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Author Topic:   Mercury 90-HP: Idle Speed Adjustment
roll02 posted 05-08-2008 09:09 PM ET (US)   Profile for roll02   Send Email to roll02  
I'm having a problem with my 90 hp mercury. When I put her in neutral she stalls out. There is an idle screw that is easy to get at. When raising the idle is the engine on? Which way do I turn the screw?
jimh posted 05-08-2008 09:40 PM ET (US)     Profile for jimh  Send Email to jimh     
What model do you have?
Casco Bay Outrage posted 05-08-2008 10:21 PM ET (US)     Profile for Casco Bay Outrage  Send Email to Casco Bay Outrage     
i think the question you should be asking is: What would cause my 90 hp Mercury model xx to stall when shifted into neutral.
roll02 posted 05-09-2008 09:40 PM ET (US)     Profile for roll02  Send Email to roll02     
your right, i just figured since she has no problem when she is reved, or in gear it was the idle.

Its a 1997 2 stroke, that came standered with a 170 montauk.

Henry posted 05-10-2008 01:08 AM ET (US)     Profile for Henry  Send Email to Henry     
I have a 2005 Mercury 90 hp 3 cylinder two stroke Saltwater version motor on my Montauk 17 and I would also like to hear some discussion regarding the idle speed adjustment on these motors.

If the motor is adjusted to the proper idle speed, about 900-950 rpm, and it stalls when shifted into gear, then there maybe other problems, eg., low compression, obstruction of fuel flow, faulty spark plugs, carburetors, air leaks.

roll02 posted 05-10-2008 09:51 AM ET (US)     Profile for roll02  Send Email to roll02     
Henry, thenks for the info I thought this was simple stupid easy. It now seems I got some work head of me.
jimh posted 05-10-2008 03:11 PM ET (US)     Profile for jimh  Send Email to jimh     
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.

L H G posted 05-10-2008 03:29 PM ET (US)     Profile for L H G    
Suggest you contact "Clark Roberts" here. He is the guy who knows all the tricks on how to make those Merc 90 2-strokes run flawlessly.

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