I have a 87 Polaris Trail Boss 4x4. Have owned it since 1988. Have just completed a top end rebuild to restore compression. Engine starts fine and idles fine. Upon increasing throttle, the engine revs up to the point of about 50% open throttle. Increasing the throttle beyond this point , the engine will not inrease in rpm . Rather you get the waaah with backpulses out the carb air intake. This problem was present prior to the top end rebuild. The air filter is currently off, so this eliminates starving of air. I pulled the fuel tank shut off valve and checked for trash in the screen & gas feed line. I've disassembled the carb, cleaned, applied compressed air to channels, checked jets, etc. Any suggestions would be much appreciated.

 

A couple of things come to mind . . .

Possibly the Reverse limiter is defective - locate the limiter (under the fuel tank, towards the front - you can get to it without removing the tank) and unplug the black wire - this will eleminate the operation of the limiter - be careful with reverse if you leave it disconnected as it will allow the reverse to be just about as fast as forward speed.

The exhaust is plugged - this happens frequently with 2 strokes that run rich on the oil. Unhook the springs the "J" pipe and exhaust pipe joint, pull the joint loose and crank it up to see what you get.

One last thing - the crankshaft seals may be going/gone allowing the higher RPM's to "suck" air, resulting in a too lean fuel mixture

 

I've got the same thing going with my sons 87 2x4 its got to be something in the ignition department its like it comes up against a rev limiter I thought about the seals but if I hit the choke it gets worse and wants to die I've messed with carb jetting and nothing keep me posted if you find anything

 

All,

After my post on the this forum, I stopped by the Polaris dealer and spoke to an "old-timer". He told me that in addition to the reverse rev limiter, there was a speed limit regulator that prevented the engine from over-revving. Since the condition was present in forward, neutral & reverse, he was betting that the speed limit regulator was defective. The Polaris part number is PO4060079 and it cost $35. I asked if there was a way to take it out of the system to test it. He told me that it was located in front of the CDI module on the front forks next to (and just under the rectifier). He said to unplug the black wire and that would take it out of the system. I did just that and the problem went away! Having just completed a top-end rebuild on the engine and just clearing this problem up, the old ATV has a fresh look on life now!