The idle construction used in Marvel Schebler Tractor and Industrial Carburetors is the most positive and satisfactory of idle systems, because it is working under very high suction and the mixture flows through the small passages at very high velocities. It is necessary to bear in mind, however, that there are times when these small holes may become plugged with particles of dirt or lint, but very seldom. If idle trouble is experienced, first check the manifold to cylinder head gasket and the carburetor to manifold gasket for air leaks. At slow idle, and engine requires only approximately 20 to 25lbs of air per hour, and a slight leak will result in a very erratic or rough idling engine.

Other causes for a rough idling engine are:

• uneven compression, caused by sticky or leaking valves
• leaking valve seats
• tappets with improper clearances
• leakage past pistons and rings
• cylinder head gasket leaking
• weak spark, or spark plug points not spaced correctly
• ignition cable covering cracked and thus ground spark, and cable not assembled properly in the distributor cap which caues corrosion and weak spark.

The spark timing of the engine is most important, and should also be checked very carefully and set exactly on the mark as called for in factory standard specifications. In fast, all of the above items must be checked very carefully to factory standards, and not just given a casual inspection.