I have a 2002 Polaris Sportsman 500 ho that I'm having trouble with.

I purchased it not running and upon doing research I found markings on the exhaust rocker and was thinking that was the issue of it not wanting to start. I ordered a new one along with a new starter. Reset the valve clearance and when I cranked it it sounded much deeper than before from the motor. After maybe 15 seconds of cranking it started and idled well, I was able to give it throttle and it all seemed well. After a minute or so of idling I gave it a good rev(maybe 50% throttle) and then let off quickly and it slowly died​​​​. When I tried to start it again it sounded much different, as if there was no compression?

timing looks to be right on
Compression is 85 psi
Exhaust cam lobe looks to be good
Replaced fuel filter
Replaced spark plug

My thought is that since randomly it now sounds as it has no compression it must be the decompression valve?

Thanks for any help!

 

Tried starting it just now, it sounded different but not like it should, fired up after 5 seconds but wouldn't keep an idle and cut off. Now has the same no compression sound when cranking.