Jul 29, 2000
South America
How To Know If Your Carbs Jetting Is Correct
First you need to make sure your idle and main jet are correctly sized, by how it runs at idle and at WOT how it runs and what color the plug and top of piston is.
1. Mark your throttle assembly and your grip with white or black dots (such as with White-Out), something easily seen while riding. Make the dots or lines side by side. Then mark your throttle housing for full throttle open. Then mark half way between those two dots on the assembly. That way while riding you’ll know what setting you are testing.
2. Go riding and set the throttle at halfway between the first and second dot and just hold it there. Note the sound of the engine and the exhaust. If the exhaust misses occasionally or sputters then that setting is too rich. Too rich always wants you to open the throttle more, like always accelerating, in order to lean it out some and have clean running and good power. Too lean would have a consistent exhaust note but would dislike suddenly opening the throttle.
3. Repeat step two for the second dot, for half way between the 2nd and 3rd dots, and for the last dot. Take mental notes for each position.

How To Correct Your Jetting
Assuming your idle jetting is correct (not rich to make up for leanness at the beginning of throttle opening) then it is the job of the needle height to kick in the needle taper at the right setting (usually around 3/16 slide open, almost 20%. Slide opening is not throttle opening since closed throttle is a certain amount of slide opening.) to have correct jetting from closed throttle to close to the 2nd dot. If you get a lean spot then raise the needle. If you get a rich spot the lower the needle. This is done with the needle clip. If the needle is lowered completely and it is still too rich there then maybe your needle jet (the hole the needle goes into) is wallowed out and too much fuel is coming up past the needle. This happens most often on 200cc or larger bikes, depending on the engine vibration. If it is wallowed out (oblong) then there is no option but to replace the jet. Note that I am recommending using the clip for setting the beginning jetting, not for half throttle. That is because ideally the half throttle jetting is done by selecting the proper needle taper. Only when the needle is near perfect for your ride would you want to use the clip to adjust its half throttle jetting but then that may mess up your beginning throttle jetting. But a racer never uses that beginning portion and so it isn’t important for them.

From there to half way between the 2nd and 3rd dot is the job of the needle taper and slide cutaway. Too much taper angle will make the jetting too rich, and too little angle will make it too lean. Too much slide cutaway will make it too lean and too little will make it too rich. You can test variations of it by drilling holes in the slide near the low center of the intake portion of the slide, or use your rotary tool to grind ridges into the bottom of the slide edge so that JBWeld will stay in place once put there. Apply it and then put electrical tape on to let it make a straight extension of the slide. How you let the slide rest will determine how it forms, by gravity of course. When hard or semi-hard you can trim it to the length you want. Usually a little bit less than the carb size divided by 10 is the minimum cutaway height.
After changing the needle taper you will need to change the main jet. If you increased the taper angle then you will need a smaller main jet, and if you decreased the angle then you’ll need a bigger jet. That is because WOT jetting is a combo of needle and main jet.

If you don’t know the taper angle of your needle you can use a digital caliper and the needle calculator at the bottom of the free spreadsheet available from Various Spreadsheet 2 Stroke Calculators
Needle listings:
Mikuni VM/TM -
Mikuni TMS/TMX -
Keihin -
Dellorto -

Reading the piston top:
It should be clean close to the transfers because the flow there of fresh gas keeps it too cool to let oil burn onto it. The rest of it should be black. If the piston top is all black then the jetting is too lean, and if it has no carbonized oil on it then the jetting is too rich. Of course that mostly relates to main jetting. But getting the main jet right is essential before analyzing the needle.

About the Needle Shroud and How It Affects Jetting
That’s what I call it and since I’ve never read anything written about it then I feel free to call it what I want to. It is the metal that extends upward from the base of the carb venturi around the needle area. Its radius is about 4mm and it only shrouds the sides and the rear of the needle. What does it do? It serves to increase the suction at the needle jet (the hole the needle fits into) which increases the jetting richness. If you need a main jet in between two available sizes then cutting the shroud down a bit may allow you to use the smaller jet of the two sizes.
Since a 6mm high shroud causes more than twice the suction than no shroud then it obviously has a big effect and should be understood. It is the suction that produces the carbs air velocity that pulls up gasoline, and a shroud produces an extra low pressure zone there.

Changing the shroud height is a way to adjust the jetting curve. Due to the needle angle each carb as it is sold is only just right for a certain narrow zone of max air velocity. But no one is told what that velocity is. Nor are they told what a better needle angle is for a higher or lower velocity. Same as in the arena of engine oil selection. The customer is left completely in the dark as if giving us knowledge would be like putting a computer in the hands of a monkey.

So let me explain. The equivalent flow area of the combo needle/needle jet and main jet is not a straight line going upwards from closed to open throttle. It bows up in the middle some. The black graph below shows the equivalent needle/main flow area. If it bows up too much and is causing mid throttle excessive richness then the black graph can be made more straight by increasing the main jet size and decreasing the suction, which can be done by lowering the needle shroud height. And if the curve bows up too little, causing mid throttle leanness, then it can be corrected by decreasing the main jet size and increasing shroud height (no easy task though).


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