My Guide to Carburetor Tuning


The graph below shows the range of effect of the slide cutaway which is something normally overlooked. Also it shows the range of effect of each of the jets.


The following is the right order for tuning:

Float Height- First make sure the bowl float turns off the gas inlet valve at the correct float height. The height of the gas in the bowl affects the carb jetting. Too high makes it run richer and too low makes it run leaner. Usually the float needs to be parallel with the float bowl mating surface when it is just barely closing the inlet valve. This picture shows where you measure the float height and the adjustment tang.


You have to know the manufacturers data for float height or go by this method: With the gas tank petcock off remove the float bowl. With the carb upright hold the float in its high position with your finger. Turn the tank petcock on and slowly lower the float till gas starts to trickle past the inlet valve. Note that position of the float in relation to the carb body. If the float "arm" is not parallel to the bottom surface of the carb then adjust the tang till that is the position at which the gas flow turns on/off. see drawing below.


PILOT JET & air screw- they control the engine at closed throttle. Adjust the carb slide stop screw till the engine has a slightly high idle speed. Turn the air screw in and then out, listening for what settings the engine runs with the highest speed. Set the screw so that you have the highest idle speed and then turn it clockwise till it just starts to lower the speed. Turn the slide stop screw out till you have a normal idle speed. Turn the engine off and see how many full turns inward the screw will go before it bottoms out. Ideal is between 1+1/4 and 1+3/4. If it's less than that then you need a richer pilot jet and if it's more than that then you need a leaner pilot jet.


SLIDE CUTAWAY- it affects the fuel/air ratio mostly when you are just opening the throttle. The goal is to feel a crisp acceleration. If the idle jet and needle are correct but the acceleration is weak then maybe the cutaway is too high. You can lower it by putting JBWeld on that edge and letting it set overnight and then trimming it in the morning. Rough up the slide surface with sandpaper before applying the JBWeld. If the acceleration feels too rich (somewhat "congested") then you can cut the slide cutaway 1mm higher or drill one or more small holes near the bottom of the slide at the intake side. Instead of lowering the cutaway you can buy another one with a lower cutaway number. Instead of drilling a hole in the slide you can order another one with a larger cutaway number.

NEEDLE- it controls mostly the mid-range of throttle opening but also affects WOT. It can be "adjusted" by raising or lowering it by raising or lowering the needle clip near its top. The main jet and pilot jet and the slide cutaway also affect it. The best way to make sure you are noticing how the engine runs at mid throttle is by marking the throttle grip and the throttle body with two white marks (such as with white-out). That way you can look and see what position the throttle is at while riding. If you get good mid range power then don't worry about anything over 3/4 throttle because that is mostly the domain of the main jet. If it runs good at mid throttle but is totally screwey at 1/4 throttle then the needle taper isn't correct for your engine and you need to use my jetting calculator to test different needles till you find one that is correct.



MAIN JET- it controls the fuel mixture ratio at mid and full throttle. In general if the ratio is too rich then the engine runs irregularly and if too lean then it runs really smooth but under-powered and too hot. You can look at the underside of the piston and if the oil is being baked on black then the piston is getting too hot. Too high compression or too advanced ignition can also cause over heating. Over heating can also cause the piston to seize. Looking at the spark plug (if it is of the correct heat range) can be helpful as a starting point. The ideal color on the ground electrode is chocolate brown. A light colored brown is too lean and a really dark brown is too rich. When I think the main jet is correct then after a couple of hard rides I take the piston out to look at its underside to make sure there is no burnt oil there. Of course you have to clean the underside before starting to jet the carb to clean off any previous deposits. What I mostly go by is how clean the engine runs and how much power it makes since being too lean or too rich will rob engine power. Here's a graph showing the change in engine heat with the change in jetting.



If you have done everything possible with the above suggestions and you can't get the engine to run cleanly/crisply all thru the rev range then you probably need a different needle.

I have looked at the promos for other jetting calculators and think my Jetting Calculator is the best because it's usable with all carbs and is fairly easy to use. Also it's final graph is not just a comparison to previous jetting but shows you how its full range jetting compares to the ideal fuel/air ratio.

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