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Boatinfo - Mercury Service Manual for 70-75-80-90-100-115 hp
After service or replacement of carburetor, turn low
speed mixture screw adjustment in (CLOCKWISE)
until it seats LIGHTLY – then back off (each
carburetor) the number of turns per model as
70 H.P. --- 1-1/4
75 H.P. --- 1-1/2
80 H.P. --- 1-1/2
90 H.P. --- 1-1/2
100 H.P. --- 1-3/4
115 H.P. --- 1-1/2
[some posts go 1/4 turn richer for startup]
This adjustment will permit engine startup.
LOW SPEED MIXTURE ADJUSTMENT
NOTE: Only the top two carburetors on four cylinder
models have adjustable idle mixture screws.
1. Start engine and allow to warm-up (run for several
minutes). Throttle back engine to idle for about
one minute to allow RPMs to stabilize.
2. With engine running at idle speed -- in water -- IN
FORWARD GEAR (prop on) -- turn low speed
mixture screw IN (clockwise) until engine starts to
“bog” down and misfire. Back out 1/4 turn or more.
3. Check for too lean mixture on acceleration.
[interpret as hesitation or bogging down on acceleration, and also a slow reduction in revs back to idle from WOT to neutral]
4. DO NOT adjust leaner than necessary to attain
reasonably smooth idling. When in doubt, stay on
the slightly rich side of the adjustment.