All About Carb Jetting

This graph shows how both the needle and main jet fuel flow areas combine in a special way to become the green graphs equivalent fuel flow area. But still the main jet has the biggest effect at full throttle because it has the smallest area of the two. The needle fuel flow area is the space between the needle and the needle jet which gets larger as the slide is raised.

Here's the calculated graph of my spreadsheet showing the final ratio* as the blue graph
and that graph compared to the ideal graph results in the red graph which shows how
your jetting is too rich or too lean through the whole range of throttle/RPM. Going by the
red graph is excellent to allow you to figure out the near perfect jetting without testing
on the street or trail. Maximum deviation should be +/- 4% of 1.0 which would show as
1.04 to .96 at H22 of the spreadsheet. So if your main riding variability is dry or humid
you can jet it for .96 for humid days which would change to be richer on dry days. If your
main variation is temperature then jet it lean (.96) for cool days so it will be richer on warm
days. Or if your main variability is elevation then jet it lean for low elevation so it will be
richer at higher elevation. But of course for most people the variations in riding conditions
will be a combo of 2 or 3 of these factors which is impossible to figure in your head but
you can use this calculator to see what will the jetting be with known jet sizes.

* ratio of fuel flow area times air velocity


Before beginning you should adjust the needle clip position so the beginning of the needle taper is at the top of the narrow part of the needle jet when the slide is a certain % open. (read more) Just remove the carburetor and use a felt tip marker to put a black dot on the needle right where the tapered portion meets the straight portion. (If you can't see the transition then just look up the needle specs and use the distance of the straight portion to know where to put the mark. Or use a digital caliper to find where the taper begins.) Then put the slide/needle into the carb and lower them into the carb till you can see that the dot is right at the top part of the narrow section of the needle jet. Then you can try to measure the slide opening or put a dot at the top of the slide right at the top of the venturi. Then subtract from the carb bore the distance from the mark to the bottom of slide and you will have the slide opening distance. (example: from top dot to bottom slide edge the distance is 29mm in a 38mm carb. 38 - 29 = 9mm. Then divide that by the carb bore to get the % of slide opening. 9 / 38 = .24 which is 24%. Most carbs work best around 25% (give or take 5%).


The exhaust pipe has an effect on jetting need because the return diffuser wave creates more suction all the way to the carburetor. And the return baffle wave increases the engines dynamic compression which requires richer jetting. A pipe made of only header and muffler does neither effect.

The newest version has these two new options:
1. You can select the type of pipe you have; racing expansion chamber, street/trail expansion chamber, non-EP header & muffler (like what mopeds have) and the spreadsheet will set the target WOT richness and compare your jetting graph with its ideal graph.
2. If your idle air screw is between 1.25 and 1.75 turns out and your main jet is right on the money to give a chocolate brown color of spark plug then you can answer Y (for Yes) to the question "idle + main jet perfect? (Y/N)". Then it will use the beginning and ending richness from your graph as correct and compare everything between to the ideal. This is the most ideal way to do it because otherwise you are limited to comparing to one of three graphs chosen by me for the type pipe used. And of course your pipe may fall between any two of those graphs. So it is highly recommended to make sure your idle jet is correct. I also realized that my formulas ratio of "air velocity divided by intake volume" is equal to using 1 over the carb area. So instead of having to know the max RPM and using sheet 4 you now don't need to know the velocity or intake volume. Surprisingly the right jetting is directly related to the carb area. Looking thru forums I also found a testimony of a guy saying he jetted his carb just right and then put it on other engines, including one twice as big. The jetting remained correct for all engines. Why? Because correct jetting just depends on the pipe used and the carb area.

This shows the two main types of needles. When selecting a needle the length of the straight section is just as important as the taper angle(s). Sometimes you can see that it is a double taper needle by holding a metal rulers edge up against the needles taperd portion.

Important!: Most gas has 10% ethanol which causes your engine to run leaner. You need to know how much ethanol is in your gas to use my jetting calculator. You can test it yourself with an ethanol tester or look on the gas pump for the sticker revealing how much ethanol is in the gas. Here is a site that explains more about this topic: After reading its page then click on "Stations" and then at the bottom of the page click onto the letters for your State to see which stations sell ethanol-free gas. All older bikes need to use ethanol-free gas for the longevity of the crank seals, carb float needle valve rubber tip, and gas lines (well you can buy ones that won't harden). Here's my page showing how to remove the ethanol: Bikes run stronger with ethanol-free gas and are jetted correctly.

Please don't despair that this process is too tedious. It is detail oriented but that is what is necessary for selecting the perfect needle for your ride. The more needles you test on the spreadsheet the easier the process becomes. Then just wait for the smile on your face once you go riding again. It's just great to have a carb correctly jetted. Say goodbye to fouled plugs and seized cylinders! (that is if you heeded my advice on engine oil)

The instructions for using the jetting calculator are at this video and these web pages:

Mikuni VM/TM Carbs, Mikuni TMS/TMX Carbs
Keihin Carbs,  Dellorto Carbs,  

Here are the graphs I derived by testing jets and a needle/needle-jet combo for actual flow (using premix) in relation to flow area. The needle jet flow area was calculated according to the diamter of the needle at the needle jet and the diameter of the needle jet hole as the needle was tested at different needle heights. As you can see the pilot and main jets have a linear relationship between flow area and flow except for being slightly offset.

Questions & Answers

(If after reading this you think it is over your head, just realize that how it works is all automatic within the program and doesn't require any of your brain power. Just watch this video.)
1) The spreadsheet expects a certain curvature of final jetting richness graph according to which type of exhaust pipe you are using unless you choose the option that your idle and main jet are correct. Of course there are other factors which is why you can only use the recommended main jet as a starting point for when you buy jets to test. If your reed valve is small then the needed richness can be as much as 25% less than what the calculator shows is needed.
2) It takes the needle data to calculate its flow area between it and the needle jet at all 8 positions of slide and calculates the flow by a formula that simulates its non-linear flow in real life.
3) Air vacuum at needle is calculated to be half way between that under the front of the slide and the back of the slide (at the cutaway). So the cutaway affects jetting all the way thru 7/8 slide open although its biggest effect is at the beginning of slide opening.
4) It calculates a virtual flow potential of the jets and needle/jet to match the graphs above. It takes the inverse of the virtual jet flow area as its resistance to flow. So the resistance of both the virtual needle/jet area and the main jet area are added together and then inverted to get the virtual flow potential of the combo of main and needle/jet.
5) The amount of gasoline flow is virtual jet flow area times vacuum there (determined by cross sectional area).
6) It adds together the gas flow from the idle jet and the needle/main jet to arrive at total jetting richness.
7) It corrects the results by elevation, temperature, humidity, % ethanol in gas, and needle jet shroud height (which amplifies the vacuum at the needle jet).
8) It compares the calculated richness graph with the ideal richness graph and then graphs the result as fractions below or above the 1.0 ideal.
Is your head hurting now? Hey, you did ask! Just don't ask about my expansion chamber calculator if you don't want your head to explode.

How accurate is the programs ideal jetting graph it uses to compare to?
Sometimes it is right on the money and sometimes not (mostly concerning the right main jet size). There are variables the program can't account for such as 1) what type air filter is used, 2) what your crankcase compression ratio is, 2) how restrictive your reed valve is. Knowing that and after seeing that different engine oils mixed with gasoline have different final viscosities I decided to include the option of whether or not your idle and main jet sizes are the best for your engine. So if you have tested different sizes and have selected the best one then enter Y (for yes) to indicate the program should adjust the ideal graph to match your jet selection. It depends on you getting the main jet size right for best power and a nice chocolate brown spark plug and putting in the right sized idle jet so that the bike idles the best (1/4 turn in from peak RPM) at close to 1+1/2 turns out of the air screw. I know this sounds iffy but the programs main purpose is to help you select the right needle at the right height after you manually get the idle and main jets right. When those are right and you tell the program they are then of course the ideal jetting graph is easily created.

How do I know this calculator is trustworthy?
Fair question. I started off using it with my 100cc with Mikuni 20mm until I got the jetting perfect thru the whole range of throttle, something I had been attempting to do for 4 years but it was an impossible task without a good jetting calculator. Then I used the calculator to check a few Dellorto carbs since I know the complete setup for them and for what engines they are jetted for. Then I have Keihin jetting databases for the Kawasaki KDX200 and Husqvarna WR bikes and I checked their data and I got good agreement. OK so that is good and as much as can be determined without an absolute complete data set. And I continue to acquire jetting data which I enter into the calculator to see if the graph matches what the rider is experiencing. Eventually I will connect a CO2 or oxygen sensor onto my header and compare the results with what the calculator shows. I expect that last phase to only enable some fine tuning of the calculator.

Can this be used for a carburetor with a Power Jet? No, it can't. Any way I think that jet is a horrible idea because once the slide is raised above its position (usually half throttle) it fully kicks in, not being gradual as the slide opens more and more. I think it would totally throw off the linear jetting and I would never put one on or buy a carb with it. If I bought a bike with it then I would remove it and increase the main jet size.

What is this programs main use? To virtually experiment with different needles till the jetting is nearly perfect. Sizing the idle and main jet is an easy and straightforward process that any monkey can do. Getting the needle right is what's hard.

How do you know this calculator compensates correctly for elevation, % ethanol, and weather changes?
Its formulas are based on data about these kinds of changes already published on the internet. You can't make stuff like that up, you have to go by already accepted scientific data. Here's my own experience: My bike was jetted perfectly with around 70% humidity. Both riding it and checking it on the jetting calculator it was right on. Next day it rained and after the rain (90% humidity) I went riding and it was just on the verge of hesitating like when it's too rich. Checked it on the calculator and it showed 4% rich. I then switched gas from 0% ethanol to 25% ethanol (which is what is typically sold here) and looked up the forecast for the next day and had to up the main jet by one size to get perfect jetting. I did that and rode it the next day and the jetting was right on. Then I switched to 10% ethanol gas and changed the main jet size by what gave perfect jetting in the calculator and riding the bike I could tell that once again the jetting was spot on. So I'm pretty confident in the calculator.

What other carbs will this work with?
Basically any 2 stroke carb with a nearly round carb throat with a slide that is flat (horizontal) at the bottom on the engine side. For needles with no published data you'll have to put a straight edge against the needle taper and if it's a single angle then use the Keihin sheet and use the mini calculator at its bottom left of the sheet to find out the taper angle. Otherwise do all the measurements and use the Dellorto calculator.

Why only 2 stroke carbs?
Because 4 stroke carbs have additional air holes in the sides of their needle jets which changes how they work and I have no way to know how to compensate for that.

Where should I buy jets/needles from?
The best is Treatland for Dollorto, and Niche Cycle and Sudco for Mikuni and Keihin. Many other places say they sell geniune jets but they are cheap replacements and can vary greatly from the hole diameter they are supposed to have.

How does a boost bottle change jetting? The normal pre-pipe-powerband power dip has a dramatically rich mixture due to the baffle wave causing the reeds to open later. When they open the piston has gained good upward velocity creating a strong vacuum and so the vacuum pulse through the reeds is pretty strong which causes a surge of gas through the jets. The YEIS boost bottle can perfectly counter that occurrence. So with a YEIS you can jet according to the jetting calculator but with a racing expansion chamber without YEIS you might want to have the jetting graph a straight line from 1/8 slide to WOT.