For several months I have been working on a problem with my 2000 Sportsman 500. The machine, after working on the clutches & rebuilding the front end, last fall, suddenly, after 10 minutes of running, refused to rev up above 1/3 throttle. After multiple posting's and suggestions, this is what I have done. New carburetor, checked for worn cam, [perfect]. Did a compression and leak down test. Found a problem with worn valves thereby low compression. Purchased new intake valves, and had the seats ground, and valves hand lapped by reputable shop. No change. Removed flywheel, and checked for timing plate, stator coil shift. It's OK. Finally, purchased a new coil wire & boot, & CDI box, figuring that it was either not advancing timing, or just limiting the revs. It's charging OK, and sparking when the flywheel passes by the pick up coil, to be expected because it starts & runs ok up to about 1/3 throttle. Other observations; it runs worse when the air box is installed without the filter. It starts a little hard but runs OK at 1/3 throttle after starting. Any suggestions would be greatly appreciated.

 

I'd still check the exhaust cam lobe to factory specs even though it may look ok. 32.726- 32.826 (1.2884-1.2924) Service limit 32.426 (1.2766) Hooking the air box up even without the air filter makes it runs worse,then could be jetting is wrong,fuel screw setting off even on the new carb unless you've verified.Plus spedos on this model has the built in limiter unlike the older models with the separate limiter modules that you could disable. You can pull the spedo harness and see if there is a difference in the way it runs. Plus you didn't mention the fuel pump. Just pull the fuel line from the carb,crank the engine over,should be either a steady stream or pulsing stream of fuel from the pump. Plus don't over look simple things like the carb slide needle plastic holder and diaphragm spring are in place properly.Can run like crap if it isn't.

 

Thank you OPT for your reply. I checked the cam first, and it appears to have been changed. The specs. were actually slightly better than new. My vernier calipers may be off slightly. Fuel pump is new. I verified that it is working properly by disconnecting supply line, and it pumped about 5 oz. of fuel before the engine stopped from fuel starvation. The spedo is an interesting thought, as it hasn't worked, in quite awhile. A little more information; Last season it began, slowly, losing top end. Still ran & started fine. I attribute this to the slow loss of compression due to the valve issue. This situation happened almost instantly. After reworking the clutches, & rebuilding the front end, I ran up and down the road, ran just fine. My son took it for a ride, and after 5 min, had to limp back to the shop. I used to build engines for drag racing back in the late 50's & 60's, and never have been as exasperated as I am now.

 

Still wouldn't rule out an internal spedo problem causing problems. Don't know if this helps,but look over how some have defeated some of the safety devices.http://www.google.com/url?sa=t&rct=j...,d.cGU&cad=rja

 

I read a post once were a guy had one running like a 3 legged dog, it turned out the be worn clutch buttons and that fixed it.

Just throwing it out there.