Carb Tuning - Page 2, By Eric Gorr

Eric Gorr

Engine Builder
Jun 29, 1999
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Fuel Jets, Air Jets, and Throttle Positions

Three circuits control the air: the air-screw, the throttle slide, and the air jet. Four circuits control the fuel: the pilot/slow jet, the spray-bar/needle jet, the jet needle, and the main jet. The different air and fuel circuits affect the carb jetting for the different throttle-opening positions, as follows:

  • Closed to 1/8 throttleÑair screw and pilot/slow jet

  • 1/8 to 1/4 throttleÑair-screw, pilot/slow jet, and throttle slide

  • 1/4 to 1/2 throttleÑthrottle slide and jet needle

  • 1/2 to full openÑjet needle, spray-bar/needle jet, main jet, and air jet
(Note: On many modern carbs the spray-bar/needle jet and air jets are fixed-diameter passages in the carburetor body and cannot be altered.)

Basic Carb Service

Nobody likes to fiddle with a carb if they don't have to. Wedged in between the engine and frame with tubes, cables, and wires sprouting out likespaghetti, carbs are a pain to work on. Carbs require cleaning just like anything else, and some careful observations can save you big money in the long run. Start by pressure washing the bike, especially around the bottom of the carb where roost from the tires and oil from the chain accumulate. Take care when removing the carb, it's easy to damage the cable. Its better to remove the sub-frame so as to enable unrestricted access to the carb. This will also make it easier to route the vent hoses in their proper positions too. When you remove the carb look at the vent hoses. Are they melted from heat or clogged with mud? If so that can cause a vapor-locking problem in the float bowl and make the engine bog.

Remove the top of the carb and disconnect the cable from the slide. Is the cable frayed or kinked? Is the rubber dust cover missing? If so then replace the cable. Now remove the float bowl, jet baffle (white plastic shroud around main jet), float and fuel inlet needle, and the air-screw. Shake the floats and listen for fluid that may have seeped inside. If so replace the floats otherwise the engine might suffer from constant fuel flooding. Check the fuel inlet needle. It has a Viton rubber tip and occasionally fuel additives and dirt damage the tip. Also check the spring-loaded plunger on the opposite end of the tip. If the spring doesn't push the plunger all the way out then replace it. Check the air-screw, there should be a spring and o-ring on the end of the needle. The spring provides tension to keep the air-screw from vibrating outward and the o-ring seals out dirt and water from entering the pilot circuit. Next check the bell mouth of the carb. Look for the two holes at the bottom of the bell mouth. The one in the center is the air passage for the needle jet and the other hole offset from center is the air passage for the pilot circuit. It's typical for those passages to get clogged with dirt and air filter oil. That would cause the engine to run rough because without a steady stream of air to mix with and atomize the fuel, raw fuel droplets make the jetting seem rich.

Once the carb is basically stripped down (pilot/slow and main jet still in place) you can flush the passages. Get an aerosol can of brake or carb cleaner from an auto parts store. Make sure you get the type with the small diameter plastic tube that attaches to the spray tip. Direct the tip into the airscrew passage. When you spray the cleaner you should see it flow out the pilot/slow jet and the air passage in the bell mouth. Next spray through the pilot/slow jet, look for flow through a tiny passage located between the venturi and the intake spigot. Spraying cleaner through these passages insures that the low speed air and fuel circuits are open and free flowing. The last area to flush with the carb cleaner is the slide bore and slide. Dirt tends to trap there, causing the mating surfaces to develop scratches that could cause the throttle to stick!

Just a small amount of water and dirt can get trapped in the tiny passages of the carb and cause havoc with jetting or even engine damage. How often should you service the carb? When it gets dirty! For example if you ride in muddy wet conditions you should at least check the vent hose. If the riding conditions are dusty and your air filter is covered with dirt, then it's a good idea to do a basic carb servicing.

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