Working on my 98 Sportsman 500. Motor is hard to start hot or cold. Will crank for long periods then backfire out exhaust. Occasionally, it will start at which time it idles smoothly, runs like a scalded dog and no indication of loss of power, smoking or backfire. Run it for a while, shut it off and hard to start again. Seems like the plugs get wet if prolonged cranking. Seems to pull start better when hot than cranking, but hard to start either way. Carb looks clean inside, float at 1/2", all passages and jets open, 142.5 main jet, no noticable wear on needle, diaphragm ok and slide moves normally when running, choke cable set to 1/8" free play. Spark is blue and jumps 3/8" gap on plug tester. Alternator putting out 14 volts at idle and runs under full load with battery disconnected. Running out of ideas and could use some technical advise. What am I missing? Thanks!!

 

Check the valves they set @ .006 while at it inspect the cam for wear on and exhaust lobe

 

OK, now I'm getting more confused.
Checked valve timing. It is OK. Checked valve lash, Intake was .005" cold, adjusted to .006". Exhaust was at .006" already. Hard to measure without a dial indicator, cam lift appears OK, getting about 5/8" compression on valve spring when valve opens. No visible wear on cam lobes. Compression climbs to 90 PSI after 4 revolutions with starter engaged, (Decompressor open?). No change when oil added to cylinder. No access to leakdown tester, but compression seems OK.
Put it all back together. Started cranking motor with starter. Nothing. As I let off starter button, motor fired and idled like new. Ran a few minutes and turned off. Restarting with starter, nothing. Pull start started motor on first pull??? Low voltage? Battery is at 12.96V drops to 11.42V while cranking. Battery is new and full of water. Connected battery charger while starting, voltage stayed above 12V and still no start.
Connected inductive timing light while cranking, no light. Let off the starter button and see one flash. Flash is consistant when using the pull start. Yet, if I remove the plug, connect a ground to it and crank the motor with the starter, the spark is fat and blue and consistant. If I put the plug back in to simulate compression while cranking and put spark tester from plug wire to ground, spark is much thinner but more intense and will jump a 5/8" gap and is still consistant.
Sorry for the drawn out details but wanted to provide enough information for some thought provoking diagnostics. Cause I'm running out of ideas.

 

Are you shutting off the fuel when you leave it ?When I do this mine seems to drain the bowl and has to crank a lot to start.Float needle leaks a little.Try a short shot of ether when it acts up and if it fires and runs you will know the problem is the float needle.Also check the air filter and the prefilter and the intake tube on the air box.The tube can suck trogether or be plugged.
You may be getting TOO technical with this.Most the time it is simple stuff that causes the problems.

 

If you're losing spark when you crank it I wonder if there is something wrong with the starter button.If I remember correctly I think the starter button was built in to the kill switch.

 

Got some Star Tron which puts enzymes back into gas as ethanol causes some condensation in them for some reason. Runs like a bat out of hell now.

 

Appreciate the responses and the ideas, I'm running out of them. The float level is at 1/2" as specified. The float bowl was full when I drained it. When the motor is running, this ATV pulls the wheels of the ground in high range. I could shut it off and wait only 5-10 seconds before retrying it, still no start. Ether doesn't help.
When I hook an inductive timing light to the plug wire, the light will flash consistantly when using the pull start. When cranking with starter, the spark is intermittent at best except after disengaging the starter, there is one spark. After adjusting the pilot jet on the carb, pull starting is pretty reliable, just won't start when cranking.

 

I was thinking about the starter button as well. Switch is normally open, infinite resistance. When actuating the button, .3 ohms. Only two wires, one 12V supply voltage and the other wire goes to starter relay. If the starter is working, the button is working too. Tried disconnecting the rev limiter, no change.

Anybody know what primary voltage to coil should be when cranking?

Guess I have to look at the stator and pulse coil next.

 

The gas is relatively fresh, in a sealed can for maybe six weeks. I'll do some research on the Star Tron for future reference. Pretty convinced at this point that if I lose spark while cranking, nothing will ignite.

 

When you find the problem let us know