If the mixture is too lean there will be a "spitting back", or a decided hesitation when the Throttle is quickly opened, and in extreme cases, the spitting back will occur at high speed. If there is no spitting back when the Throttle is suddenly opened but if spitting back occurs when the car has reached a high rate of speed then it is a good indication that the supply of fuel to the Carburetor is insufficient to meet the demands of the engine. This may be due to a low level of fuel in the tank, if gravity feed is used; to dirt or water in the Fuel System; to a too low position of the Vacuum Tank if this system of fuel supply is used; or to insufficient air pressure on the fuel if a pressure system is used. In the former case (Vacuum) where spitting back occurs when the Throttle is suddenly opened, a larger Main Jet will undoubtably cure the trouble.
If the Choke Tube is too large there will be irregular running at high speed, the exhaust "mixing them up" or sounding as though missing occurred, and at lower speeds the action of the engine will be "dead".
Other causes will be restriction of the air entering the Carburetor due to too small tubing being used, or by too sharp bends in the tubing. The cold air entrance to the hot air stove may be restricted, or the hot air outlet to the tubing from the stove may be too small.
Loose Connecting Rod Bearings - A rather sharp metallic sound
Loose Main Bearings - A dull, heavy knock with accompanying vibration
Loose Pistons - (Piston Slap) A rattling sound, most obvious when running on light load
Ignition Too Far Advanced - A very sharp knock, but without much volume of sound, cured by retarding the ignition
Carbon - Similar to Spark Knock, but cannot be stopped by retarding the ignition, particularly under hard pulling conditions
Labor Knock - Caused by motor running hot or by lean mixture.