2005 Polaris Sportsman 500 HO Carb

So yesterday I had a problem with my 500 that is starting to baffle me a bit. I do have some ideas but I want to see what you guys think. Here's it go!

I was out on my bike screwing around and it runs great. Lots of power, idles fine and runs the same as it always has. The only thing I noticed was after a short wot run and as soon as I let go the throttle it backfires. I didn't think to muck of it at the time.
Now I decide to make a run down the ice when all the problems start.
I hold the bike at wot for about 30 seconds and all is fine then all of a sudden it starts backfiring and dies. It starts back up with no problems. I took the bike home and change all the fuel over to fresh premium gas and it did not make a difference. I then cleaned the Uni filter with no difference still backfiring and dying at WOT. I changed the sparkplug and fuel filter with no change. Spark plug looked perfect! I cleaned the battery posts and checked all the electrical connections. I then pull off the fuel pump and cleaned it. It was full of mud on the vented side.
Cleaning the fuel pump did make a slight difference. The bike now does NOT backfire at WOT but it now sputters and dies at WOT after about 30 seconds.
Next I pull the carb tonight.

It seems like the floats are sticking partly shut (improper float level), only allowing the needle to let a small amount of fuel thru, causing it to run the carb dry WOT.

__________________

 

As soon as you mention 2005 I have to ask if the ECM had the recall done. ATV: Polaris Ind. expands recall of ATVs

If that brain box acts up ,all kinds of strange things happen.

 

Yep had it done a few years ago because I didn't want my garage to burn down!

 

Doesn't mean that it wont fail again. Pull the fuel line from the carb and crank the engine over and see if you have a steady pulse of fuel. If you eliminate the fuel supply and sort out the carb and still have problems I wouldn't discount another ecm starting to go out. I replaced more than one on multiple machines. Plus here's an article on the BST40 carb plus how to to set the float level.The BST-40 Bible - ADVrider

 

Kind of a pet peeve of mine are threads that people start about a problem there having with there bike and never posting a solution or resolution to the problem. So with that being said!!! Here we go.....


"UPDATE"/"SOLVED"

So yesterday I decided to get off my *** and tear into this problem. I 99% sure I got this problem solved. I pulled the carb and the carb boot off. The boot was clean with no visible cracks or dryness, looked like new! So now to the carb. I drained the fuel from the bowl and it clean clear with no water. I pull the bowl, floats and jets, everything looks perfectly clean. The needle, diaphragm and slide are clean with no damage.
I start to reassemble to look at the float height and it looks good nice and level. So I decide to put a hose on the fuel inlet tube and blow threw it while rotating the carb. BINGO! With the floats in there lowest position I had almost no air flowing past the needle and seat. I readjusted the float tang and repeated the process of tipping the carb and blowing into the fuel line and to my surprise I almost doubled the amount of air I could get past the seat while getting the float to sit level with the first touch of the needle to the seat.
I believe the problem was with the needle itself. It is an aftermarket needle and the spring tip is at least 1 mm longer than the stock one, plus the wire that hangs the needle to the float was also longer than stock.
I put the carb back on the bike and decided to adjust the engine valves before starting. Both exhaust valves were tight to the point where I couldn't get a .003 feeler gauge between the valve and rocker. The one intake was way loose (.012)and the other was perfect. After a few adjustments all the valves are now at .006mm.
I put everything back together and to my surprise the bike started after only a second or two of cranking without the choke. Huh!
The bike is idling smother with less valve train knocking and taping. I took it for a quick rip down the river and the bike ran great. Running Strong with no backfiring when I let off the throttle from WOT and it didn't start to die when I held it to the bar for 30 seconds.
So I believe I have my problem solved, but I only say 99% because I need to get out on the lake so I can hold it longer at WOT (60sec). If the bike doesn't die within 60sec then I'll be 100%.

 

What we generally called this was a complete service,plus checking for other problems! Most people attack bits and pieces here and there on their atvs instead of methodically looking at the manual and taking it step by step eliminating one thing after another. Granted somethings are tricky as what the poster mentioned above. You can't always just take things for granted because they look right.

 

Spent the day out on the ice and I can now say it's 100% fixed! No backfiring and it never skipped a beat!
Thanks for all the help!

 

I have a 1997 scrambler 500 that keeps backfiring every time i give it throttle it will idle fine
I have changed the cam put new rockers on as well did the .006 gap clearance spacing
i am running out of ideas and cant get this problem to stop ?
there is an aftermarket muffler on this unit
i have also put a new carburetor on this bike
what else could it be