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FIX-IT: How-To Rebuild a Carburetor

September 23, 2016
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If your ATV ran when you parked it but refuses to pop when you finally decide to ride it again, chances are the carburetor is a mess. You can blame it on what passes for pump gasoline today—or bad luck, karma or even your annoying little brother—but you are lying to yourself. Face it; you blew it. A carburetor is a device for mixing fuel and air that’s suitable for the feeding of internal combustion engines. The float bowl, and for that matter the fuel tank, is not for permanent and endless fuel storage. Most of the elements in pump gasoline evaporate, and you and your carburetor do not want to experience what is left after the prolonged evaporation. If you have a carbureted ATV, consider this a rule: drain the carburetor, or run it out of fuel before even parking it overnight. If you know or suspect the bike will be parked for any number of weeks, empty the fuel tank.

We had an abused Yamaha carburetor typical of those used on entry-level youth quads. This is typical of what a local shop sees—guys ride a few big weekends a year, but they leave the bike parked between those weekends. If the carburetor was left with fuel in it, there’s going to be problems. With a small, EPA-jetted four-stroke, the jets are so small that they plug easily in any case, but with the make-up of current fuels, owners see catastrophic gum-up in very small amounts of time. Here is how to deal with the mess.
You will need to remove the carburetor from the vehicle. Take care with the hoses and boots so you don’t damage anything when removing it. Typically the float bowl is held on with a Phillips screw. When you see a screw with a dimple in the head like this one, it signifies that the screw is manufactured to a Japanese industrial standard, and it does not have the same design as a typical Phillips. Even quality screwdrivers may ground it easily, so use care and a sharp screwdriver.
A screwdriver with a replaceable tip like the one pictured works best. We can only assume that these bits are made in Japan, but they fit snugly. In fact, when you remove the screw, it often fits so well that the screw is almost captive on the bit like it was magnetic.
Pull the carburetor apart and remove any of the parts that are easy to take off. Note that the idle screw is still in the body. It was locked in, and we did not want to risk damaging it. Before you remove any of the carburetor adjustment screws (that includes the idle screw), turn the screw in until it bottoms lightly. Count the number of turns for each, and write them down so you can reset them upon reassembly. When you see a carb this bad, drain the tank and remove the petcock. This petcock needed to be replaced, but fortunately it was not too expensive.
If you haven’t allowed things to get too out of hand, you might get away with a new pilot jet and using contact or carb cleaner to flush out the passages. We find that it is most successful if we simply replace the jets with new ones rather than clean the old ones. Once they are plugged up, they seem to plug more easily thereafter.
A dip-tank type of carburetor cleaner is one of the options you have for cleaning a gunked-up carburetor. In recent years, government agencies have made chemicals like this hard or impossible to find, or restricted the contents until they no longer work as well as they did in the past. You might need to soak the carb parts in this for several days.
We use a dedicated pan on a small hot plate to heat water to soak the carb in. Do not use a pan from your house or do this in the kitchen! This is a garage-only operation in a well-ventilated area. Our hot plate gets the water steaming pretty well, but it never actually boils it. Leave the carb in the hot water for about an hour. If you have a carb that has electronics on it like a TPS or Powerjet, leave the electrical parts above the water level.
Use pliers to gently remove the hot carb, then use a brush and solvent to clean the body while it is still hot. If you are working at home, get an assortment of small brushes to handle this job efficiently.
Carburetor cleaner is used to clean all of the solvent from the carburetor body. Use the plastic straw to direct cleaner through all of the orifices in the body and especially those in the intake bell of the carb. Most of these holes are restricted, and contact/carb cleaner will blow back, so wear eye protection and watch where the orifice is aimed. If needed, go back with the brush.
This is what the carb looked like after the hot-water bath and a quick cleaning. It still needs some work with smaller brushes, but the improvement is huge.
For the float bowl, we dipped it in the carburetor and parts cleaner for the same hour the body was in the hot water. Afterwards, we used the solvent and brush and blasted it with aerosol cleaner. It is much better, but would no doubt be improved by soaking it for an additional time.
K&L makes a tool with small wires to clean carburetor jets and passages with. As noted, we do not recommend cleaning the jets, but prefer to replace them with new ones. You can use a more common welding tip cleaner, but many carb passages are too small for the smallest tip cleaner.
After you probe all the openings in the body, use the aerosol cleaner with the plastic straw to blast out all the passages once more.
A few companies sell a complete carb kit, like this one from Moose Racing. It is nice to have all the parts, and with a messed-up unit like we started with, they can be a lifesaver. If the carburetor isn’t so bad, usually a pilot jet and a general cleaning will get you going.
If you have any problems with fuel leaking from the carburetor overflow tubes, don’t even mess with trying to clean the parts. Replace as many parts of the float valve as possible. You will be shocked at the prices of a float-valve kit for some bikes, but it will solve the problem. The Moose kit includes the float-valve parts.
It is rare for the needle and needle jet to wear, but it does happen with bikes that vibrate or ones that have sucked dirt. The needle is nearly always available, but the needle jet is part of the carburetor body on some bikes.
Many ATVs come with carburetors with a CV-type carburetor. They are excellent for emissions, since the throttle cable opens a butterfly valve only. The slide is lifted by a diaphragm attached to the top of the slide. The slide lifts only as fast as the engine requires it to. It doesn’t matter how fast you slam the throttle open, the diaphragm must remain pliable with no leaks to operate correctly. If your machine has a CV carb, inspect the condition of the diaphragm, and replace it if anything looks suspect.
You must also determine if the carburetor has an accelerator pump. If it does, you must also make sure that the pump is squirting when the throttle is opened. If everything is clean and checked out, the carb should be ready for reassembly. Make sure to set the air or fuel screws and the idle adjustment back to where they were. Without installing the carb, connect it to the fuel line. Turn on the fuel, then open the throttle. Watch for a thin stream of fuel to squirt from the front side of the carb. Until you see the squirt, the carb is not clean and ready to be back on the ATV.
Install that carb and see if the vehicle starts and runs well. That will let you know whether you did a good job. In some cases, with small bikes, no amount of cleaning gets the carburetor back to stock performance. In those cases, a new carburetor is required, but that is a rare event.

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