Basic Carburetor Function

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Basic Carburetor Function

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Harley Davidson Forums >> Do It Yourself Help Forum >> Fuel/Carb Related >> Basic Carburetor Function

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Basic Carburetor Function - 10/18/2005 2:11:25 PM

HDF Tech

Posts: 14559
Joined: 5/14/2005
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Keep in mind that motorcycle carburetion is a compromise and is never perfect. The Basics: Basic circuits. Enrichener circuit: Aids in cold starting and warm-up. Accelerator Pump: Aids in starting and off idle transition. Idle & Low Speed circuit: Controls idle, and helps transition to Midrange. Midrange circuit: Controls steady throttle and light acceleration. High speed circuit: controls hard acceleration and wide-open throttle. All circuits’ overlap, but each one has a primary function. Idle (throttle closed): controlled by slow speed jet and mixture screw. Midrange (steady throttle and light acceleration): controlled by needle. High Speed (Wide Open Throttle): controlled by main jet. Tuning: Base Line Tune each circuit for it’s primary function. Idle & Low Speed circuit, control 100% of idle and about 25% of right off idle low speed. The primary function of this circuit is the Idle. There is nothing else controlling fuel here. Wide-open throttle hard acceleration, when the slide/needle is all the way up the main jet is your primary controller of the fuel. The needle controls fuel for everything in-between. The needle starts to fuel right off idle with light throttle. As soon as you start to open the throttle air velocity increases under the slide, and fuel starts coming from around the needle, even before the slide lifts. Bottom line. Set the idle jet and fuel mixture screw for the best idle. Set main jet for best wide-open throttle. Set the needle for steady (cruise) and light throttle. Tuning: Ride-ability Problems When you experience motorcycle ride-ability problems that are jetting* related, try to duplicate the problem in all of the gears, and under different temperatures (engine and/or air). Remember that air temp can vary greatly from morning to afternoon, and it can take up 20 miles of riding for the bike to reach operating temperature. Decide which circuit needs adjusted. Idle or idle and light throttle: Slow speed jet/mixture screw. Steady throttle and/or light acceleration: Needle. Wide open throttle hard acceleration: Main jet. Decide if you are rich or lean. Worse hot: Symptom of rich. Worse cold: Symptom of lean. Worse in higher gears: Symptom of rich. Worse in lower gears: Symptom of lean. Make your adjustment, and then try to duplicate the problem again. It is important to only make one adjust at a time. Instead of changing the main jet it may be easier to test rich or lean first, by removing the air filter if you think it is too rich or duct taping ¼- ½ the air filter surface area if you think it’s too lean. Note: Doing the above test may cause a problem in a different throttle area, ignore this and focus on the problem area you are working on. *Do not confuse other motorcycle problems, including carburetion problems with jetting!

Source: Direct parts.com