How to rebuild an outboard motor's 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 freshened in a couple of hours, even by a fumble fingered newbie.

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 carburetor pictured above is an example of what you might find on your motor. Only, newer models 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. Ask for a discount. Sometimes they say yes.

Shake the float. It’s hollow and it should be empty. If there’s fuel inside, it leaks and extra weight drags the needle down, constantly flowing fuel. Once again the rich mixture causes problems. Sometimes a new float is included in a carburetor rebuild kit.

Usually the 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.

In most cases purging carburetor passages of sticky gum and resins is as simple as purchasing an aerosol can of 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 take home has the little red straw taped to the side. The red straw is an absolutely critical component.  I like CRC's Clean R Carb.

Remove all of the jets from the carburetor body. Work carefully. If your screwdriver blade doesn’t exactly fit the brass jet’s slot take the time and trouble to drive to the store and buy one that does. Otherwise, at best you won’t be able to remove and replace the jet. Worse case scenario you’ll “booger up” the end of the jet, a telltale sign of shoddy workmanship.
 

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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 a secret passage.

When carburetors are manufactured, machinist drills tiny passages in the aluminum casting the passages are in a chamber whose opening is sealed at the factory. Look for a freeze plug on your carburetor. Secure the crab body on the bench. Carefully, firmly, drive the tip of a narrow blade screwdriver through the plug. lever it out. Doing so gives access to those passages allowing them to be cleaned, instead of continuing to harbor crud that will work loose and plug up passages.

Next, 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 carb 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.

Remember the plug we knocked out with a screwdriver? (If your carb has one) You need a new one along with sealer available from the manufacture. Seal the plug, slip it into place. Carefully, firmly drive it until you feel it bottom out in the casting.

Reassemble the parts.  Go boating.