I have a 99 sportsman 500 thats not running properly heres what i have done. Febuary it started acting up when the motor got hot what i found was the wires to the voltage regulator were melted (they were pinched causing the short) I fixed all the bad wires and the quad still was acting up when warm now I`m thinking coil. I bough a new coil, CDI, voltage regulator, and replaced the magneto and spark coil.
Still the thing is not fixed it starts like a champ, but seems to stumble to get the rpm`s up. when it gets hot (about 15 minutes of running) it starts to back fire and dies, when i try to restart it it fires and dies no go. if i let it cool down it will start great again.
Cam lobs look good no signs of ware, 165psi compression. timming is proper.
PLEASE someone must have an idea.

 

One thing that doesn't seem right is the 165 pounds compression. If the cam detent lever,spring and detent ball are working properly the most you should show is around 50-80 pounds compression on a 500 engine. Plus did you completely go through the carb,replace the needle and seat,float arm and pin( on a 99 model the float arm should be wobbly and worn along with the float pin if it hasn't been replaced) Did you have the cam lobes measured?You just have to remove the rocker arm blocks. 1.2884-1.2940 is the range for the intake and exhaust lobes,with 1.2766 being the service limit when the cam needs replacement. The 99 model was in the middle of soft cam problems.(1996-2000 model 500 engines). Backfiring can be fuel problems,electrical,or even cam problems.Plus valve clearance is .006 intake and exhaust. You said you think it's a coil,but you bought a new one in February? Think you need to down load a cheap manual before you throw any more parts at it. OPT

 

Forgot to ask in the first post. When you say it dies when it gets hot,does the fan cycle on and off? Have you bled any air out of the cooling system by leaving the radiator cap off while the fan cycles on and off. Could be this might be part of the problem,or maybe not?? OPT

 

Thanks OPT yes the fan does cycle on and off as it should, I didn`t measure the cam lobes but when I replaced the head & piston (bent valves) it looked like new just like the day I bought it 1.5 years ago. The reason for the bent valves was a sheared keyway pin on the crank and when i set the timing i didnt know it was 45deg out. as for the cam detent I think its working good as I`m able to use the recoil start most of the time when needed only once in a while it rips the recoil handle out of my hand.
I do have a haynes manual and find the info very good and thural, as for throwing the money away on parts I can agree somewhat with what you are saying but the way i see it is its going to need the parts at some point so I may as well put them in now for piece of mind.
I was just looking at the quad with a buddy and he reminded me the last time it acted up on the trail I was messing around under the seat and the problem cleared up. So i just started it and messed around with the circuit breaker (Black bag)and the motor sounded like it wanted to idle up and run like it should, OPT do you think when the voltage regulator shorted it could have made this circuit breaker faulty??? could i just cut it out of the harness and run withpout it??? thanks for your help Randy.

 

Just split the bag and check that you have good connections on the thermal circuit breaker and there is no corrosion build up.This is the main circuit breaker and there should be one more(10 amp) breaker under the front panel for the fan,but sounds like it's ok since the fan cycles on and off. You wouldn't think a sealed part would corrode,but they can,plus just get weak with age and trip when warm ,then reset when they cool down. You can find a 20 amp breaker replacement at almost all auto stores pretty cheap. DON'T run with out a circuit breaker for electrical component protection. Best to stay with this type circuit breaker since it resets and you can reseal the pouch back up to help keep moisture out. Hope this is your problem. OPT

 

I didn`t see anyother electrical connections anywhere on the quad other than behind the cluth cover there is another pouch. thanks I`ll split the first one open and see whats going on inside it I too hope the problem is a simple a a new circuit breaker. Thanks for your help I`ll keep you posted as to my progress.

 

The sealed pouch between the clutch and battery is the main 20 amp circuit breaker that could be causing the problems! If it cuts off,then power is shut off to the entire atv. Don't think the 99 model has a 10 amp breaker under the front panel,just the earlier models. OPT