Got this 4 wheeler Thursday and ran perfect, ran it Friday night and now wont start. All it does is backfires while I try to start it. Changed plug cleaned carburetor and still same problem. Appears the bike is getting spark and gas. I checked the compression and was about 75 psi. Any help would be apprieciated. Bike also has 88 ours on it.

Thanks Ricky

 

75psi on compression is about right.Shouldn't need it,but you can check valve clearance. .006 clearance on intake and exhaust. Make sure you have only 1/8" slack at the thumb lever. Any more slack
than that the etc contacts can touch if you let off the thumb lever and can back fire or die.Cable slack adjuster is on the cable close to the throttle. I'd check for a possible split in the intake manifold that could be sucking air also,plus cleaning a carb may not always help. If it's been sitting,or no fuel preservative in the tank, the needle and seat can stick and either run rich or run lean and cause back firing.Best to install a carb kit or at least a new needle and seat if in doubt on any carb.Plus the BST40 is a finicky carb especially on float adjustment. Here's one similar to yours that shows some tips on this.The BST-40 Bible - ADVrider Plus doesn't happen often,but if a fuel filter is partially stopped or the fuel pump is weak,could be a problem. Just pull the fuel hose off the carb and crank the engine over. Should be a good pulse of fuel coming from the hose. Just several things you can check out to see if you can solve the problem.

 

Thanks for the input. I'll let ya know what I find

 

Valves are with in recommended tolerances, the contacts inside the thumb throttle are not touching, checked entire intake manifold and no cracks, removed carburetor again and needle and seat are not sticking and everything still looks brand new with in in the carburetor. removed fuel line from carburetor and has a strong pulse of fuel coming out of the fuel line as I try and crank it. And still having the same issue, bike will turn over and just backfires occasionally. It doesnt even try to crank

 

Then it's entirely possible that you could have an electrical problem if all else checks out. Best to have the wiring schematic along with trouble shooting checks and electrical specs on the stator,coil. You can check that the pick up coil(white to white/red) wires leading from the stator area to the cdi show around 97 ohms resistance.If the pick up coil checks out,you'll need the service manual for the other electrical trouble shooting and stator checks listed.Here's one for 5 bucks.2012 Polaris Sportsman 500 H.O. Service Manual PDF Download

 

This happen to me and it sound similar.

My 500 with the electric start would turn over but not start and would just back fire once in awhile and that's it.
With the pull core it would run and drive but back fire here and there.

Turned out I had a little bit of crud on the pulse coil face that meets the flywheel. I cleaned the pulse coil and it ran perfectly ever since.

The pulse coil face is magnetic and just picked up some metal filings somehow. If the gap between the pulse coil and flywheel is off I hear it also can cause the same thing.

I found this out on the internet so I,m not the only one it happened to.

 

Thanks for the input guys, I am back offshore working for a month so I will try this out when I get back home

 

after reviewing the World Wide Web the past 2 weeks, it looks like I might have sheared the keyway on the flywheel and it jumped time. I will know more about this in 2 more weeks when I get home to look into it but I have seen alot of the same issues on the same year model bikes.

 

Possible and seems like the 500 is the only one that has done that. All it takes is for the flywheel to move a little to cause problems.Takes a 5mm woodruff key if that's what happened.Hopefully the flywheel and shaft end are ok.

 

Thanks OPT, what Ive read there isnt really a permanent fix for this other than idling the carburetor up to 1150 rpms are above. If this is what is wrong might possibly put this machine up forsale after I fix for another bike