Mercury / Mariner 75 / 90 (3 cylinder) and 100 / 115 / 125 (4 cylinder) two-stroke, 65 / 80 Jet service guide

Quick answers

What two-stroke oil does a Mercury / Mariner 75 / 90 and 100 / 115 / 125 two-stroke, 65 / 80 Jet use?

TC-W2 or TC-W3 2 Cycle Outboard Oil.

What is the fuel to oil ratio for a Mercury / Mariner 75 / 90 and 100 / 115 / 125 two-stroke, 65 / 80 Jet?

Gasoline and oil 50:1 (25:1 break-in). Oil injected engines: the pump delivers 80:1 at idle and 50:1 at wide open throttle.

What gear oil does a Mercury / Mariner 75 / 90 and 100 / 115 / 125 two-stroke, 65 / 80 Jet use, and how much?

Premium Gear Lubricant. Quantity: 22.5 fl oz (655 ml) for every gear housing: 75/90 (2.31:1 or 2.33:1) and 100/115/125 (2.07:1). Fill until lubricant shows at the front vent hole, plug it, then fill until it shows at the rear vent hole.

What spark plug does a Mercury / Mariner 75 / 90 and 100 / 115 / 125 two-stroke, 65 / 80 Jet use?

65 Jet/75/90 (3 cylinder): NGK BUHW-2, surface gap; optional inductor plug NGK BUZHW-2. 80 Jet/100/115/125 (4 cylinder): NGK BP8H-N-10; optional inductor plug BPZ8H-N-10. One per cylinder. Gap: 3 cylinder: surface gap plug, no gap to set. 4 cylinder: 0.040 in (1.0 mm).

How often should a Mercury / Mariner 75 / 90 and 100 / 115 / 125 two-stroke, 65 / 80 Jet be serviced?

The factory schedule lists: Every 100 hours or once a year; Every 300 hours or 3 years. In Hawaiʻi's salt water we do a full yearly service every 200 hours or 1 year, whichever comes first, and a new engine needs its first (break-in) service.

When should the water pump impeller be replaced on a Mercury / Mariner 75 / 90 and 100 / 115 / 125 two-stroke, 65 / 80 Jet?

The factory schedule says to replace it at 300 hours. In Hawaiʻi's warm salt water we replace it every 200 hours or 1 year, whichever comes first, because a worn impeller is the most common cause of overheating.

What is the idle speed of a Mercury / Mariner 75 / 90 and 100 / 115 / 125 two-stroke, 65 / 80 Jet?

675 ± 25 RPM; idle mixture screw preset 1-1/8 ± 1/4 turn on the 75 Work/75, 1-1/4 ± 1/4 turn on the 65 Jet/90 and the 80 Jet/100/115/125.

What is the propeller nut torque on a Mercury / Mariner 75 / 90 and 100 / 115 / 125 two-stroke, 65 / 80 Jet?

55 lb-ft (74.5 Nm).

Maintenance specs

Two-stroke engine oil	TC-W2 or TC-W3 2 Cycle Outboard Oil
Fuel to oil ratio	Gasoline and oil 50:1 (25:1 break-in). Oil injected engines: the pump delivers 80:1 at idle and 50:1 at wide open throttle
Gear oil	Premium Gear Lubricant
Gear oil quantity	22.5 fl oz (655 ml) for every gear housing: 75/90 (2.31:1 or 2.33:1) and 100/115/125 (2.07:1). Fill until lubricant shows at the front vent hole, plug it, then fill until it shows at the rear vent hole
Gear oil fill/drain plug and front and rear vent plugs	Fill/drain screw (magnetic), vent screw and oil level vent screw: 62 lb-in (7 Nm) each. Replace the sealing washers if damaged; install both vent plugs before removing the lubricant tube, then the fill/drain plug
Lower unit holding pressure	10 to 12 psi (69 to 83 kPa) at the vent hole, must hold five minutes, tested before the lubricant is added; turn the drive shaft and prop shaft and move the shift shaft while pressurized
Spark plug	65 Jet/75/90 (3 cylinder): NGK BUHW-2, surface gap; optional inductor plug NGK BUZHW-2. 80 Jet/100/115/125 (4 cylinder): NGK BP8H-N-10; optional inductor plug BPZ8H-N-10. One per cylinder
Spark plug gap	3 cylinder: surface gap plug, no gap to set. 4 cylinder: 0.040 in (1.0 mm)
Spark plug torque	20 lb-ft (27 Nm)
Idle speed	675 ± 25 RPM; idle mixture screw preset 1-1/8 ± 1/4 turn on the 75 Work/75, 1-1/4 ± 1/4 turn on the 65 Jet/90 and the 80 Jet/100/115/125
Thermostat	Temperature switch closes at 190 °F ± 8° (88 °C ± 4°) and opens again at 170 °F ± 8° (77 °C ± 4°)
Anodes	Two: the trim tab on the gearcase and an anode on the bottom of the transom bracket assembly. Replace before completely eroded, never paint. Trim tab retaining screw 22 lb-ft (30 Nm); anode retaining bolt (M6 x 40) 62 lb-in (7 Nm)
Propeller nut	55 lb-ft (74.5 Nm)
Power trim fluid	Power Trim and Steering Fluid, or automotive automatic transmission fluid (ATF) if not available; level even with the bottom of the fill hole with the outboard tilted full up

Service schedule

The factory maintenance intervals, with what is replaced and what is checked at each. Rows marked Hawaiʻi are our shop's yearly service for salt water use.

1. Every 100 hours or once a year

   Replace: gear case oil, gear case plug gaskets, propeller cotter pin

   Check: spark plugs, fuel filter, thermostat, anodes

2. Every 200 hours or 1 year (our Hawai‘i yearly service) Hawai‘i

   Replace: gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

   Check: spark plugs, fuel filter, thermostat, anodes

3. Every 300 hours or 3 years

   Replace: gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

   Check: spark plugs, fuel filter, thermostat, anodes

We replace the water pump impeller at every service from 200 hours up, which can be sooner than the factory schedule, because of Hawaiʻi’s warm salt water.

Source: Mercury/Mariner 75/75 Marathon/75 SeaPro/90/100/115/125, 65/80 Jet service manual, 90-8M0127990, October 2017. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.