Mercarb Idle Circuit

Doesn’t Idle Well

  • Adjust the idle mixture. Use a vacuum gauge connected to any vacuum source off the intake and adjust the idle mixture screw until you get a smooth vacuum reading.
  • Adjust the idle mixture at idle speed only. Above that and the idle circuit is bypassed.
  • If adjusting the idle mixture doesn’t change the RPM, then you have a problem in the carburetor, such as the idle passage being blocked.
  • Check the passage from the idle mixture hole at the bottom of the bore all the way to to the venturi. This passage provides air to the fuel and must be clear. Blowing compressed air through the passage is the best way.
  • The venturi gasket needs to be sitting flat and they sometimes need to be trimmed.
  • Clear all of the small passages in the venturi.
  • Check for vacuum leaks. Spray carb cleaner around the mounting, intake and any other vacuum source. If the RPM smooths out, you have found the problem.

Off-Idle CircuitDo you have hesitation just off idle?
Ethanol Defense
Updated on 12/11/2020

Was this article helpful?

Related Articles