Johnson Service Bulletin SB-755 - Hard Starting Automatic Choke Engines

HARD STARTING AUTOMATIC CHOKE ENGINES

It is indicated, by reports from the field and our own testing, that difficulty is being encountered occasionally starting the automatic choke engines with the choke set in the automatic position. Further reports and tests indicate that the hard starting only takes place after the engine has been subjected to lay-up or occasional usage such as on weekends only.

This problem was taken up immediately with our Engineering Department and a series of tests were made to determine the cause of the problem. We have been advised by our Engineers that this problem is inherent with automatic choke mechanism in trying to arrive at a balance where you have sufficient choking effort for starting, and yet be able to prevent flooding once the engine starts. This same problem exists with the use of automatic chokes on automobiles, but is minimized by daily usage and an accelerator pump built into the carburetor assembly.

This type of hard starting is not to be confused with other types where something may be at fault in the assembly or the parts themselves. We have found this condition occurs because the gasoline mixture left in the carburetor bowl, at the time the engine is stopped, will evaporate partially, releasing all of the "lights ends" from the fuel, and what is left is not an easy starting or good running fuel. Even if the fuel line primer bulb is squeezed before starting, and fresh fuel from the tank is introduced into the bowl, the resulting mixture is diluted with the evaporated mixture and requires more starting effort.

The lack of fresh fuel thus necessitates the use of the manual choke to provide an extra rich mixture for the initial starting after lay-up. This manual choke is very stiff, and will start the engine under conditions where it cannot be started with the automatic choke. We have found in cases where engines have been standing for long periods, the manual choke will always start the engine. After the initial starting place the choke lever in the automatic position, as it will not be necessary to choke the engine any further manually. Should the engine be subject to only occasional usage it may be necessary to choke manually for the initial start each time the engine is used.

Engineering is just continuing to work on this problem. Any future developments, that might correct or ease the difficulty, will be passed on to the dealer organization.

JP:eb

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