April 21, 1961
Bulletin No. SB-838
Rough idling
SUBJECT: Poor or rough idling - trouble shooting;
40 & 75 h.p. engines
Judging by field complaints and reports on the subject engines, customers often
experience poor or rough idling which, because of the frequency of occurrence, is
accepted as the best the engine can do. We do, however, constantly check our motors
after assembly and find that they idle quite satisfactorily. Since these engines are
capable of idling better than the reports indicate, it is important that the following
trouble spots be given a thorough and systematic check.
In trouble shooting poor idling, the first thing the mechanic can do is generally
check the exterior of the motor and run-of-the-mill adjustments to be sure they are
correct. They include:
1. Carburetor adjustment (all h.p. models)
2. Spark advance (all h.p. models)
3. Synchronization of carburetor and magneto or distributor (all h.p. models)
4. Fresh gasoline with proper oil ratio thoroughly mixed (all h.p. models)
5. No pinched or kinked fuel lines (all h.p. models)
6. Fuel pump working (all h.p. models except those with gravity fuel systems)
7. Fouled spark plugs (all h.p. models)
8. No binding of internal parts (all h.p. models)
9. Breaker point adjustment and proper functioning of ignition system in its entirety.
These points are often missed; they are however important if the engine is to run as
intended at any speed.
In the event all of the foregoing points are in good condition and adjustment, the
following internal points should be checked because these problems have also often
caused poor idling.
1. OIL DRAIN SYSTEM: The oil drains on the larger engines are very critical and must
be maintained in good condition. Of particular importance is keeping the drain
screens clean of accumulated carbon and dirt, (these screens are in the reed plate
base or in the lower part of crankcase). A plugged up drain system can usually be
recognized by the motor 'loading up' after several minutes of idling.
2. CARBURETOR FLOAT VALVE: Poor idling can often be attributed to a worn or sticking
float valve. The float valve and seat assembly should be inspected at this time;
if found worn, replace and adjust float to proper level.
a. If the float valve is found sticking, clean the carburetor in its entirety
to remove the gasoline gum deposits.
3. DIRT IN CARBURETOR: Many cases of poor idling have been traced to stray bits of
dirt clogging the small passages and orifices of the carburetor. Since such
clogging can affect engine performance at any or all speeds, depending upon which
passages are plugged, clean the carburetor in its entirety and be certain all
passages are open. Exercise caution when removing and replacing needle valves so
as not to force packing material into passages.
4. BOTTOM CARBON SEAL - CRANKSHAFT BEARING: We have had some cases of engines idling
poorly because the bottom carbon seal and O-ring assembly was excessively tight on
the crankshaft and thus prevented its seating against the bottom journal bearing.
While the carbon seal and O-ring assembly is intended to fit the crankshaft snugly,
(over)