can turn some RPM has a smaller carb than a really slow turning .65. I dont know if .035" would make that much of a difference anyway, but at the moment I was looking at it it just didnt make sense.
(I know the K&B is tame in the timing department, but I would have figured the 65 would have the smaller carb than a big ol' 90)
I was hoping they would use the same carb so I could swap between the two if needed. The K&B's dont have a home, but need new carbs. I'm still working on figuring out the ST carb.