[img]i/expressions/face-icon-small-confused.gif[/img] I've tried everything I can think of but my 1999 Sportsman 500 continues to run lean and backfire after it warms up. I've read 100's of threads in the forum for any help I could find. Here's what I have done so far:

Put on new fuel pump (old pump seemed fine).
Cleaned carb/put e-clip on richest setting.
Put on different well running carb.
Checked timing and retarded slightly.
Checked valve clearance @ .006.
Pulled head and valves look perfect.
Compression checks out both hot and cold.
All vent and supply lines checked.
Changed fuel filter.
Checked thermostadt.
Measured cam lobes and they are fine.
Checked ETC.
Changed plug.
Checked rubber boot.
It runs the same with or without air box installed but I always run it installed.

No matter what I change it runs fine until warmed up and then it won't move off idle without backfiring and surging. Plug is bright white. Please any ideas welcome.

 

Check stock jetting, has it been leaned out for high altitude? SHOULD run lean and barely take throttle with air filter removed with STOCK jetting (at least down here in TEXAS!) The old saying was cdi worked or it didnt,but I've seen different. I threw a BAD cdi on GROUND,rehooked up and ran for a short while ( this was on a HONDA) WHITE plug not good, could be cdi,limiter shorting out, but check jetting for your altitude! Also COIL can be breaking down when HOT! KEEP trying,you are doing ok. Good luck

 

Have you tried running it with the choke part way on after it warms up? If that helps then most likely a plugged main jet.
when you cleaned the carb did you completely disassemble it and soak it in carb soak or did you use spray carb cleaner on it?
I am thinking that you have a carb problem, lack of fuel, wrong jet or a clogged passage. float level could be off also.

 

Thanks for the suggestions, jetting for my altitude is correct per the manual. I'll concentrate on electrical next, cdi and coil.
When its solved I'll let everyone know.

 

I tried choking it with no change other than it would tend to stall. Also tried blocking off part of the intake to slow down air flow with no help either. I did a pretty thorough job of soaking and cleaning the carb I think. But the other good running carb I put on did not help. [img]i/expressions/face-icon-small-mad.gif[/img] man this is frustrating.

 

I also connected a clear line to the bowl drain and the fuel level stayed up when it was leaning out and missing.

 

Did you spray carb cleaner on the intake boot while it was running? if there is any cracks the engine will speed up, sounds like its sucking air after it gets warm, like something is warped, maybe.

 

OK....Tried the carb cleaner on the boot, no change. Changed out the coil from my 2001 Sportsman, no change. Have new jets and needles coming Friday. Running out of options/ideas.....HELP!!!

 

More info......Hooked the timing light back up to watch what happens during miss fireing. The timing light shows the timing mark jumping all over from early fire all the way to TDC. Don't know if this is a symptum of the problem or the cause of the problem. Could this be the pulse coil or CDI acting up????

 

Does the timing light flicker on and off in sync with the missfire,if light blinks off and on, then DEFINATELY elec problem! This is an old way to check shorting out! Since its running, a resistance check may not be accurate on pulser,Plus on cdi almost all manuals say REPLACE with KNOWN good cdi. Have a friend with a 99? Pull his cdi and check,at least its a process of elimination. Good luck.