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Join Date: Jan 2011
Location: Smack in the middle of Illinois
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The correct size jets are the ones that make it run right.
There are no jetting chart that covers all the possible intake and exhaust combinations. It's up to the mechanic modifying the systems to re-jet (usually by trial and error) for the best performance. Changes to the intake affect jetting more than changes to the exhaust. NOTE: K&N filters should only be used where extreme high end performance is required. They pass more air and more dirt than foam filters. They require frequent cleaning, are difficult to clean and oil and take an extraordinary amount of time to service properly. If you ride a lot in dusty conditions (and with the top of the air box drilled, you will not be operating in very wet conditions), you should have at leas two K&N air filters. One in the air box and one ready to put in the air box. You should have a K&N filter service kit and a semi-soft bristle brush to clean the filters with when you wash them.
There are 3 fuel control circuits within most carburetors: idle, mid-range and high speed
Start with idle; does the engine start and idle well? If yes, no changes are needed to the idle circuit. If no, can the idle be corrected by just adjusting the idle fuel mixture screw? If yes, make the adjustment and move to the next adjustment. Note; the engine has to be at normal operating temperature to accurately set the idle - if the fuel screw has to be turned out (counter clockwise) more than 4 full turn from lightly seated, increase the size of the pilot jet by 1 increment. The original pilot jet is a #38 - next larger one is probably #40 - there is probably no need to ever go larger than a #42 and very few modifications would ever require going that large.
With the engine idling satisfactorily, adjust the mid-range: the mid-range is for throttle openings from about 1/8 to 3/4 throttle. The adjustment is made by raising or lowering the slide needle. The typical slide needle is held in position with an E-clip and has 5 grooves the E-clip can be put in. Raising the clip leans the mixture and lowering the clip enriches the mixture. Warm the engine to normal operating temperature, ride the vehicle with a steady throttle setting in 3rd, 4th or 5th gear (typically about 1/2 throttle) - if the engine runs smoothly (no surging or blubbering), no adjustment to the needle is needed. If it surges, it's usually lean and the needle should be raised. If it blubbers (like it running with the choke on), then the needle needs to be lowered. If the slide needle in your carb is not adjustable, you will need to change needles or (if it's lean) you can shim the needle up using small thin washers. Once the setting is right, then move on to the main jet.
The main jet is the easiest adjustment to make. The carb does not have to be removed to change the main jet.
The main jet is for full throttle operation. The main jet is the only fuel control when the throttle is wide open and the engine is running near the top of it's RPM range. Get about 5 or 6 main jets size 150, 155, 160, 165 and 170. Warm the engine to normal operating temperature. Take the vehicle out where you can operate it in 5th gear at top speed. The engine should accelerate to maximum RPM cleanly to top speed and at top speed, it should not surge (lean) or blubber (rich). Jet up till it blubbers and then back down on the jet size till it runs without blubbering. You might need jets larger than 170 and 170 might be too big. Again, it's trial and error till you get it right.
Once you are done, you are not done - the jet requirements can change with fuel, air density, humidity and ambient temperature. You will have to fine tune each time you ride until all the circuits are working together. If you have a bog off idle, adjust the fuel screw out 1/8 turn at a time till the bog disappears and/or adjust the accelerator pump linkage so there is no lag in the accelerator pump shooting fuel into the air stream as the throttle is opened.
Always use fresh fuel, 92 octane or better, try to avoid ethanol blended fuel, try to always get fuel from the same source, avoid octane boosters and have fun.