i have reciently instaulled a pro circut slip-on on my rhino 660. it ran like a top with dynajet cdi, k&n replacement filter, then i added the pipe, went up a main jet size and now it stumbles on itself at really high rpm (7000-8000). not when its accelerating but if i try to maintain speed or let engine break takeover at th ehigh rpm's. any sugestions?

 

If you only went up one jet size you may still be too lean. Here is what I run at 6000 ft altitude, and I have a clamp on style K&N, with the restrictive stock airbox snorkel removed, and a Yoshimura slip on exhaust, ... so pretty much the same mods as you.

fuel screw in bottom of carb is 1 3/4 turns out

washers on needle have been switched so the thick washer is on the bottom and the thin washer is on top (opposite how they come stock)

160 Mikuni main jet

So if you went up one jet size, you are at a 147.5 main jet, which may be too small for your mods, at elevations below about 9000 ft.

It may also be the idle circuit, since you notice the problem when you back out of the throttle at cruise speed. Be sure to adjust your fuel screw with the engine warmed up and running. It is tough to get at it without tilting the carb, but you can use one of those interchangable screwdriver tips, and just turn it by hand instead of putting it in the socket on the screwdriver. First drill out the small brass plug that covers the fuel screw, then turn it all the way in, and then back it out two full turns. That will be a good starting point, and you should be able to go another half turn back in, or up to another full turn farther out, to get the setting you need. Set it so that you have the best idle, and you are good to go.

Take it for a ride after adjusting the idle circuit, and if it doesn't fix the problem you can try switching the thick and thin needle washers, which gives the engine just slightly more fuel in the midrange circuit. It is about the same as moving 1/2 of a clip adjustment on an adjustable needle, being as the stock needle does not have any other adjustment possibilities.

Then take it for a ride again, and if you still have the problem the main jet needs to be richened up. Don't forget to check the color of the plug to get an idea if you're too rich or too lean.

DV