ORIGINAL: Mr Cox
I don't understand the meaning of top and bottom here. The larger number is the outer diameter of the base and smaller number is the bore of the carb barrel. The two engines then use the same bore in the carb which is not that strange when they are quite close in engine size anyway. The difference of the base diameter is just so that it fits the case, and has no influence on fuel draw or anything.
Mr. Cox. I didnt read the table properly. When I've measured carbs before, I've measured the dia. of the venturi on top of the carb (above spraybar) and also the bottom (below the spraybar) and also the O.D of the "neck" and most often I've found the measurement of the venturi of the carb is always smaller than the bottom. I assumed thats what the two numbers meant. I didnt take into account the size of the "neck". I havent had my 2 cups of coffee yet.
ORIGINAL: rcdude7
ORIGINAL: 1QwkSport2.5r
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.
.340 = 8.5mm and I think this is on the large end for the .65 sportster. It might be okay but I would bet the engine will handle better with a 7 or 8mm as this what other plain bearing .60's use.
I dont know if they used a different venturi size on any other sportster engine, but mine came from the RCMAT target drones for the military. As I understand it, they were basically meant to be thrown in the air and ran WOT to be shot down during target practice. My sportster carbs do not throttle well. The midrange is so rich it often stalls. (They have the early style K&B carb with the needle valve on the carb body) Randy at Mecoa said a new carb is in order, but a different spraybar/mixture disk might cure the trouble.
8.5mm seems small for an engine this size IMO.. I have several car engines, with the .28 being the largest and having a 9.10mm venturi. (I suppose it needs it to turn up to 28,000rpm.