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You take your main jet that is for a CF=1, say a 200.
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If it is 80 F at 5000 ft, you calculate a new CF, say 0.9 (this is just an example). (this is a good article we found on the net) Take 0.948(Denver atmospheric pressure at 70 degrees) x jet size(60) = new jet size(56.88) round up 57 would be the proper jet size to start at for a stock scooter using a 12mm carburetor.
2 STROKE TUNING IN COLORADO HOW TO
How to determine the proper jet size on a Chinese scooter:Ĭarburetor diameter(12mm) x 5 = 60(or number it calculates) x 0.9 = sea level jet size In most cases screw adjustments won't correct the lean back firing, installing a one size larger pilot jet will do the trick for higher elevations (Colorado).
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2-2.5 turns out on a fuel screw is proper for Colorado. Fuel screws are towards the front of the carburetor and air screws are toward the rear of the carburetor. Any more than 3 turns out go one size larger on the pilot jet. Try richening up the fuel mixture by turning the fuel screw out or the air screw in to allow more fuel. If the motor is back firing at decel this is a good indication the pilot jet is to lean. Be sure you are full throttle under load for at leased 200 yards or so. At this point you can do a plug chop to verify jetting. This will give you a good place to start. Here is a table made from the formula: the temperature and altitude with give you the correct atmospheric pressure, the numbers below have been pre calculated. Main jet tuning: perfect air fuel ratio is 12.5-1ĭenver formula for 70 degrees at 5300 feet is : 0.948 Needle Jet - this is what the jet needle pops in and out of (between 15 to 60 percent of throttle.Jet Needle - this controls the fuel when you open and close the throttle (between 20 and 80 percent power).Main Jet - this controls the fuel when you open up the throttle (between 50 and 100 percent power).Pilot Jet - this controls the amount of fuel when idling.The carburetor consists of four components: