What's that noise coming from the clutch?
So, why does my engine make that noise?
<disclaimer> There is probably nothing wrong with your clutch. There is seldom any real trouble from clutches on the EX250. They all make noise, even when brand new. 'Normal' is somewhere between barely noticeable and totally annoying. It's a relatively expensive fix, so this is not something everyone needs to run out and do... until it starts to sound like someone is pounding on the motor with a sledgehammer. </disclaimer>
The Ninja 250 engine is noisy by nature. Some of the noise is from the valve train, some from the cam chain, and yet more from (in certain cases) the clutch basket.
The clutch basket noise is the one that tends to get the most attention, since it can be quite loud and noticeable at idle. It's a low knocking sound and can sometimes be mistaken as a rod bearing knock.
So what's the deal with the clutch basket? For reasons yet to be explained, the Kawasaki clutch basket design has hard rubber/plastic bumpers inside the clutch basket assembly, which can disintegrate over time. This allows excessive slop in a system designed to smooth the power delivery from crank to transmission. This slop translates into ugly noise from the motor.
To determine if the noise you're hearing is from the clutch basket, start your bike, hold the brakes tight, put it in gear, and with the brakes still applied, let out the clutch until the motor starts to slow (known as 'slipping the clutch'). If your clutch basket has some slop, the drag will pull the bumpers solidly to one side and the noise will temporarily go away.
This issue isn't really a problem unless it gets excessive.
Let's have a look
This is a picture of a clutch basket. On the left you can see a red arrow pointing to a pin, which corresponds to the same pin on the right. The plate connected to the pin(s) on the right is free-floating and connected via the pins to the aluminum clutch plate cage. The two gears are connected to each other and are solid. The large gear is the primary drive gear, which is driven by the gear on the end of the crank opposite the flywheel. The smaller gear drives the oil and water pumps.
This clutch basket has been dismantled and destroyed in order to see the inner workings. The cushion bumpers are in the 6 holes on the right, with the pins extending through the middle of each.
The red arrows show space between the bumpers and the hole due to the misshapen bumper material. This is where the slop takes place. The green arrows show two holes where the bumper has disintegrated and is completely gone!
Closeup of the misshapen bumper.
So, where did the material of the missing bumpers go? See the little gap? Mike Norman from G-Force pointed out that he was finding tiny black pieces of stuff in the oil screen and eventually traced it to this gap. The gap shows up after the bumper allows sufficient movement to allow the hole in the clutch plate cage to move over the bumper hole in the back plate. Once there's movement, and the rubber bumpers start to come apart, they can disintegrate into tiny little pieces that fall out of the gap.
Why does this happen to some clutch baskets and not others? Duke has two motors which are about the same age (1999 and 2001). One is perfectly quiet - no noise. The other clutch basket is clearly shot. Is the cause from certain oil? Overheating? Sitting unused? Gas in the oil breaking down the material? Bad material from the factory? Bad design? The verdict is still out...