posted 05-10-2008 03:11 PM ET (US)
On a two-cycle motor the idle speed is usually adjusted by a combination of factors.Throttle plate
Generally the throttle position will be at the stop, or minimum throttle. This results in a closed throttle plate. Most engines have some sort of idle air bypass, often just a hole in the throttle plate.
Ignition Timing
In order to slow most two-cycle engines to a engine speeds of 500 to 1,000-RPM, the ignition timing has to be retarded. In the classic outboard motor the ignition timing is generally accomplished by mechanically slewing the timer base coil assembly with respect to the crankshaft rotation. Typically there are baseline references for the ignition timing. You will need a timing light and a tachometer to make these adjustments. You should verify the ignition timing is at the specified advance or retard.
Carburetor idle jet
The fuel-air mixture for the carburetor is generally controlled by a series of fuel orifices (called "jets" in outboard motor parlance). In some carburetors these orifices are of a fixed size, while in others there are adjustable orifices which can affect the fuel-air mixture ratio at idle. If you have an adjustable orifice there is generally a tuning procedure given. A classic procedure is to adjust the orifice to produce a noticeably lean and rough idle, then to increase the orifice until a noticeably rich and rough idle occurs, and then to split the difference.
More modern motors tend to have several sets of jets or orifices which are utilized at different throttle settings. These orifices are often not adjustable, although in some cases the size of the orifice can be changed to produce better results, particularly when the engine is not being run at or near sea level.
It is also possible for these orifices to become clogged.
Fuel recirculation
Two-stroke engines also use a system of fuel recirculation in which puddled fuel which has collected in the air inlet path is routed back to another cylinder to be reintroduced into the fuel-air stream. If these recirculation hoses become clogged, or the check valves associated with them fail, the idle speed performance of the motor can suffer.
Spark cut-out
On some motors there are ignition cut-out switches associated with the transition from a forward or reverse gear into the neutral gear. This neutral shift switches are intended to momentarily spoil the ignition in order to ease the shifting, however, I think they are rare on a 90-HP motor.
Throttle linkage
The throttle linkage is often rather complex. Initial movement of the throttle from the idle setting often only moves the ignition timing link, and the carburetor throttle linkage is not picked up until a certain RPM is reached. The details of the throttle linkage adjustment are often complex. You need to consult the manual for your engine to make any adjustments.