How To Jet Your Carb

You can only begin re-jetting your carburetor if the following conditions are met:


Top end is in good condition.

Bottom end is in good condition as well as crank seals, spark plugs, air filters, reeds and so on.


Mark your throttle housing and grip in 1/4 turn increments. Use a marker, razor blade, etc. Now get the engine to operating temperature by riding around.

Once the motor is warm, use a nice long straight with no one around if possible and try and use the all the throttle . This puts a good load on the motor and is an accurate test of performance, and will show any jetting issues and, if there are, try to notice if the problem gets worse or better as the motor heats up.


Now try to locate the throttle opening at which the problem exists.

See details just below:


Rich Jetting Symptoms and Solutions


Motor won’t idle and idle set screw is ineffective. Try leaning out the air screw by turning it counter-clockwise a quarter turn at a time. If the air screw has no effect, install a leaner pilot jet and return the air screw to 1 turn out. Proper air screw settings are usually between 1 and 2 1/2 turns out. The correct pilot for your scooter is one that allows instant off-idle throttle response, and allows the air screw to be effective between 1 and 2.5 turns out. If the problem still exists, check and clean the choke circuit and carb vent lines.


Engine stumbles off idle to 1/2 throttle, then cleans up. If everything above is correct, check the needle. Stock position is usually in the middle. To lean out the needle, you need to raise the clip. If the needle is dropped all the way lean and the problem still exists, try a leaner needle.

Engine will rev through the mid range then becomes gurgley (technical term) at full throttle and power is sluggish. This is a text book example of a rich main jet. This problem often occurs when an air filter becomes clogged and gets worse as the motor heats up. Choose a main jet with a smaller number and lean out the air fuel mixture one step at a time. If the problem still persists even though it’s improved a little, reinstall your original main jet and lean out the needle one position. Now fine tune the main jet with plug checks


Lean Jetting Symptoms and Solutions


Motor hesitates off idle with a “Boooooooooowang” sound. This is a lean symptom and often occurs when a motor is cold. Try turning the air-fuel screw clockwise a quarter turn richer and check throttle response again until engine revs without hesitation. If the motor “hangs up” or doesn’t come right back down to idle, install a richer pilot, and reset air screw.


Motor knock knock knocks at idle when hot. Try solution above. If problem persists, perform a “compression test” to check for ignition crank seal leakage.


Engine revs clean and crisp but runs really hot and lacks power. This symptom indicates a lean main jet and/or needle. Use a richer main jet and/or needle setting. This is the most common misconception about two stroke jetting: When an engine runs excessively hot and lacks power, it is on the verge of seizing. If you are out on a long run or rally and don’t have the right main jet, try raising the needle by lowering the clip. It won’t be exact but at least you won’t seize.


Note: To be certain that your problem is lean, pull on the choke and see if the problem gets better or worse. If better, your problem is a lean condition.


Shiny Red Tuning

Unit 24, Merretts Mill Industrial Centre






Tel: +44 (0)1453 298212


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