I've just bought a really clean 95 300 2x4 and i am tryind to get a few bugs out of it. I've cleaned the carb, replaced the air filter with a stock one and replaced the belt too. The carb has a 165 main jet and a 40 pilot. The bike seems to knock just occasionally when I decellerate sometimes.It doesn't do it all the time just now and then and usually when the engine is down in the lower rpm's. It sounds like a snap or a knock. Am I hearing preignition or detonation?If so how do I cure it? I don't want to burn a hole in the piston.The plug looks fine. The bike has excellent compression and has been taken care of before I bought it.I don.t have to run a high octane fuel do I since the comp. is only around 6 to 1?

 

Don't know for sure but I want to think that what you are hearing could be piston slap. My engine has always done it. Its always stayed the same no matter what I've done to the engine. I guess its normal. These 300 engines have some of the sloppiest clearances of all the polaris motors.

It could also by a rod or pin or a bearing for that matter. I'd have to hear it to tell you for sure.

 

Well I drained the gas and re-filled it with 92 octane. The sound is gone!! I guess it was detonation. I called the guy I bought it from and he said he always ran the "supreme gas" in it until the prices started to rise this summer. He switched to "regular" and also noticed the sound.

 

you might think about going down one heat range on plug and also ck exhaust for being plugged which will cause overheating and loss of power 400 are very common to have this happen especaily with aftermarket tires. mud is thrown into the exhaust outlet then is baked to a hard clay and will restrict the outlet to damn near nothing. the machine will be low on power and will moan bad on accel.