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Ask Jeff Policky Your Own Question
Jeff Policky
Jeff Policky, Motorcycle Mechanic
Category: Motorcycle
Satisfied Customers: 2425
Experience:  Yamaha Gold, Suzuki Gold, Honda Bronze, Polaris Bronze, BRP
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I have a 99 polaris sportsman 500. Has compression, fuel,

Customer Question

I have a 99 polaris sportsman 500. Has compression, fuel, and spark. Using a timing light, the spark is firing after the timing and TDC marks on the flywheel. CDI black wire is disconnected to eliminate switches. I have swapped out the CDI to another matching ATV and it works fine. Verified timing of crank vs. cam. Checked compression (110 lbs). Keyway on crank good. Ohms tested Stator and pulse coil....OK. Coil Ohm tested fine. Spark will jump 10-15mm.
Also, has pressure firing back thru the carb while cranking over. Valve clearance is .06 as per specs on intake and exhaust. Timing chain/gear marks all line up.
Customer dropped it off barely running...I cleaned the carb and it ran fairly well for 5-10 minutes then died, never farted since.
Submitted: 1 year ago.
Category: Motorcycle
Expert:  Wayne S. replied 1 year ago.

Worn exhaust cam lobe and rocker arm.

Customer: replied 1 year ago.
Checked it already. The ATV acts like the timing chain has skipped a couple a couple teeth, but the crank and cam timing marks are still in sync. Any chance this has a two piece crank and it has slipped??
Customer: replied 1 year ago.
Also performed a leakdown test with goo results. While adding air pressure to cylinder, I rotated the engine using a breaker bar on crank nut through 2 full revolutions to verify the valves were all opening and closing....I got good air flow thru the intake and exhaust ports as you would expect while rotating the engine.
Expert:  Wayne S. replied 1 year ago.
I am going to open up your question again so others can answer.
If I think of something else to check I will get back to you.
At this point all you can do is recheck things. Start with checking the vacuum pull on intake, fuel supply and spark.
Check ignition timing, exhaust flow and battery if it uses one.
Bypass the fuel with starting fluid and try to get it to fire.
Should have spark while under compression as well.
I hate it when engines are so stubborn when everything seems right.
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Customer: replied 1 year ago.
Thanks Wayne. I have tried the starting fluid as well to no avail. I have been working on these (ATVs, sleds, motorcycles) for many years and this one is keeping me awake at night. :(