Two-Stroke Smoking More Than Usual - Troubleshooting Guide

These are not problems:

• First start after winter storage. Fogging oil makes heavy white smoke for a few minutes. Let it burn off before you judge.
• Cold starts. Smoke is heavier until the engine warms up.
• Break-in. Many motors run extra oil during break-in. Tohatsu, for example, uses a 25:1 mix during break-in instead of the normal 50:1 and notes the engine will smoke more than usual. See Engine Break-In.

If the smoke clears once the engine is warm and under load, it is very likely normal.

On premix engines, too much oil is the most common cause of extra smoke. Look up the ratio in your manual; do not assume 50:1 applies to every motor. Then check how you actually mix:

• Use a marked mixing bottle, not a guess or a "glug."
• Know the true size of your tank. A "6-gallon" tank that you fill with 4 gallons gets too much oil if you pour in a 6-gallon dose.
• If someone else filled the tank, ask what ratio they used.

If the ratio was off, run the tank down and refill with a correct mix.

This catches many buyers of used boats. If the engine has working oil injection and you also premix the gas, the engine gets oil twice. Tohatsu notes its oil-injected engines mix the oil automatically, so you do not premix with them. Find out what your motor has before the next fill. If a previous owner disabled the injection, you must premix, and the oil injection system's lines and warning horn need to be dealt with properly. See Two-Stroke Premix Conversion.

Use only two-stroke outboard oil with the NMMA TC-W3 rating, or the special grade your manual names. Never use automotive or small-engine oil. Mercury says TC-W3 oil helps reduce smoke and controls deposits that foul spark plugs and stick piston rings. Tohatsu adds that its TLDI direct-injection engines need semi-synthetic or full synthetic oil that meets or beats TC-W3, so check your manual.

Long idling or slow trolling lets oil and fuel puddle inside many older carbureted two-strokes. When you finally throttle up, you get a big puff of smoke. It is common and usually harmless. Run the engine at a higher speed for a few minutes after long trolling to clear it out. If the motor smokes heavily all the time, even after a good run, keep going down this list.

A choke stuck on, an electric enrichener (primer) that keeps feeding fuel, or a fast-idle lever left up makes the engine run rich. That shows as black, sooty smoke, a fuel smell and wet plugs. With the engine off, make sure the choke plate opens fully and that any choke or primer button returns to its rest position.

With the engine off and cool, remove the plugs and lay them in order. Compare them:

• Wet and oily: too much oil, too much fuel, or a weak cylinder.
• Black and sooty: running rich (choke, carburetor or fuel issue).
• One plug very different from the others: that cylinder has a problem. Note which one.

Replace fouled plugs with the exact type in your manual. See Spark Plugs.

If your engine has oil injection, write down the oil tank level and engine hours at each fill. If it suddenly uses noticeably more oil than before for the same running, the injection pump may be delivering too much. Adjusting or testing the oil pump is usually a dealer job, because it is calibrated to throttle position. Owners who see smoke rise with no obvious cause are often told to have the link between the throttle and the oil pump checked: on some engines a worn or broken linkage part can leave the pump feeding more oil than it should. The service manual for your model has the check. See Two-Stroke Oil Injection.

A carburetor with a sticking float or worn needle valve can flood the engine with fuel, causing black smoke and fuel smell. Stale fuel can also make the engine run poorly and smoke. Take a sample of fuel in a clear jar and look for water or a cloudy layer. If the fuel is old or cloudy, drain it and refill with fresh fuel; Yamaha points out that no additive will restore stale fuel or remove water. If the carburetor overflows or the motor smells strongly of raw fuel, see Carburetor Cleaning. Any fuel leak means stop and repair before running.

Thin white vapor on a cold, humid morning is just condensation. Thick white smoke that keeps coming after warm-up, especially with rough running or a sweet smell, can mean water is getting into a cylinder, for example through a failed head gasket. On a PWC or outboard that was swamped or flipped, water in the engine is also possible. Stop running it and have a mechanic check. See Submerged Engine Recovery if the engine was dunked.

An engine that runs hot can scuff or score its pistons, which leads to smoke, lost compression and power. Check that the telltale (pee stream) is strong on outboards, or that the cooling outlet is flowing on a PWC. A weak or missing stream means stop. See Engine Overheating.

Running too cold matters too. Yamaha notes that deposits, especially from salt water, can make the thermostat or poppet valve stick open so the engine never reaches its proper operating temperature. If the extra smoke showed up as the water turned cold in fall, or the temperature gauge sits lower than usual, have the thermostat checked (see Thermostat Replacement). Also make sure it's really smoke and not steam on a cool, damp morning.

If the smoke persists at cruise speed after the checks above, a compression test tells you if rings or pistons are worn. Readings much lower on one cylinder than the others point to internal damage. See Compression Test. West Marine notes that worn rings tend to come with a gradual loss of performance, not just smoke.