17-08-2013, 12:51 PM
Carburetor Adjustment
Carburetor Adjustment.pptx (Size: 781.38 KB / Downloads: 50)
Adjusting The Idle Speed And Mixture
On some float-type carburetors, you can adjust the air-fuel mixture and engine speed at idle. Check for an idle speed screw designed to keep the throttle plate from closing completely, and an idle mixture screw that limits the flow of fuel at idle. If your carburetor contains these screws, proceed below.
With the engine off, remove the air filter and air cartridge.
Locate the idle mixture screw and turn it clockwise until the needle lightly touches the seat. Then, turn the screw counterclockwise 1-1/2 turns.
If your carburetor has a main jet adjustment screw at the base of the float bowl, turn the screw clockwise until you feel it just touch the seat inside the emulsion tube. Then, turn the screw counterclockwise 1 to 1-1/2 turns. Replace the air cleaner assembly and start the engine for final carburetor adjustments.
Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, turn the idle mixture screw slowly clockwise until the engine begins to slow. Turn the screw in the opposite direction until the engine again begins to slow (image A). Finally, turn the screw back to the midpoint.
Adjusting The High Speed Mixture
Some older carburetors contain a high speed mixture screw, near the throttle plate and opposite the idle speed screw. Under load, the high speed circuit increases air flow through the throat. Setting the high speed mixture involves running the engine until it is warm, stopping it to adjust the high speed mixture and then restarting for final adjustments.
Run the engine for five minutes at half throttle to bring it to its operating temperature. Then, stop the engine.
Locate the high speed mixture screw and turn it clockwise until the needle just touches the seat. Then turn the screw counterclockwise 1-1/4 to 1-1/2 turns
Cold Idle Speed Adjustment
Connect a tachometer to the engine.
Warm The engine to operating temperature.
Set the emergency brake.
Following the service manual, set the idle cam to hold the throatle open for a fast idle.
Turn the fast idle screw until the techometer reads with in specification.
It is aprox 750 to 950 rpm.
Rich Mixture
Rich Mixture means the carburetor is delivering too much gasoline. Typical symptoms of a rich mixture are:
Poor fuel economy
Sluggish acceleration
Choke not needed from cold starts
Sooty or black spark plugs
Sooty or black muffler end pipes
Strong smell of gasoline when machine is at idle
Uneven running (will often slow from regular idle rpm's and stop)
Lean Mixtures
Lean Mixtures means the carburetor is delivering too much air. Typical symptoms of a lean mixture are:
Backfires as the throttle is closed (primarily during coast-downs)
Lurching acceleration
White or light grey spark plugs
Requires excessive amounts of choke to run/start
White or light grey muffler end pipes
Bluing (on chrome systems) of the exhaust header down-pipes
Incorrect Adjustment
Incorrect Adjustment applies to carburetors that have incorrect adjustment of the air/fuel screw. Incorrect adjustment can produce any of the previously noted symptoms. The following symptoms are typical of an adjustment problem:
Poor overall performance
Rattling sounds from the clutch
Engine tends to stall easily
Erratic acceleration
Poor fuel economy
Misfires and/or backfires