Marine 2-stroke outboard maintenance and troubleshooting

2-stroke outboards (Yamaha Enduro, Mercury Sport 2S, Tohatsu MFS) are common on smaller tenders (5-25 hp) and workboats because they're light, simple, and easy to service. Fuel: pre-mix at manufacturer ratio, usually 50:1 (Yamaha 2M and up) or 100:1 for newer TCW3 oils. Use TCW-3 rated 2-stroke oil ONLY, automotive 2-stroke oil coats the spark plug and dies quickly on a marine outboard. Service intervals: (1) Every 50 hours or annually: spark plug (single plug for most small outboards), water pump impeller inspection, gearcase oil check. (2) Every 100 hours: full water pump service, thermostat inspection, spark plug replacement, decarbon the exhaust ports (spray a marine decarbonizer into the intake at high idle until the engine smokes). (3) Every 300 hours: rebuild the fuel pump diaphragm (if the outboard has a mechanical fuel pump), inspect reeds, check compression (should be within 15 percent between cylinders on multi-cyl). Common failures: (a) Won't start, dead spark plug (99 percent of no-start on 2-strokes), followed by carburetor jets clogged with ethanol residue. (b) Runs rough at low speed, smooths out at high, carburetor idle jet clogged, disassemble and clean. (c) Blue smoke, normal for a 2-stroke (that's the oil burning), but heavy blue smoke = ratio too rich (too much oil in the mix) or piston ring wear. (d) Water in the gearcase (milky oil), bad seal, replace immediately. (e) Salt residue in cooling passages after every use, flush with fresh water on the earmuffs, or the aluminum block gets pitted within a season.