ATV Carb Cleaning: How-To

Sport ATV, UTV, Utility ATV — By ATV Mag on August 19, 2008 at 12:00 pm

A poor running ATV can be as frustrating as your in-laws crashing your humble abode for a week with your drunk, stumbling brother-in-law Ned in tote. Nobody wants to deal with Ned and his constant pestering. Worse yet, trying to make a quick escape from the burdensome in-laws on an ATV experiencing lack of power, off-idle stumbles and tough starting can be equally as humbling.
Since we can’t help you with your in-laws and Ned, who is sitting on your favorite recliner belching and scratching, we’ll cover an area of expertise we can aid you with: tuning your ATV’s carburetor for a clean escape next time the in-laws are in town. While we often refer to the carb as the heart of an ATV — it meters the flow of fuel and air — don’t let it intimidate you. Cleaning and tuning a carb is a task you should be able to handle if you have an ounce or two of technical skills. A few frosty 12-ounce beverages to keep Ned at bay wouldn’t hurt either.
If you don’t feel confident in your skills after reading this, have an experienced technician or friend walk you through the process or do the work for you. To coax them into helping, take away Ned’s beverages and offer them to said helper.
For this particular carburetor clean-up, we used a 2003 Honda Foreman Rubicon and its gummed-up 33.5mm Keihin CV carb. While carbs differ from model to model, the following instructions should give you a basis to start your service. If your ATV has a twin carbureted engine, you’ll obviously have to do this process twice. However, it’s important to note we advise you to avoid mixing parts between the two carbs, as each carb will have a slightly different construction that might not be easily identifiable to the naked eye.

Getting Started
If you are a neat freak, have a piece of cardboard on hand and slide it underneath your ATV before removing the carb. This will help you avoid cleaning up any gas spills or messing up a clean shop. Turn the fuel petcock to the off position and then ensure Ned isn’t looking over your shoulder cracking a cig. Next, locate the float bowl drain screw on the bottom of the carb and drain the fuel from the carburetor. Generally this screw is at the bottom of the float bowl and can be accessed by a screwdriver.
For our Honda, we removed the seat and the left and right side plastics, consisting of about five plastic grommets per side and one screw, to have enhanced access to the Keihin carb. Next, we removed the air intake and airbox to allow for more wiggle room when removing the carb. Procedure for your model will vary.
Now, before removing hoses from the carb, we grabbed the paint marker, scratch paper and pencil. We marked each hose and wire with a number and wrote down exactly where the number corresponded to on the carb. Most locations are intuitive, but this step will save you time during installation.
To free the carb from the assembly, we had to remove the air and fuel lines and two large hose clamps that connect the airbox to carb and carb to engine intake. Also, our 2003 Rubicon had two electrical connections that needed to be unplugged. Access to the throttle linkage required removal of a black plastic cover on the carb and one screw.
Stage I
For those of you who plan on doing a basic carb service, you might be able to remove the float bowl without removing the carb from the ATV. For that reason, we’ll start with our carb cleaning how-to at the float bowl as Stage I.
A float bowl filled with gasoline residue, rust and other small particles can often be blamed as the culprit for a rough idle and other maladies. Rotten fuel can dry up and form a thick goop in the float bowl if it has been sitting for extended periods of time. This goop invades the jets and causes all sorts of issues with the fuel/air mixture.
Locate the float bowl on the bottom of the carb and remove the four screws affixing the bowl to the carb with a tight-fitting screwdriver. If you are having issues breaking the screws loose, spray with WD-40 or Liquid Wrench and let the fluid work for 30 minutes or so to avoid stripping the screws — screw heads are generally fairly soft on carbs and are easily susceptible to stripping.
Once the bowl is removed, inspect the gasket for wear or rips. The gasket should protrude above the rim. If it doesn’t, it should be replaced. Now, for those of you who plan on soaking the carb in cleaner overnight, remove the main jet and main jet holder, slow jet, float and corresponding seat washer, fuel needle and fuel needle seat.
With the jets removed, it is a good time to inspect them for clogs and clean them. Soak the jets in carb cleaner. After letting them soak, grab a toothbrush and clean the jets with the soft bristles invading the tiny holes throughout the jets. It is important to use only a soft bristle brush. Wire or similar material can actually widen the holes and you will accidentally re-jet your ATV’s carb. Don’t be that guy! Leave that role to Ned.
If your carb’s float bowl has built-up residue and particles, you will want to soak it in the gallon can before cleaning. Our Rubicon’s bowl had a thick layer of goop built up that required a lengthy soaking period to break the goop loose and dissolve it. The Keihin carb’s float bowl had tough-to-reach crevices so we used compressed air to help blow out any of the gunk lingering in the bowl.
Stage II
If you plan on seeing this cleaning all the way through like Ned finishing that case of beer, this stage is for you. Our next step in the carb cleaning involves removing the diaphram and metering assembly. At the top of the carb, locate the circular cap retained by four screws. Remove the screws and carefully lift the cap off so as to not allow the spring underneath to shoot out uncontrollably.
Now, carefully remove the diaphram and metering assembly. To remove the metering needle, use a screw driver or nut driver to turn the plastic retainer and free the needle. Inspect the diaphram for wear or rips. Be careful with the diaphragm as this is one of the most expensive carb components. Before setting the diaphram down on your work bench, invert the diaphram so it looks like a mushroom. This prevents the diaphram from taking any unnecessary abuse while outside the carb. To clean the diaphragm assembly, spray cleaner on a shop rag and wipe off any varnish. Try to avoid spraying cleaner on the diaphragm as it is made from rubber and the abrasive cleaner can destroy it.
For our Honda Rubicon model, the carb comes with a primer. We removed the primer by backing out two screws. Also, ensure the O-ring seal isn’t damaged. The orifice directly inside the carb from the primer in our Rubicon had a significant amount of residue and goop. Cleaning the primer is an important step on this particular model because the priming action pushes residue through the carb network and can really impact performance.
Next, remove the choke assembly and idle adjustment assembly. Both are fairly easy to remove. To disassemble, remove the screw at the top of the plastic tube for each assembly. Once the screw is backed out, the tube can be pulled off the actuating shaft. Removing the choke plunger will be the most difficult task here, but it can be removed by lifting up while rotating the retaining fork.
Now, it’s time to call upon that ounce or two of technical skill you have. Removing the throttle shaft assembly can be a nerve-racking task, and many forgo it because removing the throttle plate can be difficult. We recommend removing the throttle plate and shaft if you plan on soaking the carb body in cleaner.
The plate can be removed by backing out the two Phillips screws that hold the plate in place. This is the most important step. The screws are staked, meaning the tips of the screws were widened so they keep a tight fit to the shaft. If you can’t find a tight-fitting screwdriver, avoid this step so you don’t strip out the screws — and use spray cleaner opposed to soaking. Before removing the plate, note the side facing out as this side must be facing out when you re-install it. If you do remove the plate, the shaft on our Keihin carb could be pulled out by removing a C-clip on the top of the shaft and then carefully removing the felt seal with a slotted screwdriver. While removing the shaft, note where all the springs and linkages go.
Now the carb body is ready to be soaked in cleaner. After the carb has rested in cleaner for about a half hour, the residue and particles should easily break free with compressed air, a rag and toothbrush. Q-tips can also be used to reach smaller crevices. Use the spray cleaner and hit all the orifices and crevices in the carb to remove any lingering particles.
With the cleaning complete, it’s time to reverse the process and install all the components back on the carb body. Ensure you don’t overtighten critical components like jets and the metering needle.
When the service is complete, you should have an easier starting, more responsive and powerful engine. You may be surprised by the power that returns – power you forgot your ATV had. It will be useful power too, for your getaway the next time Ned is in town.
Tools For The Job
You won’t need many tools or supplies for a carb service, but there are a few key things you should have on hand before starting the job.
• The most obvious is carb cleaner. Don’t skimp on generic or unknown brands. Ask around and find a cleaner that does a good job of breaking down and dissolving built-up residue quickly. We’ve had success with Gunk and B-12 Chemtool. You will want a spray carb cleaner and a gallon can to soak parts in overnight — if you plan on doing an in-depth cleaning. The gallon cans usually come with a metal bin that you can suspend parts in, as sediment will break up and fall to the bottom of the can. If you have access to a parts cleaner, even better.
• Soft bristle toothbrush. This is a helpful tool when cleaning jets and scrubbing interior portions of the carb.
• Several small plastic bins or cups to keep important parts separated.
• Compressed air. A can of compressed air will work if you don’t have access to an electric air compressor.
• Shop rags. Carb cleaner is a very abrasive chemical, so have a few rags on hand for any spills or oversprays that might come in contact with vital rubber components.
• Paint marker along with scratch paper and a pencil.
• Jets if considering re-jetting due to altitude or adding a performance exhaust system.