98 polaris sportsman 500 (ethanol, gas, fuel, engine)

[Dustyroad70]

I have the above. I started it the other day after a period of a few months inactivity. It started fine, ran smooth, but, when I attempt to accelerate, it kind of coughs and backfires, no power, no acceleration, when I let off the gas it goes back to idling fine. I am not the original owner, but, I have had it for probably 8 years and it used to have powerful acceleration. I have not abused it at all. It was low on gas, I put fresh gas in it to no avail. Any good ideas?

[sunsprit]

If it ran fine when you last ran it, then it's likely a fuel flow/delivery problem. The fuel pump could be slow to deliver fuel, the carb float needle/seat could be stuck to a small flow, could be condensation in the float bowl, etc. Drain the carb ... there's a screw on the lower left side of the float bowl, and see what comes out. You may yet need to remove the float bowl and clean out debris/crusty stuff and free up the float to allow fuel flow, or clean out the mainjet.

Be aware that there's a problem with the original camshaft exhaust lobe in this machine, which wears out and typically takes out the exhaust rocker arm, too. It will give you virtually the identical running symptoms you have now with this engine. The only way to know for sure is to remove the valve cover and visually inspect the camshaft and watch the rocker arm movement when pulling the engine over with the pull starter (spark plug removed to make it easier). If the exhaust rocker arms don't appear to travel as much as the intake rocker arms, there's an exhaust cam lobe failure. Not difficult to replace, typically $400 in parts; lots of videos on YouTube showing how it's done.

[Toyman at Jewel Lake]

I'd agree with a carb problem, sounds like a plugged pilot jet. Take it apart, clean and run a small, fine wire through the various jets and make sure they are not plugged.

Today's gas contaminated with ethanol is hell on carbs, especially if it sits in a vehicle for any length of time. At the least, once you get in running right, shut the fuel valve and run all the fuel out of the carb before letting it sit. Better is to add a fuel stabilizer of some type.

You might try draining all the old gas, and replacing it with fresh stuff, maybe with some carb cleaner. I wouldn't count on it, but you might get lucky.