Yamaha Rhino runability issue?

S

Super Cleat

New member
I am very familiar with these machines, but one of my customers units has me scratching my head. its a mildly modded 660 rhino with a slip-on and aftermarket airbox. It seems to "stumble at high r's under acceleration. valves are set, carb is clean and jetted, I have tried 2 known good cdi box's, new plug, plug cap, primary coil and pulse coil test good, and the unit has fresh 91 in the tank . where do i go from here guys? I have lost about 2 days labor on this glorified golf cart and don't really know where to look next.
please no stupid comments.
Thank you both for your time and advice, a leak down test led me down the right path. I had to recut the exh valves to correct the problem. Its still odd in my opinion that the valve seats could cause this runability issue. but its out of my life now, yay!
 
Remove all of the vent hoses attached to the carburetor - they slow down the air entering the float bowl, which will slowly starve the engine of fuel (run lean).

The aftermarket air box may be allowing to much air in (lean condition).
Try closing parts of the airbox intake with duct tape - close off the intakes
in increments.
I've had the same problems when drilling to many holes in airbox covers (as per specifications).

Install the stock airbox, air filter and stock jets to check if that repairs
the problem - then you'll know if the problem is fuel related.

Try adjusting the needles in minor increments (not a clip at a time), but with thin washers that come supplied with jet kits.

Merry Christmas!
 
Does sound like fuel starvation at high rev under load. You state that the carb is jetted [correctly] so a check of the float level is in order [assuming proper fuel flow].
 
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