The power valve was designed to give bottom end power to a high state of tune 2stroke,, If the exhaust port is cut high . it gives big power increase ,as in the TZ& YZ race bikes of the 1970s, but you have to slip the clutch on take off ,as there is little bottom end power, So the valve was introdused to road bikes to give drivability in towns etc, as the valve stayes closed , when power is needed, the valve opens the exhaust port hieght- and big power is released,,,
Sunday ride-out was to the air museum . i was on the X7 . as DT250 split the front sprocket in half,,,on way to Indian rest. on Sat eve, to much power,,,
Spitfire-