Johnson Service Bulletin SB-933 - Idle Adjustment for 28 and 40 HP Engines

Issued January 7, 1963, this bulletin provides Johnson's factory-recommended procedure for adjusting idle speed and carburetor mixture on 28 and 40 horsepower outboard engines. It explains the effects of lean mixtures and excessive idle speed.

Johnson Motors Service Bulletin letterhead
January 7, 1963
Bulletin No. SB-933 Factory Recommend Procedure
SUBJECT: Idle adjustment for 28 and 40 H.P. Engines.
Apparently not all dealers are familiar with the procedure for adjusting engine idle accurately on the 28 and 40 h.p. motors. Engine idle is, as you know, regulated by two adjustments; 1) carburetor mixture control, and 2) idle stop screw adjustment. Unless a routine procedure is followed when making these two adjustments, chances are considerable time will be consumed to obtain the desired engine idle, or, after continued efforts the engine is unjustly accused of "just not being able to idle".
This bulletin therefore described how to adjust engine idle properly and why proper idle adjustment is so essential to the motor.
PROCEDURE FOR PROPERLY ADJUSTING ENGINE IDLE.
1. Allow the engine to reach normal operating temperature.
2. Adjust the idle stop screw IN so that engine speed is approximately 800 RPM (using tachometer). Adjust the carburetor mixture control knob (slow speed) - mid point between the leanest and richest settings at which the engine will still run.
3. Re-adjust the idle stop screw so that the engine is now turning approx- imately 600-700 RPM.
Always allow enough time for the engine to respond to a richer or leaner mixture control adjustment. Generally 15 to 20 seconds is ample to notice any affect of mixture adjustment change on the engine.
WHY THE IMPORTANCE OF IDLE ADJUSTMENT.
A "too lean" mixture adjustment can affect performance by prematurely activating the engine cut-out switch. This causes intermittent firing or no firing at all on the bottom cylinder, will cause the engine to run rough and eventually lead to spark plug fouling on this cylinder.
Another adverse affect of too lean a mixture is inadequate lubrication, since the engine is not being fed enough fuel/oil mixture to properly lubricate its moving parts.
Some have commented that a leaner mixture seems to give a smoother idle. THIS IS INCORRECT. To a degree, leaning the mixture creates a slightly faster idle and since vibration is purely a function of RPM - the engine will idle smoother because it is running faster. Exactly the same result can be achieved with a proper mix- ture control setting by simply increasing engine speed with the idle stop screw.
SB-933 Page 2
Remember that the correct mixture setting is halfway between the "too rich" and "too lean" positions, and, that smoothness of idle depends upon engine speed which is easily adjusted with the idle stop screw. Also keep in mind that if engine idle speed is too fast, accelerated shifter clutch dog wear will become evident as a result of shifting at too high a speed.
JOHNSON MOTORS
Robert Compton Service Department
RC:eb

Technical reference

The procedure begins at approximately 800 RPM while setting the carburetor mixture, then calls for a final idle speed of approximately 600-700 RPM. Johnson advises allowing 15-20 seconds after mixture adjustments to observe the engine's response.