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Mercury 25 / 30 / 25 Jet two-stroke (429 cc) service guide

Oil, gear oil, spark plugs and the service schedule for the Mercury 25 / 30 / 25 Jet two-stroke (429 cc) (25 to 30 hp, two-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: Models listed: 25 MH, 25 ERC, 25 MLH, 25 ELRC, 25 EH, 25 ELH, 25 ELPT, 25 ELHPT, 30 MH GA, 30 MLH GA, 30 ELRC GA, 30 ELPT, 30 ELH GA, 30 ELHPT, 25 Jet MLH GA, 25 Jet ELH GA, 25 Jet ELPT. The cylinder head anode moves at serial 0N162743 and below (inside the head) / 0N162744 and up (outside the head).

Quick answers

What two-stroke oil does a Mercury 25 / 30 / 25 Jet two-stroke use?
Mercury or Quicksilver Premium 2-cycle TC-W3 outboard oil, or another NMMA certified TC-W3 oil.
What is the fuel to oil ratio for a Mercury 25 / 30 / 25 Jet two-stroke?
50:1 (2%) standard; 25:1 (4%) break-in, in the first tank of fuel. Automotive unleaded (regular), no more than 10% ethanol.
What gear oil does a Mercury 25 / 30 / 25 Jet two-stroke use, and how much?
Mercury/Quicksilver gear oil API GL-5, SAE #80-90; Premium or High Performance Gear Lubricant. Quantity: Approximately 350 ml (11.8 fl oz) in the gearcase specifications, the maintenance section and the gear housing section; Fill at the fill/drain hole until it appears at the vent hole.
What spark plug does a Mercury 25 / 30 / 25 Jet two-stroke use?
NGK BPR7HS-10 or Champion RL82YC10, qty 2. Gap: 0.9 to 1.0 mm (0.035 to 0.039 in.) in the ignition specifications.
How often should a Mercury 25 / 30 / 25 Jet two-stroke 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 25 / 30 / 25 Jet two-stroke?
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 25 / 30 / 25 Jet two-stroke?
900 ± 50 RPM in gear, 1050 ± 50 RPM in neutral; 25 Jet 950 ± 25 RPM; idle mixture screw 1-1/2 ± 1/4 turn.

Maintenance specs

Two-stroke engine oilMercury or Quicksilver Premium 2-cycle TC-W3 outboard oil, or another NMMA certified TC-W3 oil
Fuel to oil ratio50:1 (2%) standard; 25:1 (4%) break-in, in the first tank of fuel. Automotive unleaded (regular), no more than 10% ethanol
Gear oilMercury/Quicksilver gear oil API GL-5, SAE #80-90; Premium or High Performance Gear Lubricant
Gear oil quantityApproximately 350 ml (11.8 fl oz) in the gearcase specifications, the maintenance section and the gear housing section; Fill at the fill/drain hole until it appears at the vent hole
Gear oil fill/drain and vent plugsInstall the vent plug before taking the lubricant tube out, then the fill/drain plug
Spark plugNGK BPR7HS-10 or Champion RL82YC10, qty 2
Spark plug gap0.9 to 1.0 mm (0.035 to 0.039 in.) in the ignition specifications
Spark plug torque20 N·m (177 lb-in)
Idle speed900 ± 50 RPM in gear, 1050 ± 50 RPM in neutral; 25 Jet 950 ± 25 RPM; idle mixture screw 1-1/2 ± 1/4 turn
ThermostatStarts to open at 52 °C (125.6 °F); fully open at 75 ± 1.5 °C (167 ± 3 °F); valve lift 3 mm (0.12 in.). The component views label the thermostat housing "60 °C (140 °F) thermostat"
AnodesTwo: one in the cylinder head (inside the head on 0N162743 and below, on the outside on 0N162744 and up), anode bolt (M6 x 16) 6 N·m (53 lb-in), and the gearcase trim tab, anode screw 6 N·m (53.1 lb-in). Replace before completely eroded, never paint. Reaching the inside anode means removing the head and fitting a new head gasket
Power trim and tiltManual trim, 6 positions, on most models; PT models have power trim.

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: 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: thermostat, anodes

  3. Every 300 hours or 3 years

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

    Check: 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 25/30 HP/25 HP Jet 2-Stroke Outboard service manual, 90-8M0131803, February 2023. 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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