Selecting The Right Carb Needle


This method of determining the right needle and main jet comes from two things: 1) the realization that even at full throttle the flow area around the needle (between it and its jet) has some force of flow restriction and so upper RPM jetting is dependant on both the needle and the main jet. 2) the jet sizes are relational to the carb size more than anything else, no matter what engine the carb is attached to. The second most important factor is the design of the exhaust pipe. The red graph below shows a KDX200 jetting that is close to the ideal ratio of 1 thru most of the throttle opening which indicates the correct jetting. This one just needs one size smaller slide cutaway to richen the 1/8 and 1/4 slide opening jetting. So now with my method you will have a good baseline to work from, no more guesswork and no need for 20 bike runs to get the jetting sorted.



So the following graphic shows the correct way of thinking about every replaceable carb component. Dell'Orto says that that at full throttle the needle flow area needs to be more than the main jet flow area. So if the old chart is true and the needle has no influence after 75-85% throttle opening then why did they say that? (If you use the JD Jetting calculator you can also see that both the needle and the main jet affect WOT jetting.) The only logical answer to that question is that the needle still does have influence on the gas flow at full throttle. As long as the needle is in the needle jet area then it presents a flow restriction. Notice how the needle width and clip position have the peak of their influence at 3/8 slide. And the slide cutaway at 1/2 slide. If the main jet is changed with a needle taper change to keep WOT richness correct then the % of jetting richness change at WOT reduces from +/- 6% to +/- 1.5% and its peak of influence is at 5/8 slide.


So here's my page on the correct way of carb jetting. My method emphasizes that the right sequence is starting with the idle jet. The old fashioned method utilized the needle clip position to adjust the mid range jetting, whereas my correct method recommends its height to be set so that near 1/4 slide opening the beginning of the needle taper is right at the top of the needle jet. The mid range jetting is then determined by the needle taper angle and the main jet. That is the fully correct way to go about it. I have to admit though that for race bikes it may not be important unless you also use them for trail riding. That is because the jetting from before 1/2 throttle opening is relatively unimportant when racing. For street and trail bikes this method of jetting can make a world of difference because often on the street and trail the throttle is gradually opened and so jetting throughout the whole throttle range is important.

Here's two reports from happy users of this software:
"This software works very well for carb jetting. It gets you into the ballpark and is almost spot on first time. I setup 2 bikes (CR250 and a CR500) with it and afterwards only had to do some minor adjustments with the air screw. Keihin PWK 38mm Airstrikers on both bikes. It helped in the selection of the needle, main, and pilot. You could try different combinations and it would show you if the combo was too lean, rich, or on the money. I took what I learned from the information and put it into the bike and it worked very well."

World Championship testimony: Thomas in the Nethlands wrote about the results of using my jetting calculator:
"Just some nice input about the bike (1965 Husqvarna 360 piston port with a Mikuni 32) that I sent you info on earlier and you corrected my input, and sent new corrected one to me, and where red line was rather straight and nice. I was just informed that that bike (driver said jetting was great on the sand track) was one of the bikes in the Swedish team winning world championship in Netherlands yesterday (Sept 21 2019) for vintage bikes before 1965 and driver was over 72 years!" [link]



How/why I developed this method: I have about 10 years tuning 2 strokes and can usually get the jetting close enough but recently I bought a bike and changed the idle and main jet so it ran really good and gave a perfect colored plug and ran really crisp all thru the rev range. But it had an odd lack of effect past 3/4 throttle. I finally figured out that the needle taper was too steep which fully engaged the main jet too soon. So I wanted to figure out a way to help others who have jetting anomalies. My bike is a commuter bike and it's very possible this is a trick the factories use to limit power on the street. So this will help many people who are trying to get the best power out of commuter bikes. Street/trail bikes require really good jetting thru the whole throttle range. Also it will help people who are putting a different carb on their ride and have no idea what size jets and needle to start with. Carbs are sold with a certain set of jets and needle which only work well on some engines.

I can also help you choose a new carb according to your engine size and max RPM. Each carb is setup for a certain max RPM and engine size so if you buy a carb set up for a motocross bike but you want to use it for an enduro bike then you will have a headache jetting it.


Click here to read about how to use my jetting calculator for Mikuni, Keihin, Dellorto, and almost any other carb.

Click here for a list of all my 2 stroke spreadsheet calculators.


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