How to Rebuild a Carburetor

How to Rebuild a Carburetor

If your outboard’s carburetor hasn’t been cleaned for a season or two, it’s probably as gummed up as the underside of a 5th-grade classroom desk. Today’s fuel can deteriorate in a matter of weeks thanks to ethanol, and a single day on the water with old fuel can be enough to gunk up the works. A blast of carb cleaner helps sometimes but rarely resolves the situation.
How do you know if your carburetor needs a full-blown rebuild? Signs include an inability to idle without stalling, generally running rough and fuel starvation. Blisters between the fingers on your right hand ­­— from tugging the starter cord for hours on end — are another dead giveaway.
Luckily, rebuilding the carburetor on most outboards is a piece of cake. Though it might sound like a complex operation, it’s actually rather simple, and the term “rebuild” basically means giving it a thorough cleaning, then replacing the gaskets and a few simple components. Ready to cure your carburetor’s ills? Then order a rebuild kit from your manufacturer or Sierra (, and roll up your sleeves.


Step 1: Amputation

First pull the fuel line and linkages. Then look for the bolts securing the carburetor to the engine and remove them. On most small outboards (such as the 15 hp Mercury four-stroke shown here), you’ll find two bolts holding the carb in place. If they’re corroded and don’t spin off relatively easily, hit them with a squirt of PB Blaster and let it soak in for an hour or two, to loosen the bolt. Now remove the carburetor from the engine and take it to your work space.

Step 2: Dissection

Set up a clean, flat working area, cover it with a towel and disassemble the main parts. You don’t need to remove all of those little cams and screws; just expose the float and float bowl, the needle and the carburetor body. Also, remove any rubber or plastic parts that aren’t replaced by parts in your rebuild kit, or they may be damaged during Step 3.

Step 3: Disinfection

Many people try to take the easy route by spraying the parts with a regular can of aerosol carb cleaner — and we’ll use one of these later — but to ensure you get the job done right, you need to submerge the parts in a solvent bath. Get a large can of GUNK Carb Medic or a similar solvent/cleaner that has a metal strainer basket inside. Place the carburetor parts into the basket, then drop them into the cleaner and reseal the can. Let them percolate for half an hour, pull the basket and let the parts drain. Then drop them back in for another half hour or so. This will clean them far more thoroughly than a mere spritz and a prayer.
Remove the basket again, let the parts drain thoroughly and set them out on a towel. If there’s visible crud on any of the parts, scrub it off with an old toothbrush and give that part another dunk or two in the cleaner. Then use that canned carb cleaner, with the little red extended nozzle attached, to blast out every little hole, passage, nook and cranny you can find. Set the parts on the towel and allow them to drain again, then use a can of compressed air to blast out any remaining carb cleaner.

Step 4: Rejuvenation

Now it’s time to replace all of the old parts with the new ones in your kit. Note: Don’t attempt to save money by inspecting and reusing the old parts and gaskets that look OK and ordering just those you need instead of an entire kit. You’re only talking about a few dollars, and it’s not worth risking the entire operation to save a buck or two.
Replace the float and needle first, since they need to be secure before you can replace the gaskets and O-rings. As you push each of these into place, reassemble the parts they mate with to hold them in place.

Step 5: Reunification

With your carburetor completely cleaned and reassembled, go ahead and bolt it back into place on the engine. Replace all of the linkages, and you’re ready to take the boat out for a test run. Sometimes the engine will run like new for a few minutes to an hour, then begin running rough again. This is a sure sign that you dislodged some gum or gunk, but it remained in the carburetor and has become stuck in a new spot. You’ll have to remove and disassemble the carburetor and clean the parts again, but the new float, needle, gaskets and O-rings can be safely reused.
To prevent the carb from gumming up again quickly, treat your fuel with a stabilizer that reduces fuel deterioration, such as Star Tron. And every few months, give the carburetor a blast of carb cleaner to keep more gunk from building up in the future.