Carburetor runs fine except at wide open throttle.
Assuming the engine compression is OK, the plugs are good and other ignition parts are good I suspect a lean condition.
- Test the fuel pump pressure – around 5 lbs, but check you motors manual to be sure.
- Look at the float level. It should be level when you have the top turned upside down.
- Check the float drop. Around 1″ depending on the application.
- Move the float up and down to make sure it isn’t catching somewhere. It doesn’t take much.
- Check the power piston. It should move up and down freely.
- Does the power valve pin move up and down freely? Also make sure it lines up with the power piston plunger.
- Look under the power valve for any foreign material.
- Check the power valve itself for foreign material.
- Do you have the correct carburetor on the engine. There are some differences between the Mercarb carburetors that could affect power.