My Guide to Carburetor Tuning


The graph below shows the range of effect of each of the adjustable parts of a carburetor. This data comes from my jetting calculator.


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 is important to maintain stability when going over bumps and also has a slight effect on the jetting. On my VM18 a change of 2mm float height required a half turn adjustment on the idle air screw. 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. Note that the angle should be just enough for the tang to touch the spring loaded needle without compressing its spring.


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 significantly. That should result in a 1+1/4 to 2 turn air screw position. If less than 1+1/4 then you need a larger idle jet. If more than 2 then you need a smaller idle jet. Then turn the slide stop screw till you have a normal idle speed.
The normal tendency is to tweak the idle adjustment too rich to help compensate for weak throttle response (usually due to the needle clip in too high a position) as you crack the throttle open quickly. But if you follow this method you can have good jetting all the way thru.




SLIDE CUTAWAY- it affects the fuel/air ratio up to about half throttle. The goal is to feel a crisp acceleration. If the idle jet and needle height are correct but the acceleration is weak then either the needle/needle-jet clearance is too small or the cutaway is too high. You can lower it by putting JBWeld on that edge, putting electrical tape on its outer surface to keep it flat there, and letting it set overnight and then trimming it in the morning. To keep the JBWeld from falling off later you need a very rough metal surface and to cut grooves in the slide that the glue can root into (see picture). 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. Actually the needle clearance has a bigger efect than the cutaway but the cutaway is much easier to modify.



NEEDLE CLIP POSITION - Its effect is mostly up to 3/8 slide open (40% throttle open) and should not be used to adjust 3/4 throttle position because that should be adjusted with needle taper.
Look at these 3 graphs for a single taper needle showing too little, just right, too much slide height at taper beginning. Just right is for the needle taper to start lessening at 1/4 slide (.25 x carb bore).

Now look at these 3 graphs for a dual taper needle and how setting the needle height correctly (middle graph) evens out the first half of the graph.




NEEDLE ANGLE- it has its effect from 3/8 slide to WOT. It first needs to have its height adjusted in my jetting calculator so that there are no lean/rich irregularities before 1/2 slide open. Then the main jet needs to be adjusted for good WOT running. Then if the mid throttle running isn't correct then a needle with a different taper needs to be installed. For richer mid throttle running select a needle with more taper angle. For leaner running select one with less taper angle. 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) at half and full throttle. That way you can look and see what position the throttle is at while riding. You can use my jetting calculator to test different needles till you find one that gives a good jetting graph.




MAIN JET- it has a progressively stronger effect from closed to full open throttle but you test it at 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. This photo shows correct jetting because the oil is baked on black except for near the transfers and boost port. A completely black piston top reveals too lean jetting.

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 if using a synthetic oil. Engine oils with more crude petroleum oil (group 1 & 2 oils) will leave more of a black deposit. (Also using race gas will cause the deposits to be grayish.) A light colored brown is too lean and a black 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.



From the instructions of the JD Jetting spreadsheet (no longer sold) there is this which backs me up when I say that the needle and main jet have some effect at 1/8 slide open:

"Needle diameter near the tip affects full throttle jetting and this is taken into account. The main jet also interacts very slightly down as low as 1/8 throttle."
"The needle straight diameter also has an effect at closed throttle. The pilot jet will overlap with the needle straight diameter as the throttle is slowly opened. A fast or inconsistent idle when the bike is hot can be a result of too large (lean) of a needle straight diameter or a pilot jet too small."

My Jetting Videos:
About 2 Stroke Jetting, especially needles
Benefits of a Carburetor Jetting Calculator
Using the Jetting Calculator to find a good substitute needle. (example of a PWK 28 carb that doesn't offer needles with a small enough taper angle)
Using Jetting Calculator #5

Click here to see all my 2 stroke videos


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.

If you are too inexperinced to know by riding if it is running lean or rich then go by the plug reading. Just be aware that if you want a mid throttle reading then you need to keep the throttle at midway while going uphill and not open it all the way while riding to color the plug. This is the best guide for plug reading:


Click here for my page about reading the plug.

Proceed to my page about Jetting Calculator

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