How to Rebuild an Outboard Motor Carburetor

Rebuilding an Outboard Carburetor is Easy
Here's How, Step-By-Step

outboard motor carburetor diagram

A vintage Johnson/Evinrude/Tillotson two-stroke outboard motor carburetor

It's no secret gasoline has a maddeningly short shelf life. So after even a few weeks the lighter molecules evaporate leaving behind sticky fuel that gums up carburetor float bowls and clogs the narrow jets and passages. If that's the bad news, then the good news is that outboard motor carburetors can be rebuilt in a couple of hours, even by a fumble fingered newbie. In essence rebuilding simply means cleaning the gum and varnish out of the float bowl, jets and tiny passages.

The procedure to rebuild an outboard motor carburetor is straightforward, but consider shooting a gallery of pictures along the way to ease reassembly. Why rely on your fallible memory when digital cameras are omnipresent? The outboard motor carburetor pictured above is an example of what you might find on your motor. Only, newer outboard motor carburetors won't have the high or low speed adjustable needle valves.

Begin by detaching the primer bulb fuel line from the engine. Under the cowling, disconnect the fuel line that runs directly the carburetor. Disconnect any linkage connecting the carburetor to the engine. Unbolt the carburetor from the intake manifold. Usually there are two bolts or nuts and studs.

Disassemble the carburetor. Laying the component parts on a clean shop towel makes it easier to see individual parts and group them. Inspect the inlet needle. Its up and down motion lets fuel in or shuts off the flow. If the needle tip is grooved gasoline trickles in no matter what the position of the float. Overflow en-richens the fuel to air mixture, causing rough running and wasting fuel. A replacement needle and matched seat are a matched set. To buy parts bring the make, model number and serial number to the parts counter. Be sure to ask for a discount, since you are doing your own work. Sometimes they say yes.

carburetor rebuild kit

Important to note, outboard motor carburetor repair kits are available both from the manufacturer and also from Sierra International, a leading supplier of marine engine and drive parts for nearly 40 years. Another good online source for parts is Boats.net and it offers a generous discount prices for parts (outboards, inboards and stern drives).

Shake the float. It's hollow and it should be empty. If there's fuel inside, it has sprung a leak and extra weight of the gasoline sloshing around inside the float drags the needle down, so the carburetor constantly flows fuel. Once again the rich mixture causes problems. Sometimes a new float is included in a carburetor rebuild kit. Usually the paper or rubber gasket between the float bowl and the carburetor body can be reused. But for a paltry few dollars it's not worth risking a leaky fuel bowl. Similarly, always replace the paper gasket or O-ring that seals between the carburetor base and the intake manifold. If it leaks, errant air flow causes an engine to run lean. Poor running and engine damage result.

CRC carburetor cleaner

Good news: It is no longer necessary to soak a carburetor overnight in a bucket filled with a caustic solution. CRC and number of other chemists offer aerosol carburetor cleaner that let you rebuild an outboard carburetor in less than an hour. Be sure to wear rubber gloves and eye protection. Also, vitally important, don't let the spray soak any of the rubber gaskets (float bowl) or they will swell, growing too large to fit in the bowl gasket groove. Hint: have a spare bowl gasket in your spare parts kit.

In most cases purging carburetor passages of sticky gum and resins is as simple as purchasing an aerosol can of the aforementioned carburetor cleaner. Depending on where you shop the cost ranges from 3- to 5-dollars. Regardless of the brand name, make certain the can you pick up to take home has the little red straw taped to the side. The red straw is an absolutely critical component that allows you to direct the spray into a jet and internal carburetor passages. I like CRC's Clean-R-Carb.

Now for a secret: many otherwise correctly executed carburetor rebuild jobs work well for an hour or two, then seem to go sour. Seemingly inexplicably, the outboard reverts to rough running. The reason is as simple as the sea is salt: You didn't get all the gunk out. So the engine ran long enough for the fuel to dislodge that gunk and plug up the jets and passages all over again. To prevent this frustration, completely disassemble the carburetor and make sure you de-gum every little passage and orifice. See accompanying sidebar

Don rubber gloves and eye protection. Work in an area with lots of fresh air, but no breeze. Insert the notorious little red straw in the carburetor cleaner spray nozzle. Hose down the float bowl, the underside of the carburetor body. One after the other insert the straw into each one of the passages. The cleaner is a solvent that rapidly dissolves gum and varnish. The pressurized flow blows dissolved gook right out of the castings.

Then, reassemble the parts and go boating.

Author Timothy Banse has published articles in Popular Mechanics, All Chevy, Pickup Van & 4-Wheel Drive, Mecanica Popular, Motor Boating, Yachting, Mar y Vela and many other magazines and newspapers from around the world. He writes about cars, trucks and tow vehicles and marine-engine technology.