Tips from Army Motors
Low Speed Jet Carb Trouble
A QUICK WAY TO DETERMINE whether the trouble is in the low speed jet of a carburetor, the next time you have an engine that runs roughly and unsteadily at low speeds, is to interchange the wires from No. 3 and No. 4 spark plugs, start the engine and race it for just a second. The resulting backfire will blow out any foreign matter and the engine will idle smoothly (if the trouble is in the slow speed jet) This is not a permanent cure because the foreign matter will still be in the carburetor but it is a short way to find the trouble.
Carter WO 539S
Text from Army Motors, October 15, 1940.
Images from TM 9-1826A, February 11, 1944.