Where to find any info about power valve operation specifically on the Suzuki RG 250 of the late eighties? I would appreciate any information at all on the different types of power valves used on two stroke exhaust ports (old or new), up to the types being used at the moment. As far as I know, the RG250 used the same mechanism as the RG125. This required a resonant chamber cast into the cylinder head directly above each exhaust tract. A simple plug valve was fitted between a hole in the roof of the exhaust tract and a hole in the floor of the resonant chamber. At low rpm the plug valve was held in the open position, effectively lengthening the exhuast system and lowering its resonant frequency. At high rpm (typically 9000 rpm) the plug valve was rotated 90 degree into the closed position.The plug valve was indeed actuated by a stepper motor and cable linkage. In the UK the RG125 was sold with no plug valve and no stepper motor to 'restrict' them to 12bhp. To remove the restriction, most people cut a length of copper household plumbing tube and hammered it into the hole where the plug valve should have been. Early '80s Yamahas had a very slightly more sophisticated system. The power-valve DID change the exhaust port timing by adjusting the effective height of the top of the exhaust port. This was achieved by having a gate valve shaped in profile like the 'sector' of a circle. This could rotate through about 20 degrees and alter the height of the port. I say only slightly more sophisticated because until the TZR series of road bikes the movement of the valve was still either 'on' or 'off' at about 9000 rpm.

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