- Check for vacuum leaks, intake manifold to cylinder head, carburetor mounting gasket.
- Uneven compression. Do a compression test on the cylinders.
- Leaking valve seats.
- Valve tappets out of adjustment.
- Weak spark, bad wires, plugs, points, distributor cap.
- Bent or scored idle mixture screw.
- Idle mixture passage partially plugged.
Lack of Power
- Carburetor is wrong for the application. Carburetors are designed to handle engines with certain HP, stroke and displacement. Marvel Schebler carburetors have often been swapped out thinking that since they bolt down they are OK for the engine. NOT true for reasons already stated.
- Equipment tends to sit around for long periods of time. Unless you are using non ethanol gas, the ethanol has been absorbing water and corroding the inside of the carburetor.
- Dirt in the carburetor can cause all kinds of problems, clogging passages.
- Float level off – Adjust the float to specifications. Be sure to replace the bowl gasket. Gaskets are crushed. Re-used too many times will cause a leak between top and bowl.
- Venturi installed upside down will cause air/fuel disruption.