Whats the smallest jets available for the 42 Mikuni HSR?

Mikuni main jet size 60
Mikuni pilot jet size 10

Standard jets for the HSR42 run from 150 to 210 in increments of 2.5.

If you are running a 150 you better be at altitude or it won't last long.

3rd position to start, move it up to lean it out. Air/fuel should be around 1-1/4 turns out. Runnin' a lil fat is not that big of a deal. Are you having any performance issues? What size needle & seat are in it?

Btw: what size pilot are you running?

Not sure on the needle and seat, I think it has a 15 pilot jet. Engine performs well for what it is, mild port job on the heads and stock intake, carb, and exhaust. BTW the heads have not been shaved. Overall the Mikuni has better drivability, but at WOT the stock carb ran just as good. I suspect the Mikuni is a little too much for a basically stock engine. Thanks

Performance is about efficiently getting air through the motor. With the M-carb you are more efficient on the intake/atomization but the stock muffler is not letting the engine breathe (air out) as well as it could. A higher flow muffler will let you see better performance out of the set up. Most of the jetting information you see on here is with a high flow exhaust so its possible you are running a little fat now. If you are too fat and loosing RPM after 3/4 throttle, then step it down a size.

Also note- std pilot is 25

The majority of you rrunning will beon the pilot jet, enrichment screw and needle. I am finding out that going down on the pilot and adjusting the enrichment screw to add a little more fuel along with a smaller main work pretty well. I am currently running a 22.5 pilot, needle on the second notch from the top with a 165 main. Enrichment screw turned out a little over 1 turn. Stock motor could take a 17.5 to 20 on the pilot with a 160 main.

The needle position only helps tune the transition from idle to main jet. Lowering the needle via raising the clip will allow less fuel to enter the system as you increase throttle, and raising the needle via lowering the clip will have the opposite effect. Once you reach a certain point the needle is completely off seat and doesn't have an effect any more. Don't know exactly where that point is, but surely at wide open throttle the needle is irrelevant.

O.k., took the carb apart and it has a 160 main jet, 97 needle, 25 pilot, 60 pump nozle, and the needle clip is in the 3rd position from the top. Idle mixture screw is set a 1 1/4 turns out. From 1/2 throttle to WOT there is hardly any change in rpm, maybe 100 rpm diff. No hesitations, it just seems flat from 1/2 throttle on. I did try a Yost Power Tube with a 165 main jet and it did not perform any better.