All About Carb Jetting

How the Needle and Main Combine

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.

Jetting Graph
Here's a calculated graph from the spreadsheet showing the final ratio as the blue graph and that graph can be visually compared to the ideal grey graph. This shows how your jetting is too rich or too lean through the whole range of throttle opening. Above the 1.0 line is too rich and below it is too lean. The slide open positions are shown on the bottom scale. Going by the grey graph is excellent to allow you to figure out the near perfect jetting without testing on the street or trail. You can virtually change the jets, needle, and slide cut away to see how it changes the graph. That 3% richness at the beginning is stndard and can't be changed so don't try. Up to 1/4 slide open is a rapid transition period and usually OK if the idle jetting is good.


[This is a backup method to use if you doubt the calculation of the program using the needle and slide specs]
Before beginning you should 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 needle into the slide and measure the distance from the bottom of the slide to the mark. If the narrow part of the needle jet (the needle hole) is recessed down from the bottom of the venturi then measure that and subtract it from the slide to taper distance to get the mm the slide is open at taper start. My Mikuni VM18 is recessed 1mm and my friends Dellorto is .5mm.

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. Typically the dual tapered needles are of the last type shown here which richen the jetting between closed and open throttle which is best for race bikes.

Two important measurements are the needle shroud height and the recess of the needle jet (atomizer) from the base of the venturi (A37 on the VM sheet). That last measurement is from the base to the narrowest opening of the jet. Notice that they curve right there so don't measure to the jets top. Normally it's around 1mm.

The measurement of the inlet bell diameter affects how much the idle jetting is still in effect at WOT. The bigger the diameter, the more it stays in effect which is why Dellorto's are much more that way than Mikuni VM's.

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 these web pages:

Mikuni VM/TM Carbs, Mikuni TMS/TMX Carbs,
Keihin Carbs,  Dellorto Carbs,  Old Carbs
(various other carbs)  

And here's its usage video:

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.

This shows how the air velocity affects suction/gas-flow:     And my graph showing how the needle shroud height affects jetting:

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 these videos: #1  #2.)
1) The spreadsheet relies on you first determining by trial and error what the perfect idle and main jets are. By using the calculated effective flow area for both in relation to the middle slide area it figures the jetting richness for 1/8 slide open and fully open and plots a straight line between the two which you can use to compare to the full graph of jetting according to the other entered data (needle, needle height, needle jet, slide cutaway)
2) It takes the needle and needle jet data to calculate the flow area between them at all 8 positions of slide and calculates its effective area by a formula that simulates its non-linear flow in real life (discovered by my own testing).
3) It takes the inverse of the combined inverses of needle area and main jet area to find the effective flow area of the two combined.
4) Jetting richness of needle/main is calculated to be the effetive flow area multiplied by the ratio of front slide area divided by middle slide area, then all that divided by the carb bore area. So the cutaway affects jetting all the way thru 7/8 slide open.
5) The jetting richness is further multiplied by factors derived from altitude, % ethanol, humidity, temperature, and needle shroud height (which reduces the flow/vacuum at the needle jet).
6) The idle richness is calculated using the jet flow area, air velocity, front slide open area, and WOT carb area.
7) It adds together the jetting from the idle jet and the needle/main jet to arrive at total jetting richness which is plotted on the graph as the blue line.
8) It compares the calculated richness graph with the ideal richness graph and then graphs the result of the difference between the two as a red line graph.
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.

Why should I trust a calculator when any tuner will tell you that each engine is unique and will need different jetting?
Yes it is true that two similar bikes will need different jetting and so the calculator takes the safest route by first requiring that you have already followed the procedure to find the perfect idle and main jet. Then enter in all the details, including the needle, and the calculator will plot a straight line between the jetting richness of 1/8 slide open to fully open. Then it plots the graph for your current jetting and you can see how it varies from the straight line in between closed and open throttle. Then you can virtually try different needles, needle height, needle jets, and slide cutaways till the jetting line is as close as possible to perfect.

What is the best way to get a plug reading?
Click here for my page on plug reading. Most ideal for getting a plug reading is going up an incline WOT that prevents you from going higher in the RPM range than you normally go. On a flat road there is little resistance to speed so with a long run at WOT the engine goes as absolutely far as it can with RPM to still be able to sustain the speed. But that usually winds up being a leaner situation than you want to read on the plug. So if you test on a flat road winding the engine out as far as it will go (because of holding the throttle wide open to fully test the main jet) then jet for a plug with coloring a bit lighter than chocolate brown.

How do I know if my mid throttle jetting is lean or rich since both cause a power loss?
If it is lean then you are forced to slowly open the throttle or else it will bog. If it is too rich then if you slowly roll on the throttle it will sound congested and may even sputter, and it prefers that you open the throttle up more.

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.

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. But if it has an electronic circuit that gradually increase its flow as the slide is raised then it may work although it would still be like legs on a snake. But if you have a carb with a power jet and want to help me integrate it into the software then let me know and in return I can help you get better jetting.

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?
There is an "old carbs" sheet for evaluating any other 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 to see if it's a single or double angle taper. Most of the old carbs use single taper. Then use a digital caliper to measure its two diameters and taper length and enter those dimensions into the calculator.

Is this only for 2 stroke carbs?
Not really, just for carbs with a throttle slide. Some 4 stroke carbs have additional air holes in the sides of their needle jets which lessens the fuel flow but since the program is relative to what is known to be the correct main jet size then it should still work as long as the carb uses a slide instead of a butterfly valve. The second air velocity calculator on the Velocity sheet is for 4 stroke carbs. But it asks for volumetric efficiency which only a rare person would know about their engine because it depends on valve timing and RPM. I would put .95 for a race bike, or .85 for all other bikes.

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 (especially Jets-R-Us) 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 which the YEIS boost bottle can perfectly counter. Its effects are mostly right there although it may also cause you to need an idle jet that is richer.