The 1988 XR600R uses a new decompressor which eases the required starting force and also reduces starting kickback. This system is used on all 1988 4-stroke singles of 250 cc and larger engines. It is more compact and lighter than previous systems and is maintenance Free. On the XR600R, it reduces the required starting force to that of a 250 cc engine.
The system uses two separate decompressor cams: one is for reducing the necessary kickover force (decompressor cam) and the other reduces starting kickback (reverse decompressor cam) . Both of these decompressor cams slip onto the drive end of the camshaft. The decompressor cam is loosely keyed onto the camshaft which allows it to shift positions as the engine speed increases (after startup). The reverse decompressor cam rotates on a machined surface on the camshaft but it can only rotate in ono direction because of a one-way roller bearing.
At TDC (on compression) the right exhaust rocker arm is contacting the normal cam lobe’s heal (concentric circle) so that valve is closed, as would be normal. But on this model, the decampressor cam opens the exhaust valve a slight amount to reduce kickstarting force. After the engine starts, centrifugal farce moves the position of the decompressor cam and it be comes flush with the normal cam lobe’s heel.
Reverse Decnmpressor Cam
The reverse decompressor cam is stationary during normal engine operation. It doesn’t turn when the engine is being started or when it is running. Its one-way roller hearing allows it to free-wheel whenever the engine is running in the normal direction.
But if the engine kickbacks during startup, the reverse direction locks-up the one-way roller bearing which moves the lobe of the reverse decompressor cam into position [it opens the exhaust valve). The reverse decompressor cam automatically re-turns to its normal position when the engine stops turning from the kickback.