A buddy just gave me his polaris 425, which runs great, but he said the plug will foul a least once a month. Based on the look of the fouled plugs, it looks like oil foul. Any suggestions on how to fix this problem?

 

Do you get a blue puff when you start it, or does it puff smoke while you ride? a puff on startup is usually valve seals leaking by, constant smoke is worn rings or a score in the cylinder wall. Bad thing is, for either problem, you have to pull the motor apart to either get at the head and servive the valves, or rering or bore/piston the jug. If its running good and you dont have time or money now, a sparkplug once a month is not a bad trade off for some fun riding for now. But it will need to be fixed eventually.
Just out of curiosity, he's not mixing 2 stroke oil in the gas is he? the 425 came out when Polaris was heavy into 2 strokes in all their quads, maybe he forgot......lol.

 

Doesn't smoke at all. Just that it's a pain to always change out the plug being it's under the gas tank.

 

That's unusual, to foul a plug in a month or less, you would think there would be a little smoke out the exhaust. Does the quad seem to use more gas than usual? is it a thick, shiny looking deposite on the plug, or a black, sooty looking deposite? A weak spark would cause a plug to foul over time, but you say it runs fine, so i doubt that is a problem. What grade gas is run in the quad? premium or regular 87 octane?

 

The 3 plugs that I looked at were from 3 years ago. It was like a flat black deposit on the plugs. He was using a regular octane.

 

Sounds more like running rich, since it seems to run fine, I'm pretty sure the jetting is ok, maybe the float level is too high and extra gas is getting into the combustion chamber,causing plug fouling, or the air filter is plugged up or if its an aftermarket foam one, maybe its over oiled, causing it to run rich? hard to say without actually seeing the quad and plugs, but thats about all I can think of right now.

 

Just replace the carb needle and seat and make sure the float arm is parallel with the carb body! The 425 Magnum was bad on needle and seat wear and fouling plugs. Probably wouldn't hurt to replace the float arm and pin if feels wobbly and worn when you tear into the carb. OPT

 

Also the jet needle and needle jet is worn out. This happens all the time. If you ride a lot these have to be replaced about once a year on the 425. If not they will run very rich.