I came across something odd on Perry's carb website. I went to look up a perry carb for my ST S90K (just for the heck of it) and also my 2 K&B sportster .65's. What I found odd was the table calls for a 4500 carb for my S90K which has a .340 top and .590 bottom and the sportster calls for a 4600 which is .340 top and .625 bottom. I am guessing this is to promote fuel draw?

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.

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.

.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.

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.

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.

The diameter above and below the carburator barrel is a way to play with the mixture at the low end mainly, as I see it. At low throttle settings it affects the mixture in a similar effect as an airbleed hole (and screw) will do. On engines without an airbleed hole (or low speed needle) the upper whole is often larger to reduce the fuel mixture at low revs. Some instead have a notch in the throat or on the barrel to create the same effect, i.e. to close downstream first.

Regarding coffé, I'm on my forth for today but it doesn't help much...

I understand. Thanks. Aside from Perry, are there any other carbs that anyone knows of that will fit the sportster and not cost $50 apiece?

I have a 32oz french press that I fill once. I guess that is 4 cups, but my coffee cups are 16oz, so..

The coffee people say something like 5 or 6 ounces is a 'cup' but my cup is the same size as yours.

The selection of affordable replacement carbs is not very big. Perry is one of the few who give any measurements. For most others you have to depend on the kindness of RCU members to provide the info.

Yeah, when it comes to coffee and tea (and the like) a "cup" is 6oz, however a true "cup" is 8oz.

I'd asked around awhile back if anyone had a carb (and I listed the neck/venturi size) laying around that they didnt need and I never got any bites. K&B still sells their remote-needle carb for $50, and there's the Perry carb for $47. I could try a new mixture disk, but at 13$ apiece + shipping, I didnt think it to be worth the gamble in case it didnt work and I had to get a whole carb anyway. I might as well just get new carbs for them. I'm still working the bugs out of the ST S90K. If the rain ever lets up today I'll go out and try running it again. (The last time I tried running it, it wouldnt run for crap. bad fuel I think)

Hi!
If you have 13mm neck diameter on your engine , MVVS has several carbs (7mm, 8mm, 9mm, or 10mm. I can custom make any size between those if you like).

Unfortunately, the O.D of the carb neck on these K&B's is 15.8mm. The venturi is in fact 8.5mm. I'd probably use the same size venturi, but I'd want better adjustability of the idle/midrange mixture than I do with the current carb. If I had the ability to machine small parts, I'd just make a new mixture disc and just move the spraybar orifice a little bit to have better adjustability of the midrange.