1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 106Get this manual
IDLE SPEED ADJUSTMENT
Adjust engine idle RPM as outlined in Section 2C "TimingSynchronizingAdjusting"MAIN (HIGH SPEED) JET ADJUSTMENT
The carburetor is equipped with fixed high speed jet and normally no adjustment is requiredHowever, extreme changes in weather (temperature and humidity) andor elevation may result in too lean or rich fuel mixture at wide-open-throttle, which may require change in the high speed jetA smaller size main jet will lean the fuel mixture, and larger size jet will enrichen the fuel mixture
Normally, for every 1000 feet (304.8m) of altitude increase, main jet size is reduced by 2However, this does not always hold true for all situations; different boat, motor, load and ambient air temperature conditions may require different jets I.E going jet size richer than recommended for particular altitude may improve accelerationIt may also be noted that no noticeable improvement in engine performance was achieved by changing the standard main jet at altitudes up to 5000 feet (1524m)Above 5000 feet, however, changing jets did improve performanceSeveral additional changes to improve engine performance at higher altitudes are as follows: 1Idle timing should be advanced 2-3 degrees at 5000 feet (1524m) and above2Prop engine at the top of the recommended RPM scale +150 RPM3Reset idle mixture screws for best idleWith standard idle mixture setting (sea level) considered to be at the 12 o'clock position, adjust the mixture screw leaner for all models as follows: 5000 ft(1524m) o'clock 7500 ft(2286m) o'clock 10000 ft(3048m) o'clock