These idle mixture adjustment tuning procedures apply the Mercury outboards manufactured between 1965 thru 1989—specifically 45-HP to 115-HP models. It’s highly recommended to follow the idle mixture adjustment procedures in the factory repair manual. A downloadable version of the factory manual is available from this site. Procedures are applicable to the following models:
Loosen the idle screw, located to the left of the throttle until the throttle plate is closed: the throttle-shutter positioner will not touch the taper of the idle speed screw. Loosen the jam nuts on the throttle-cable sleeves, just below the throttle cam. Move the throttle to the “Idle” position.
MERCURY OUTBOARDS
45 hp 4 cylinder 1965-1968
50 hp 3 cylinder 1986-1989
500/50 hp 4 cylinder 1965-1985
60 hp 3 cylinder 1984-1989
650/65 hp 3 cylinder 1972-1976
650/65 hp 4 cylinder 1965-1971
650, 700/70 hp 3 cylinder 1976-1973
70 hp 3 cylinder 1986-1989
75 hp 4 cylinder 1984-1986
80 hp 3 cylinder 1987-1989
800/80 hp 4 cylinder 1969-1983
850/85 hp 4 cylinder 1973-1977
90 hp 3 cylinder 1987-1989
850XS/90 hp 4 cylinder 1976
100 hp 4 cylinder 1988-1989
115 hp 4 cylinder 1988-1989
Testing and diagnosis helps determine problems and possible causes. Its important to perform extensive testing and troubleshooting prior to any major tear down or repair work. This ensures that the full extent of the problem is understood beforehand and may prevent any unnecessary repair work. Its recommended you use a factory repair manual when performing repairs on any vehicle, additional safety precautions, or technical data maybe included.
Idle wire adjustment
Completely push in fast/primer knob and fully rotate it clockwise.
Shift engine into neutral position.
Check the fast idle speed by fully rotating the primer knob to clockwise position. Refer to the service manual for your model for fast idle speed RPM specifications. On most motors fast idle speed should be between 1500 – 2000 RPM.
Start-up the Mercury outboard engine and allow it to warm up to normal operating temperatures (let it run for several minutes). Throttle the outboard motor back to idle for about 60 seconds to all the RPM to stabilize.
Completely push in fast/primer knob and fully rotate it clockwise.
Allow the engine to idle in forward gear, the turn low speed mixture screw counterclockwise until the Mercury outboard motor begins to load up from over-rich mixture.
Slowly turn the low speed mixture screw clockwise until all cylinders are firing evenly and engine speed increases.
Continue turning carburetor mixture screw clockwise until too lean a mixture is obtained and the Mercury outboard motor slows down and misfires.
Now set the low speed mixture screw halfway between lean and rich.
DO NOT adjust the carburetor leaner than necessary to attain reasonably smooth idling, this can cause internal engine damage. When in doubt, set mixture slightly richer rather than too lean.
Reinstall the access plug into opening in carburetor air intake cover.
how do you adjust the idle mixture on a Mercury outboard motor?
Adjust the idle stop screw on the side of the carburetor near the choke cable, turning the screw with a screwdriver until the vacuum gauge is reading about 600 rpm. Turn the idle mixture screw counterclockwise 1/4-turn at a time until you hear the engine begin to stall out, and turn it clockwise 1/4-turn.
How do you adjust a 2 stroke outboard carburetor? the best practice to adjusting a two-stroke carburetor is to use minute, slow turns. there are two screws for adjusting the carburetor: a low speed set screw and a high speed set screw. the low speed screw adjusts idle, and the high speed screw adjusts the fuel mixture at higher revs.
FAQ
How do you adjust the idle on a 2 stroke Mercury outboard?
Turn the idle speed screw on the carburetor linkage to adjust the motor’s speed to between 700 and 800 rpm. Put the motor in “Forward.” Turn the mixture screw clockwise until the motor begins to misfire. Turn the screw counterclockwise one-quarter turn or until the motor no longer misfires.
How do you adjust a Mercury outboard carburetor?
The idle speed screw (arrow 1) is located on the primary linkage. The idle metering adjustment screws (arrow 2) are located on the primary metering block on most Holley carburetors. A typical starting location for these idle mixture screws is between 1 and 1 ½ turns out from fully seated.
Where is the adjustment for idle screw located?
The idle speed screw (arrow 1) is located on the primary linkage. The idle metering adjustment screws (arrow 2) are located on the primary metering block on most Holley carburetors. A typical starting location for these idle mixture screws is between 1 and 1 ½ turns out from fully seated.
