What I’ve Learned About Carb Jetting

What I’ve Learned About Carb Jetting

  1. The needle/needle jet clearance affects the idle mixture because some fuel is pulled up past the space between the needle shaft and the needle jet at closed throttle. When the needle jet gets wallowed out the tuner can be driven crazy trying to correct the idle mixture because the extra needle jet source is throwing the idle mixture off. But this normally happens only on 250cc engines and above.

  2. Since the idle jet, air screw, throttle slide, needle, needle jet, and main jet are almost always overlapping in their effects there is a dire need to use a jetting calculator to figure out how to get perfect jetting thru the whole throttle range. Also because anything within 5% of ideal jetting feels OK to us (although that is a big 10% range) although it isn’t perfect.

  3. The most important factor in jet sizing is the carb bore area. Second is the design of the exhaust pipe. Air velocity also has some influence. On one story on a forum a guy jetted a carb right for his bike and then put it on two other engines, one being twice as big. He said the jetting felt right for each engine.

  4. The hardest part about jetting is getting the right needle in it, although most people just leave in the original needle and just change the clip position. They usually settle on a jetting compromise, something that isn’t too bad but isn’t perfect. I’d rather do it right and find the perfect needle for it by using a jetting calculator.

  5. Triple taper needles are usually terrible. They almost always screw up the jetting curve to be too lean mid throttle. Keihin only uses single taper needles and Dellorto and Mikuni VM only uses single and dual tapered needles. Single or dual tapered needles do a better job of maintaining a linear response of jetting throughout the whole range of throttle opening. The exception to that rule is the Keihin PWK 28 which has a selection of needles with tapers that don’t go lower than 2.5 degrees which is still too much. See my latest video on how I had to find a 1.5 degree Mikuni needle to substitute for it. That gave a near perfect jetting graph.

  6. The needle height should only be adjusted to start “kicking in” (increasing the flow area at the needle jet) around 1/4 slide opening. It should never be used to adjust the mid range jetting at the expense of minimal throttle jetting. That’s what other needles are for, to get the taper right to match what the engine wants for mid range. If you raise the needle too much then it kicks in before 1/4 throttle and makes it too rich there. If you lower the needle too much then it kicks in too late and around 1/4 slide open the jetting is too lean.

  7. Places that sell needles couldn’t give a flying fuck about you knowing all about their needles so you can make an informed decision. They just want you to buy as many needles as possible like shooting in the dark till you find something you like. I offered to the three main sources to measure their needles for them free of charge and they all turned me down. I had to buy a few of their non-standard needles to measure them myself to put the data in my jetting calculator.

  8. Jets-R-Us supposedly sells “genuine” Mikuni needles but two out of three that I measured had their taper angles way off from what their letter codes said they should have. RESULTS ON MEASURING TMX NEEDLES : 6GDY12 should have angles 1.75/1/6.5 degrees but I measured 1.15/1.83/3.7 degrees. 6DGY04 should have 1/1.75/6.5 degrees but I measured 1/1.83/3.45 degrees. Their 6CEM3 was pretty close to the .75/1.25/3.25 degrees it should have. Now I only buy from Niche Cycle or Sudco.

  9. Carb sizing isn’t as easy as using the common formula that we have seen on the internet. There is a range from high velocity racing carb sizing to medium velocity street/trail carb sizing and so there is no one formula for all since it’s all about max air velocity and what type of riding you do. A free calculator (spreadsheet) can be downloaded from www.dragonfly75.com/moto/carbsizing.html

  10. Different engine oils with different viscosities mixed at different ratios all had the same flow rate thru carb jets in experiments I did. I wanted to know if those differences affected the needed jetting. A 25:1 ratio has 96% gasoline and a 50:1 ratio has 98% gasoline so the 2% difference is hardly anything. But I didn’t chck any high viscosity oils at lower ratios than 25:1 so there is a possibility that would make a jetting difference.

  11. Correct main jet sizing is a combination of 1) spark plug color (although type oil and gas used affect that also), 2) carbon pattern on top of piston (piston should be clean near the transfers) and black everywhere else, 3) lack of carbon underneath the piston crown (carbon there indicates it is getting too hot), 4) how the engine runs (including exhaust note).

  12. If you have to size the main jet too big in order to keep the engine from seizing then you should consider lowering the compression or retarding the ignition or increasing the stinger diameter. “Too big” is noticed by the engine response and exhaust note seeming too rich.

Go to www.dragonfly75.com/moto/carbtuning.html to read how I tune carbs to perfection.

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I am surprised:
No one commented yet
No Air Density Gauge
No EGT or Head Temp
No Weather Station
No float height

I find #3 interesting
I have had a little experience with # 10

What jetting calculator are you using if not http://www.dragonfly75.com/moto/selectNed.html

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Im too dumb to comment Im still reading this and learning

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It seems most of this is more relevant to motor bikes, rather than karts. Any discussion of carb needle swaps, carb bore sizes, oils, or actual carb part swapping is largely moot since a lot of that is spec’d for us in the rules and we can’t play with it.

Also curious about the lack of consideration for EGT or ambient weather…


I like having 2 twisty things to turn. Piece of cake :rofl:


Will comment once I have time to read through. Definitely interested in the topic!

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Until you turn the twisty things a bit far on a cold day :joy:

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On a what day??? :rofl::rofl::rofl:

Concerning using the gauges -
None of the points listed are dealing with the fine points of tuning on race day other than the suggestion to use a jetting calculator to get the needle, needle jet, and slide cutaway right. On the calculator the mid range results are relative to the main and idle jets. So get those two right for a typical day where you live and then figure out the 3 sizes needed.
Then the gauges at future dates can be used to fine tune your whole setup. But first you need to establish that baseline.

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I just use the Rotax jetting app. Bob’s your uncle.

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Interesting reading I’m sure, but I stopped reading when you said “pull”. When it comes to fuel and air, everything gets pushed, there’s no pull! I believe there’s a real difference, if you want to learn how carburetors work.

This seems like semantics to me. If the end result is an air/fuel charge occupying an area of low pressure, then why should we care whether it’s being pushed or pulled?

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I tend to agree, people have different methods and theories but often get the same results. It’s true of course that atmospheric pressure pushes, vs there being a “suck”…

But the question is, what difference does that detail make when it comes to matching the volume of air with the volume of fuel for given conditions

@alvinnunley, what did you find?

After high school I decided I wanted to be an electrical engineer. The money wasn’t there, so I decided I would go to Skyline Junior-college. One of my first day in school the professor got up front and told us that when he went to school electricity flowed from positive to negative, but now it’s said that electricity flows from negative to positive. Point was, he wanted us to think negative to positive whenever he said positive to negative. That was my last day in junior-college. Pretty much like saying whenever I say suck, think push.

Nothing gets sucked, and if you think that way, you’re never going to understand exactly how a carburetor works. What I’m saying is an actual fact, not an opinion, or a special way of saying the same thing. Oh, and by the way, it’s not just saying the same thing just in a different way, saying suck is wrong. When you say suck, you’re implying that a vacuum has some kind of power of attraction, it has nothing of the sort. Now you may understand the difference, that’s good, but most people don’t, they don’t even think about it.

You would be surprised at the number of people who have engaged me in an argument whenever I have pointed this fact out. They tend to make it pretty obvious that I’m wrong. I would venture to say this, “the idea that a vacuum can suck is almost universal”. The idea that a venture he can create a vacuum and suck fuel into the carburetor is almost universally accepted. The idea that a a carburetor can suck fuel out of the fuel tank is almost universally accepted.

The idea that the atmospheric pressure can push fuel into the carburetor is almost universally rejected. Of course I have to exclude all the people who went to school and learned the proper way of describing it. Even then, I’ve seen some of them use the term suck.

Even to this day, the idea that fuel gets pushed into the carburetor is not understood. It’s amazing the amount of work you can get done by nothing more than a atmospheric pressure of 15 psi. I could write a book about this I think, sorry.

I think we generally agree that it’s a push vs suck.

My question is more about what practical impact have you found by making the distinction between suck vs push.

How has it helped you in your jetting decisions?

What’s the correct way to describe it? It seems a subtle distinction. But I am no engineer or even a wrencher.

My goodness I have to speak up :sweat_smile:

I spent quite a few months of my many months in trade school learning about gas laws and thermo-dynamics in relation to internal combustion, gas turbine and pure jet engines. The only engines out there who’s air consumption can be considered to be pushed into it are gas turbine and pure jets, and that’s only by virtue of speed and ram air effect, the air isn’t ‘pushed’ by atmospheric pressure, it’s ‘pushed’ by driving the vehicle into it.

You can see yourself just put your hand over the intake to your carb, you’ll feel the suction. Your hand isn’t being pushed onto the intake.

What does the carb do? It’s job is to inject atomiséd fuel into the incoming air so your engine, which needs both fuel and air (oxygen specifically) can burn it.

Your engine sucks in a given volume of air, this volume doesn’t change, ever. What does change is the DENSITY of the air - the number of air (inc oxygen) molecules occupying that volume. Adjusting the screws and jets doesn’t change that, it changes the amount of fuel that gets added with the aim of achieving air fuel/oxygen mix that uses up all of the oxygen to burn the fuel whilst having just enough unburned fuel left over (in a 2 stroke) for lubrication, since the 2 stroke uses the fuel/oil mix for lubrication. That’s why the engine will overheat and seize if you lean it out too much and burn ALL the fuel you’re allowing in - you don’t give it anything for lubrication.

That said, if it helps you understand how a carb works (post withdrawn by thinking of the air being pushed into, knock yourself out, but to try and convince me different is like telling me the earth is flat !

So why is it when you start with a fuel line from tank to carb that is empty and you cover the carb inlet with your hand you can feel suck and it primes the line way faster than just cranking the engine over with an open inlet? Where is your push theory in play there?

Sitting here with a big smile on my face.
What I’m saying is not a theory. Have you ever heard the phrase “fact displaces theory”.
Your hand is getting pushed down by the surrounding air pressure. If you think otherwise, you’re wrong.
The piston is pushing the air, that’s in the cylinder, out the exhaust valve. Fresh air cannot get into the cylinder because your hands over the carburetor, increasing the vacuum every time the engine turns over. Of course there’s a limit.
That vacuum is in the carburetor too, so the outside air pressure is pushing the fuel through the fuel line to the carburetor.
It may sound implausible to you, but it’s true, every word of it.

To a previous poster; turbine/jet engines will run when the plane is not moving, explain that. You explain to me; what force does a vacuum have/use to pull your hand into the carburetor.
I really shouldn’t be seeing vacuum, rather I should be saying; lack of pressure. Anytime there is a differential in pressure between 2 areas, it’s often referred to as a vacuum in one area versus a pressure in the adjacent area. Take for instance opening a bottle of Coke, when You Unscrew the cap, air (whatever) escapes. So if you’re looking at the inside of the bottle as normal pressure, then the outside is the low-pressure area. A low-pressure area is often called a vacuum.

Turning over the engine creates a vacuum in the throttle body, because it’s displacing air from the crankcase. With that displacement there’s a pressure differential created between it and the atmosphere which is somewhere around somewhere around 14 PSI.

The fuel is pushed rather than pulled.