Hissing / Airleak Noise

I just finished a top end rebuild on my 04 - 250, Everything seems to be running great, no problems with power, idling, anything. However I seem to be getting some kind of hissing or airleak noise. The noise mostly occurs on the deceleration of the motor, it may occur also on acceleration however I cannot hear it over the exhaust. To me it sounds like the rad cap was leaking, but there was nothing there and I dont seem to be losing fluid.

I alos put a new EBC clutch in the bike, the clutch needs to be adjusted as it is a bit grabby, could this be making the noise? I also installed new Boysen reeds are they prone to any problems?

Any ideas of what to check and how to check would be appreciated.

Anyone have any ideas?? I would really appreciate some input.

I wouldnt suspect the boyeson reeds... and I doubt you'd hear an air leak? I could be wrong however. Ive delt with some pretty bad air leaks and never noticed any difference in sound. If you want to know if you have an air leak pull the plug and take a look to see if your lean.

Ive never experienced the type of noise your experiencing from a clutch but Ive heard some strang ones so maybe that is your issue. How many hours are on the bike since you rebuilt the top end and clutch?

This was the first ride since the re-build. I have had it running in my garage and some blasts up and down the road.

coolant on the pipe?

coolant on the pipe?

Good suggestion, never thought about that!

When my 125 made hissing noises it was because of a bad water pump seal the bike ran low on coolant causing a hissing noise out the weep hole which I thought was water evaporating in my pipe. check your coolant level

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