Power Valve Adjustment | Back End | Tricks of the Trade
Two-stroke exhaust power valves provide loads of adjustment but the trick is knowing what to do.
6mm T-bar, ground screwdriver bit to fit adjuster screw
Two-stroke exhaust power valves provide loads of adjustment but the trick is knowing what to do, which direction of rotation of the adjuster does what, how many turns does what and which non-adjustable spring does what.
The KTM two-stroke power valve has two springs. One is non-adjustable and controls the revs at which the power valve is fully open and the other is adjustable and controls the revs at which the power valve starts to open.
Step 1:Â To change the non-adjustable coloured spring, remove the two bolts on the cover.
Step 2:Â The non-adjustable coloured spring on the inside of the larger spring controls the revs at which the power valve will be fully open. The standard yellow version allows the power valve to be fully open at 7900rpm.
Step 3:Â There are alternative red and green springs which alter the revs at which the power valve is fully open.
The red spring makes the powerband much more aggressive and the green spring makes it much less aggressive.
The larger, outer spring has its preload controlled by an external brass screw. This spring controls the revs at which the power valve starts to open. In the standard position the valve will start to open at 5600rpm.
Step 4:Â To adjust the brass screw you will need to grind a square tool to fit into the head of the screw.Â It is quite difficult to find a screwdriver bit to fit into it nicely as it is an unusual size.Â That is, unless you have the KTM factory tool.
Step 5:Â Turning the brass screw one full turn outwards will allow the exhaust power valve to open 150 revs lower, creating a smoother power curve.Â Turning the adjuster screw one full turn inwards will make the power valve open 150 revs higher.Â This will create a much more aggressive powerband.
TIP:Â Always count the number of turns from all the way in before you start playing with the adjuster screw.
The power valve affects two-stroke performance greatly, so incorrect adjustments can cause the bike to run very poorly.