Johnson Service Bulletin SB-745 - Automatic Choke Service and Adjustment

November 11, 1959

Bulletin SB-NO.745

Subject: 40 and 75 H.P. AUTOMATIC CHOKE - SERVICE AND ADJUSTMENT.

The automatic chokes on the 40 and 75 H.P. electric starting engines are preset at the factory for best overall performance. If performance of the auto- matic choke is not satisfactory, the following procedure for locating the difficulty must be used. 1. Check choke valve shaft for binding: Binding of the choke valve shaft can be corrected by lubricating the bushing with light machine oil or by straightening the shaft if it has been accidentally bent. If the binding cannot be corrected by oiling or straightening, it will be necessary to replace the shaft (always use new screws when reinstalling either throttle or choke butterfly valves). Make certain that these screws are properly staked to prevent loosening. Before loosening any screws and/or removing the choke assembly, read item #2 below. 2. Check choke assembly. With a scriber, mark the position of the choke assembly with respect to the water chamber - (illustration #1). Remove the three choke assembly attaching screws and remove the choke assembly from the water chamber. Inspect the choke assembly for loose, distorted or broken bimetal thermal element. If any of these conditions exist the choke assembly must be replaced. Check the choke assembly shaft for movement by turning the shaft clockwise as viewed from the bimetal side (illustration #2). The shaft should rotate through an angle of approximately 45° before reaching the internal stop. When the shaft is released it should return to the original position without binding. If it does not return without binding, the choke assembly must be replaced. When reinstalling the choke assembly, make sure that the hook on the bimetal element is properly positioned on the chokeshaft lever (bimetal must contact left hand side of arm (illustration #4). If items #1 and #2 do not locate the source of the trouble, proceed with item #3.

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3. Engine Warm-up. This requires running the engine for a minimum of ten minutes at not less than one-half full power to stabilize engine temperature. Note- To obtain correct operation of the automatic choke, the engine temperature control thermostat must be functioning properly. If the thermostat is blocked open, the engine will run cool and the choke will be too rich on the follow-through after the engine starts. The automatic choke water jacket temperature, after warm-up, is uncomfortable to the touch. After engine warm up proceed with the following steps. 4. Place the manual choke lever in "OFF" position: 5. Adjust carburetor needle valves: Operate engine at full power and adjust the high speed needle valves. Return to idle and adjust the low speed needle valves. Recheck idle adjust- ments after engine has idled 3 or 4 minutes and correct if necessary. Do not proceed to the next step until the engine is operating smoothly. 6. Place the manual control choke lever in the "Automatic" position: This should result in no change in the operation of the engine through the speed range. If the operat- ion of the engine does change do not attempt any adjustments of the automatic choke at this point, but proceed to the next step. 7. Check position of choke shaft arm: With a warm engine idling at approximately 600 R.P.M., push the choke shaft arm on the manual override end of the choke shaft to the full open position (illustration #3). Release the choke shaft arm and measure the amount of motion with a 7/32" diameter pin attached to the arm. This movement can be less than, but must not exceed 1/8 inch. If the motion exceeds 1/8" over choking will take place, and the following should be checked.

a. Check vacuum line between choke assembly and manifold for leaks and/or obstructions. b. If no trouble is located under (a) proceed with step #8.

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8. Adjust Choke assembly: If, after the preceding checks and adjustments have been made, the choke is too rich or too lean, loosen the three screws which attach the choke assembly to the water chamber and rotate in the direction indicated on the choke body. If this is not sufficient adjustment, an additional amount of adjustment, may be obtained by loosening the three screws which hold the water chamber to the carburetor (illustration #2) and rotating it in the direction required as indicated by the arrow on the choke body. Before making any of these latter adjustments, mark original location of choke assembly and water chamber with a scriber for reference purposes. When reinstalling choke assembly make sure that the hooked end of the bimetal thermal element is properly positioned on the choke shaft arm (bimetal must contact the left hand side of arm illustration #4). Be sure to tighten all screws after making final adjustments. If the motion described in step #7 can't be brought into the correct position thru the above adjustments, the choke assembly must be replaced. Never attempt to adjust the choke without: a. Adjusting the carburetor needle valves properly. b. Making sure that engine temperature control is functioning properly. c. Making sure that engine temperature is normal. NOTE: When replacing the 40 H.P. shroud it is possible to knock the manual choke lever in the "ON" position. Check after shroud is in place.

JOHNSON MOTORS

Jack Penland Service Department

JP:eb NOTE: POST ON RD AND V4 INDEX.