How the power circuit works
As the engine gets up to higher speeds, more fuel is needed, which is provided by the power jet. At idle the vacuum is at it’s highest which pulls the power piston up allowing the power jet, or power valve stem to move up, cutting off fuel through the power jet. As rpm increases and vacuum to the power piston drops, the stem drops down and pushes the power jet stem down, opening up the flow of fuel. This allows extra fuel to flow into the main jet circuit giving that extra kick of fuel.
This particular carburetor does not use a gasket on the bottom of the power valve.
Lack of power at high speeds
- Power piston binding – it should move up and down freely. Remove the piston by flicking the stem up and down to break the staking. Use something like silicon spray lubricant to clean out the piston well.
- Power piston spring missing, or bad – replace the piston spring.
- Power piston stem bent – the stem needs to be straight, or it will miss the power valve stem.
- Power valve (jet) is dirty – use carburetor cleaner to clean until the stem moves up and down freely. Blow through all of the holes. The power valve is all brass and there is no rubber to replace.
- Fuel pump pressure is low, or the float adjustment isn’t correct.
- Blow through the bottom of the power jet while moving the stem in and out. All the way out should cut off the flow of air.
At idle and low speeds, the power piston is held in by vacuum. as the vehicle speeds up, vacuum drops, which allows the spring loaded piston to move down into the float bowl pushing the power valve open and allowing more fuel to flow. The only thing that moves the power valve piston is vacuum, so the piston needs to be very free. You can try using carburetor spray cleaner, or as we do Silicon spray lubricant. Spray the piston area as you move the piston up and down, or twist it one way, then the other. Using your finger push down on the spring end, then remove your finger so that the rod snaps up. Doing this repeatedly will make the assembly pop out of the top. When reinstalling the piston, lightly stake the side so that it stays in the cylinder.