How to Clean the Carburetor on a Small or Older Outboard

Before taking anything apart, make sure the problem is not somewhere simpler. Check the kill switch clip, the spark plugs, the fuel tank vent, the primer bulb (it should get firm), the fuel line for kinks or soft spots, and the fuel filter. Run the engine from a portable tank of fresh fuel connected straight to the engine. If it suddenly runs well, your problem was fuel or fuel supply, not the carburetor.

Many small carburetors have a drain screw at the bottom of the float bowl. With the engine off and cool, put a container underneath, open the screw a few turns and let the fuel run out, then close it snugly. Old drain screws are small and soft and can snap off in the bowl: if one will not turn with gentle pressure, stop rather than force it.

Pour the drained fuel into a clean glass jar and let it settle. Water shows as a clear bubble or layer at the bottom. A cloudy, jelly-like layer means the ethanol has phase separated: drain the tank too, because no additive can make that fuel usable again. Refill with fresh, treated fuel, squeeze the primer bulb and try a start (on muffs or in the water). Mild varnish sometimes clears after an hour of running with a fuel system cleaner, but badly gummed jets will not.

Disconnect the battery (on electric-start engines) and remove the kill switch clip so the engine cannot start. Disconnect the fuel line at the engine. Lay out a parts tray and keep the fire extinguisher close.

Take photos from every side before you remove anything. The throttle and choke linkages, the primer or enrichener hoses, and the routing of every small hose are easy to forget. Photos save hours.

Remove the air intake cover or silencer. Disconnect the throttle and choke linkages (note which hole each rod goes in), any fuel and primer hoses, and the wires of an electric choke or primer solenoid if fitted. Undo the mounting nuts and lift the carburetor off.

Keep it upright so the fuel in the bowl does not run everywhere. Plug or cover the intake opening on the engine with a clean rag so nothing falls inside.

Remove the bowl screws and lift the bowl off. What you find tells you a lot:

• Green, brown or amber film, or gel: stale fuel and varnish. Cleaning should fix it.
• White, chalky crust: corrosion from water in the fuel. Light deposits clean off; deep pitting may mean a new carburetor.
• Water droplets or rust: clean the tank and replace the fuel filter as well, or the problem comes back.

Slide out the float hinge pin and lift the float out with the needle attached. Look at the needle tip: a ring-shaped groove or a hardened, cracked rubber tip means it will not seal, and the carburetor will flood or leak. Shake the float next to your ear: if you hear fuel inside, the float is leaking and must be replaced.

Unscrew the main jet and any removable idle (pilot) jet with a screwdriver that fills the slot completely. Some jets are pressed in and are not meant to come out; clean those in place.

If your carburetor has an idle mixture screw that is not capped, gently turn it in while counting the turns until it just seats. Write the number down, then remove it. Never force it tight: that damages the tip and the seat. Many emissions-era carburetors have a sealed or capped mixture screw; leave that one alone.

Spray carburetor cleaner through each jet, each drilled passage and the small holes in the throat, then blow them through with low-pressure compressed air. Hold the jets up to a light: you should see a clean, round hole.

Do not push a wire, pin or drill bit through a jet. The hole size is precise, and scratching or enlarging it changes how the engine runs. If a jet will not clear with cleaner and air, replace it.

If you use a soak-type carburetor dip, remove every rubber, plastic and foam part first and follow the cleaner's label for soak time. Some carburetors have sealants or coatings that long soaks can damage, so check your service manual. Rinse, then dry everything completely with air.

Install new gaskets, O-rings and the new needle and seat from the rebuild kit. Do not reuse old gaskets: they are flattened and will leak. Compare each new part to the old one before installing it; kits often include extra pieces for other versions.

Now is also the time to replace a primer bulb that will not stay firm and any fuel line that is cracked, stiff or soft. Old line can collapse inside and starve the engine, which looks exactly like a dirty carburetor. Use only marine fuel hose printed with its USCG type and the SAE J1527 standard, never automotive or garden-store hose; BoatUS recommends Type A1, the most fire-resistant and least permeable kind. A hose that smells of gasoline when you wipe it with a dry rag is breaking down.

Older engines need extra attention here. Mercury advises frequent checks of every fuel-system part on engines from 1990 or earlier for leaks, softening, hardening, swelling or corrosion, and warns that some fiberglass fuel tanks built before 1991 may not tolerate ethanol fuel (resin can be drawn out of the tank and sent to the engine). If your boat has an old built-in fiberglass tank, ask its maker or a pro before you blame the carburetor again.

Install the jets snugly (they are soft brass, so do not overtighten), then the needle, float and hinge pin. Check the float height or float level with the method and measurement in your service manual; a float set wrong makes the engine run rich or lean even with clean jets. Fit the bowl with its new gasket and tighten the screws evenly and gently.

Turn the idle mixture screw back in to the number of turns you wrote down.

Mount the carburetor with a new mounting gasket. Reconnect linkages and hoses exactly as in your photos and make sure the throttle and choke move freely and return on their own.

Reconnect the fuel line and squeeze the primer bulb until firm. Watch the bowl gasket, the drain screw and every fitting for a minute. Any drip means stop and fix it before you crank the engine.

Two things owners often see right after a rebuild:

• Fuel spits or pours out of the main jet into the carburetor throat as soon as the bulb is squeezed: the needle is not sealing or the float is set wrong, so the bowl overfills. Also check that the fuel hose went onto the fuel inlet and not onto a bowl vent fitting.
• Fuel dribbling out around the throttle shaft: usually the same overfull bowl running down the bore, not a leaking shaft. Fix the float level first.

Either way, do not try to start the engine with fuel overflowing; take the carburetor back off and correct the float and needle.

Run the engine only in the water, in a test tank or on flushing muffs with the water on. Expect it to take a few extra pulls or cranks while the bowl fills. Let it warm up, check that the cooling water stream (telltale) is flowing, then check the idle. Fine-tune idle speed and mixture only as your manual describes.

If it still runs poorly, the cause may be a second carburetor, the fuel pump, an air leak at the mounting gasket, ignition or low compression. That is the point to involve a mechanic.

• Treat every tank with a marine fuel stabilizer and buy fuel from a busy station.
• Use fuel with no more than 10% ethanol. E15 and E85 can damage marine fuel systems.
• Before storage, follow your manual: many owners run the carburetor dry or drain the float bowl so no fuel sits in it. Honda, for example, points owners to the Storage section of the owner's manual, which covers stabilizer and draining the fuel tank and carburetor, and some of its small portables have a float bowl drain screw for exactly this.
• Replace old fuel hoses and keep a spare water-separating filter on board.