I have an 02 500 ho. had a wire break to rectifier (sp) witch caused the fan to qiut and it overheated enough to blow the head gasket. checked head and all looked ok . when i got it together it now back fires in air box. so i changed springs thinking they were flouting, not that still doing it. after reading alot of other post im not sure it is the cam but maybe the valves. even after retarding the timing to have exahst stay open more and intake open later it starts fine but still bakefires under throttle. what are some of ya'lls thoughts? i'm thinking either bad valve or improper seating...............

 

did you actually check the head w/ a flat edge to make sure it wasnt warped? i agree that its valve or cam related. did you check the cam to make sure the lobes werent worn down, and how did the valves look? just a couple of questions.

 

Sounds like either the cam timing is off or the intake is hanging open a bit.
A simple compression would tell you a bunch. A leak down would tell you even more.

 

this will sound bad but i well sooo stupid after doing a lot of thought i stopped to day and picked up a cam just in case i needed it since shops are closed on sunday and monday. last night after reading here i realized after watching mine run with the value cover off the exhast valves werent moving much. so when i got home i really checked the cam and guess what................. my lobe wasn't flat it was gone and now at midnight i'm saying sorry for dobting anyone that ever brought up that it was the cam. i guess it was denial but that was it. now i think my timing is a little off now but not enought to make a diffrents still cranks easy but has little throttle lag but could be the fact it has been sitting up for a year and some but thanks to everyone and to anyone that has this problem CHECK THE CAM it's not that hard THANK YOU AGAIN

 

Congrates dude!!! Now go out and ride!!! Might be a good idea to check the valve lash after a ride.
About the timing? Are you talking about the cam timing? If so, it better be dead on.
Good call stendori!!! I tip my 12mm Snap-On combination wrench to you!!! :')

 

Hey just to put it out there but if any of you ever run into this again and the cam is still good.

The valve springs loose there tension when they get that hot and it will act like a bad cam as well.

I am just putting that out there as a tech that had ran across this and have heard others that had the same thing.

 

just to get lucky is a good thing but to have his wheeler up and running is awesom. enjoy ride safe.

 

i have had 6 machines like yours mostly rangers but a few atvs. Make sure you inspect your rockers arms for damage they most likey are. Spend the extra money and replace them and both sprockets and chain. And get the vavle lash dead on