Carb Maintenance-April’99

If you made the mistake of letting your quad sit all winter without draining the fuel from the carb, you may now be paying the price. That price could be a rough idle, a flat spot in the midrange, fuel running out of the carb’s overflow vent or a machine that just wheezes and comes to a halt. Guess what? You have a fuel-flow problem?caused by leaving fuel in your carb for more than three months without running the engine.

In that time, gasoline’s lighter elements will evaporate, leaving the heavy, varnish elements behind. These elements can congeal, harden and even dry into a greenish-white powder. None of this is good for your carb’s complex internal workings.

All ATV carburetors have replaceable main jets, needle jets, slides, idle mixture screws, floats and float valves. This is fortunate, because often these parts are plugged, clogged or frozen with hardened gunk that can’t be removed?even if you soak the carbs until the body turns into Silly Putty! Mikunis also often have replaceable float-valve seats, although Keihins have a seat machined into the body. Unfortuanately, some carbs use pressed-in low-speed pilot jets, and these are non-replaceable?if they are clogged and cannot be cleared, the carb is history.

When replacing carb parts, you must know the year, make and model of the ATV and the carb’s series number. This is usually stamped on a flat plate on the side of the body. Always know the number stamped on the jet you want to replace. You will find these numbers on all main jets, pilots, slides, needle jets and jet needles. They must be exactly the same. Close is not good enough, unless you don’t mind replacing plugs or pistons on a regular basis.


Shut off the fuel, drain the float bowl and remove the carb. Next, remove the float bowl. Inside you will find the floats that pivot on a pin (Keihin) or a dual-armed brass piece (Mikuni) that pivots on a pin. This controls the float valve by moving up and down with the fuel level. As fuel enters the bowl, the floats rise and push up on the float valve. The valve is forced into its seat, closing off the flow of fuel when everything works correctly.
The largest tower in the center of the body contains the main jet, which in most cases, is threaded into the needle jet. The jet needle, attached to the bottom of the slide, moves vertically, through the center of this tower, metering the amount of fuel allowed into the carb’s bore from about 1/4 to 3/4 throttle.

Remove the main jet with a flat-blade screwdriver or a hex wrench, depending on model. Note that some mains are surrounded by a shroud which keeps fuel pooled around the jet?even when the fuel is “sloshed” around the float bowl during riding over rough ground. Above the main jet is the needle jet.
Remove the needle jet. Some models are screwed into the body and come out from the bottom. Others are pushed up through the carb’s bore and out the slide tower. Resist the temptation to use a hammer to drive it out! The plastic handle of a screwdriver works best.

Forward of the main jet tower is the pilot or slow jet. On some models it is recessed deep inside a tower, while on others it is screwed into the top. Others are pressed in and are not removable. These jets are usually the hardest to remove from the carb body, especially if they are “glued” in place with dried fuel. Because there is a hole located in the middle of the jet, you can enlarge the hole with a small drill bit and use a very small pointed spiral Easy-Out to screw it out. In the worst case, you can keep enlarging the hole until the walls of the jet can be collapsed. Remember, jets are replaceable?the carburetor body is not.

Two adjustment screws are located outside of the float bowl. The idle speed screw is usually found on the side of the slide tower and doesn’t have to be removed. The other place to find it is on the external throttle linkage, usually located on the right side of the carbs body under a plastic cover.


On most Keihin carb models, the idle mixture screw is located directly under the bore ahead of the float bowl. Not there? Then try on the side of the carburetor, above the float bowl, either just ahead of the slide tower or just behind. When you remove the idle screw, be extremely careful of the needle point. If it gets damaged, it has to be replaced. After the jet is removed, be sure to remove the spring, brass washer and sealing rubber O-ring.
So you know what item you’re actually removing, remember this: If the adjustment screw is located ahead of the slide tower, turning it out will richen the idle mixture by adding more fuel. If it is located behind the tower, it leans the mixture by adding more air.

Caution: Wear safety glasses for the following procedures! With all the jets out, use carb cleaner in an aerosol can, with an extension straw attached, to spray into each air-bleed tube located at the rear of the carb inside the bell housing. One air tube feeds the idle mixture screw. The other feeds the slow jet. You are looking for the cleaner to squirt out the jet towers to indicate that they are clear. Next, spray into the slow-speed tower. Cleaner should come out of a small hole located just forward of the large opening that contains the needle jet. Spray into the idle mixture screw tower and fluid should emerge from a small hole in the bore, forward of the slide, and almost to the end of the bore.
If your carb uses a bystarter piston enrichener, as opposed to a butterfly-style choke, then you must spray into the brass tube that hangs down from the body. This tube feeds fuel to the bystarter. If all the passageways are not free of debris, low-speed carburetion will be off and the machine will be very hard to start in cold weather.

For really nasty clogs, aerosol sprays sometimes don’t have the power to blow out a dried gas clog from the passageways of the body or small jets. Soaking the carb body in carburetor cleaner usually has little effect on these small tubes and holes because the cleaning fluid is so viscous it can’t enter. Sometimes high-pressure air isn’t even enough. If you can’t spray it out or blow it out, you may be buying yourself a new carb.


The jets themselves can be soaked in carburetor cleaner to remove buildup or to soften a glob of varnish. After soaking the jet for several hours in carburetor cleaner to soften the clog, use a K&L Supply jet cleaner (408) 727-6767. Don’t waste your time soaking the carb’s body. Soaking rarely cleans the super small interior passageways. They are just too small for the “thicker” carburetor cleaner to get into. All you will accomplish is having a nice shiny exterior and still have a plugged interior. Mains, pilots, jet needles and slides can be purchased separately. Everything else is usually available only as a kit. So, if you lose the tiny O-ring behind the idle mixture screw, for example, you will be forced to purchase a complete kit.


Float level adjustment doesn’t require a special tool. You just need a ruler that measures in millimeters. Hold the carb body at about a 30-degree angle from vertical to remove most of the float weight off the spring pin at the top of the float valve. If the spring pin is depressed, then the measurement will be off.

With the end of the ruler resting against the float bowl gasket, set the ruler lightly against the float (Keihin) or the float arm (Mikuni). Measure across the top of the float or float arm. If it isn’t correct (see your manual), then use a small flat-blade screwdriver to bend the flat piece of metal directly above the float valve’s spring pin until the height is correct. Plastic floats are not adjustable and do not need adjustment.


If that rock-hard varnish was not inside the jets in the first place, you wouldn’t have to go to all the extra work and expense of cleaning, poking or replacing, right? So, don’t let it harden in the first place! If the ATV is not going to be operated for more than six weeks, shut off the petcock and run the motor until it starts to run out of gas. Now operate the choke and nurse it along at low rpm until it dies. With nothing in the jets to solidify, it can now sit for years without damage.

What about fuel conditioners? They may prevent the fuel from deteriorating chemically, but they won’t do anything to stop evaporation. It’s wiser to remove all the gas from carb by using the motor’s suction. That’s why just draining the float bowl won’t work. It requires the vacuum created inside a cylinder when the piston is on the downward stroke to suck the last droplets of gas from the tiny nooks and passageways of the carb’s body.
Your carb is quite expensive for its size! So don’t be caught with a “wet” carb during the off-season.

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