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%.

First you need to ride your bike and find the maximum speed in any desired gear. (Top speed for you is just how far you normally wind it out, not the absolute maximum speed the engine is capable of.) Usually when you upshift is the maximum speed for that gear. Put on a bicycle speedometer and calibrate it for your tire size if your bike doesn't have a speedometer. Then just follow these steps:

Enter the recorded top speed:
Enter top KPH at F18 or top MPH at F24 on the 4th sheet of the Jetting spreadsheet to the right of the carb velocity calculator.
Measure your rear wheels outer circumference:
Push the bike till the valve stem is at the very bottom. Mark the ground next to the tire equal to where the stem is. Sit on the bike and roll forward till the stem is once again at the very bottom. Mark the ground there and measure the distance between the marks. If you measured in inches then put that value at G24, or enter the wheel circumference in meters at G18.
Measure the crank rotations per wheel rotation:
Put the bike in the gear that you were in when you recorded the max RPM. Take off your ignition cover and prop the bike up so the rear wheel is off the ground. Take the spark plug out. Spin the wheel till the valve stem is at a spot that you can easily remember (such as visually equal to the top edge of the swingarm.) Mark the flywheel and the cases so the two marks align. Slowly turn the rear wheel while counting the complete turns of the flywheel. Do this till the valve stem returns to its original position. A sloppier but easier way is to hold a pencil in the spark plug hole and count the times the piston hits it. Write down the crank rotations per wheel rotation and put that number at E18 or E24.
For metric the top RPM will display at H18. For imperial the top RPM will display at H24. Then just enter the top RPM at C7 of the carb velocity calculator.

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.


On the Mikuni sheet there is an option at E9 for entering the shroud height
that extends above the needle jet. This half shroud rises above the opening
of the needle jet and causes more suction there to draw up fuel and so a smaller
main jet is needed. The only carbs I know of with high shrouds are
Mikuni VM18/22/24/26 and Keihin PWK (not 28mm) and the PJ and PE series.
The spreadsheet tries to compensate for it but you may still have to use the
correction factor option at G2 to normalize the graph.

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 continue from here for :
MIKUNI VM Carbs, TM Carbs,  KEIHIN Carbs,  DELLORTO Carbs,  

Questions & Answers

Why is there a correction factor entry box if this jetting calculator is complete?
After seeing that different engine oils mixed with gasoline have different final viscosities I decided the Jetting Calculator needed a correction factor to compensate. I decided not to let the calculator compensate because only about half the oils publish the data needed to find their viscosity at different temperatures. It depends on you getting the main jet size right for a nice chocolate brown spark plug and then enter a correction factor to get a perfect jetting at 7/8 to full throttle which you can see the value at H22 and I22. 1.0 is perfect jetting. Then you can keep that correction factor the same while you play with the calculator using different needles and main and pilot jets. Just be sure to keep using the same oil. Well you could change it but then you'd have to find the right correction factor for it after getting the size just right for the perfect colored plug.

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 couple of Dellorto carbs since I know the complete setup for them and for what engines they are jetted for. One showed 1.0 at 7/8 throttle and one showed .99 which is close enough. Then I have a jetting database for the KDX200 and I checked their data and I got the expected variation at 7/8 throttle without knowing all the weather data or oil data, just elevation and fuel/oil ratio. OK so that is good and as much as can be determined without an absolute complete data set.

What is the basis for your calculations, how you determine if it is rich or lean all thru the range?
I started off testing different theories using the data sets I have and finally determined that correct jetting is jet flow area times carb air velocity (because the relationship of suction to velocity is linear) equalling 28 (at I15). Knowing the engine size and max RPM I could figure the max air velocity and use that at WOT. By experimentation I saw that at lower throttle settings at the exit side of the slide the velocity stays near to max velocity and the velocity at the needle varies from that according to the slide cutaway. But since that ratio decreases as the the throttle slide lowers then I had to come up with reference values for each throttle setting. Doing that allowed the red graph to show if it was rich or lean all thru the throttle range. Trust me, I am a perfectionist and if I ever find that this method is not universal then I will chunk it but so far it does appear to work on all carbs.

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 perfectly 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 calculator 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 Mikuni or 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.