How to Adjust the Carburetor on a 1986 Yamaha ATVby Leslie Renico
Your 1986 Yamaha ATV's carburetor is responsible for keeping the mixture of fuel and air in your engine at the proper level. If the carburetor stops doing its job, your ATV's engine will either have a lean mixture -- not enough fuel for the air involved -- or a rich mixture -- too much fuel for the air involved. If you have a lean mixture, you'll hear your engine sounding like it's revving higher, but you won't see a similar increase in power. If it's too rich, your engine will make a sputtering sound, or act like the power is coming and going.
Identify the faulty circuit in your carburetor by noticing where the throttle is when you notice signs of trouble. The pilot circuit controls the carburetor between zero and one-quarter throttle; the mid-range circuit controls it between one-quarter and three-quarter throttle; and the main circuit controls it from three-quarter to wide-open.
Adjust the pilot screw using your screwdriver, if the symptoms of carburetor trouble happen when the throttle is between zero and one-quarter. Let the engine idle and turn your pilot screw clockwise (or in) until the idle gets rough. Turn the screw counterclockwise (or out) through the range of smooth idle, until it starts to idle roughly again. Find a place roughly halfway between those rough idles as your ideal adjustment. If the symptoms of carburetor trouble indicated that the mid-range or main circuit was at fault, move to Step 3. If you have to give the screw more than two turns in either direction, it may be time to change the pilot jet.
Move the jet needle if the carburetor's trouble happens when the throttle is between one-quarter and three-quarters. Lower the needle to enrich a lean mixture; raise the needle to thin out a rich mixture. If adjusting the needle does not fix the situation, it may be time to change the mid-range jet. If the symptoms of carburetor trouble indicated that the main circuit was at fault, move to Step 4.
Change the main jet if the symptoms of carburetor trouble occur between three-quarters and full (wide-open) throttle. If you want to thin a rich mixture, replace your main jet with a smaller one; if you want to enrich a lean mixture, replace it with a larger one.
Items you will need
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