Most commonly this is caused by a vacuum leak, or the carb isn’t getting enough fuel.

• For the air leak, spray carburetor cleaner around the base of the carb, the intake manifold, and the throttle shaft. If you hear a change in RPM, you’ve found a leak.
• Marvel Schebler carburetors use a gravity fed fuel system and problems are popping up due to today’s fuel. The needle sticks on the float valve and there isn’t enough pressure from the gas to open it.
  • If you have a needle that allows for a needle-float clip, make sure that is connected. When the float drops, the clip pulls the needle out of the seat.
  • If you don’t have a clip, clean the viton needle tip with mineral spirits until you don’t see anymore black residue come off.
  • In worst case scenarios, customers have had to install a mechanical fuel pump and inline pressure regulator.
• Be sure the power adjusting needle isn’t turned all the way in.
• Any filter between the tank and the carburetor should be changed
• Make sure the venturi is right side up. The rounded lip would be up.