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Mercury / Mariner 40 four cylinder and 30 Jet two-stroke (721 cc) service guide

Oil, gear oil, spark plugs and the service schedule for the Mercury / Mariner 40 four cylinder and 30 Jet two-stroke (721 cc) (30 to 40 hp, two-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: 40 (4 cylinder) and 30 Jet two-strokes: 4 cylinders in line, 44 cu in (721 cc), 2 carburetors: serial 0C159200 and above (United States), 9570305 and above (Belgium), 0A748269 and above (Canada). Gasoline/oil ratio 50:1; The 30 Jet has a jet drive in place of the gear housing (section 6B).

Quick answers

What two-stroke oil does a Mercury / Mariner 40 four cylinder and 30 Jet two-stroke use?
Quicksilver 2-Cycle Outboard Oil; in an emergency a 2-cycle outboard oil rated BIA TC-W or TC-WII.
What is the fuel to oil ratio for a Mercury / Mariner 40 four cylinder and 30 Jet two-stroke?
50:1. Oil injected engines also run 50:1 in the fuel tank for the first 15 gallons of fuel (break-in) and after extended storage.
What gear oil does a Mercury / Mariner 40 four cylinder and 30 Jet two-stroke use, and how much?
Quicksilver Gear Lube (or Quicksilver Super-Duty Lower Unit Lubricant); do not use automotive grease. Quantity: 12.5 fl oz (370 ml), 2:1 gear housing; fill until lubricant flows from the vent hole with no air bubbles, then drain about one fluid ounce (30 ml) to allow for expansion.
What spark plug does a Mercury / Mariner 40 four cylinder and 30 Jet two-stroke use?
NGK BUHW-2, qty 4.
How often should a Mercury / Mariner 40 four cylinder and 30 Jet two-stroke be serviced?
The factory schedule lists: First 20 hours, salt water; First 25 hours; Every 100 hours or once a year before storing. 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 40 four cylinder and 30 Jet two-stroke?
The factory schedule gives no replacement interval. 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 40 four cylinder and 30 Jet two-stroke?
600 to 700 RPM in forward gear; idle mixture screw preset 1-1/4 turns out.
What is the propeller nut torque on a Mercury / Mariner 40 four cylinder and 30 Jet two-stroke?
55 lb ft (75 N·m), then bend the tabs against the nut.

Maintenance specs

Two-stroke engine oilQuicksilver 2-Cycle Outboard Oil; in an emergency a 2-cycle outboard oil rated BIA TC-W or TC-WII
Fuel to oil ratio50:1. Oil injected engines also run 50:1 in the fuel tank for the first 15 gallons of fuel (break-in) and after extended storage
Gear oilQuicksilver Gear Lube (or Quicksilver Super-Duty Lower Unit Lubricant); do not use automotive grease
Gear oil quantity12.5 fl oz (370 ml), 2:1 gear housing; fill until lubricant flows from the vent hole with no air bubbles, then drain about one fluid ounce (30 ml) to allow for expansion
Gear oil fill and vent screwsFill screw (magnetic) and vent screw: 60 lb in (6.8 N·m), marked on the gear housing diagram; new gaskets on both. Install the vent screw, then quickly install the fill screw
Spark plugNGK BUHW-2, qty 4
Spark plug torque20 lb ft (27.1 N·m)
Idle speed600 to 700 RPM in forward gear; idle mixture screw preset 1-1/4 turns out.
AnodesTrim tab and galvanic corrosion inhibitor: check for damage or deterioration from salt water. Trim tab bolt 20 lb ft (27.1 N·m)
Propeller nut55 lb ft (75 N·m), then bend the tabs against the nut
Power trim fluidQuicksilver Power Trim and Steering Fluid, or ATF Type F, FA or Dexron II; level to the bottom of the threads in the fill hole with the outboard trimmed full up. The system is pressurized: open the fill screw only at 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. First 20 hours, salt water

    Replace: gear case plug gaskets

  2. First 25 hours

    Replace: gear case oil, gear case plug gaskets

  3. Every 100 hours or once a year before storing

    Replace: gear case oil, gear case plug gaskets

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

    Replace: gear case oil, gear case plug gaskets, water pump impeller

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 30 Jet, 40 service manual. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.

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