YZ 250 2 stroke bottom end knock?

[scottiedawg66]

Hello. I have a yz250 two stroke, so does a buddy of mine. They both make a knocking sound when they are at very low RPM. lower than idle, like when getting through a very technical section. His bike has done it for almost 3 years, and mine has been about a year with no problem. Does anyone else experience this, am I about to split my cases?

 

[FNG]

When is the last time either of you did a top-end rebuild?

 

[scottiedawg66]

thats a good question. my bike still has good compression so I have never done one...

sounds like you are thinking piston slap?

 

[Patman]

When was the last time the crank / rod/ etc... were inspected?

 

[scottiedawg66]

bought her used. I have put about 18 hours on it and since the compression is still good so I have not even done a top end.

 

[mideastrider]

Is that real slow and pulling. Could be a spark knock your hearing. Just a thought.

 

[dirt bike dave]

IMO, some bikes make this sound under that situation because a small pocket of unburned fuel gets ignited in the header pipe of the expansion chamber.

Pretty hard to prove this theory, though. But if two similar bikes have done the same thing for 1 - 3 years with no ill effects, it is unlikely both are about to grenade because of it.

 

[Chili]

I've merged your threads, please do not cross post to multiple forums.

 

[scottiedawg66]

IMO, some bikes make this sound under that situation because a small pocket of unburned fuel gets ignited in the header pipe of the expansion chamber.

Pretty hard to prove this theory, though. But if two similar bikes have done the same thing for 1 - 3 years with no ill effects, it is unlikely both are about to grenade because of it.

that is why I have been riding it! i was just curious if anyone else has a yz250 that does this. it def. sounds the same on both bikes, and it sounds like the bottom end which is why I have not just done a top end for the hell of it. Usually that (among other things) is the first thing I do when I am going through a bike I got used.

 

[dirt bike dave]

My guess is the only riders who are going to notice it are those who lug around tight technical trails. That is a minority of riders.

I'm one of those guys, and have heard similar noises on my bikes before on really tight trails. Never ridden a '95 YZ, though.

If the motor is not under a heavy load, it is probably not detonation.

 

[scottiedawg66]

this is not detonation. my bike is an '05, buddies is an '02. this is in very slow situations almost stalling speeds...like low idle rpm.