Carb Jetting 101: Terms, Tips and Jetting Theory

[Dammit!]

A jetting sticky thread is long overdue so here goes.

Resident experts, help me out here. If you spot a typo or something I'm just flat wrong about, let me know or if you have some useful info, post it. Just keep it as newbie friendly as possible. The subject matter is confusing enough on it's own. I'll be adding to/editing the original post as needed.

If you need a service manual, you can download one for most models here.

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Terms and Definitions

Jetting: The term comes from the two main circuits that control fuel flow in a carburetor. The pilot jet and the main jet.

Main Jet: This is the main fuel circuit in a carburetor. It's a common misconception that this circuit only effects engine performance at wide open throttle. The main jet actually has a trickle down effect on other adjustments and should be adjusted first.

Needle: The needle is attached to the carb slide and is raised and lowered when you push and release the throttle. When you let off your throttle, the needle lowers into the main jet circuit, cutting off fuel flow as it goes down.

Needle Clip: This is the small circular clip that is used to adjust the needle position.

Pilot Jet: This is second jet in your carb. Much smaller than the Main Jet and typically has no effect beyond approximately 1/4 throttle. It's used to adjust your mixture at idle and low throttle.

Air/Pilot Screw: The air screw works in combination with the pilot jet. You can consider it a fine tuner for the pilot circuit. It's most obvious effect is noticed by throttle response.

Float: The float controls fuel level in the float bowl at the bottom of the carburetor. It has no effect on jetting but can cause some symptoms that can be easily confused with a jetting problem. If the fuel level is too low for example, it can cause a bog similar to a lean condition.

Float Seat/Valve: Basically the valve that stops fuel from flowing into the float bowl when the float reaches a specified level.

Rich: A "rich" condition is what occurs when you have too much fuel in your fuel/air mixture. Symptoms of a rich condition can be a rough running engine (sputtering), a black spark plug, a wet plug or excessive plug fouling.

Lean: A "lean" condition is what occurs when you have insufficient fuel in your mixture creating an over abundance of oxygen. Symptoms can be a rough running engine (bogging), a white or light gray spark plug and sometimes can result in overheating or even severe engine damage.

Sputtering: This isn't exactly a technical term but is commonly used to describe how an engine runs when jetted too rich. For example, using too large of a main jet will cause more fuel to enter the combustion chamber than it can efficiently burn. This will result in a "sputter" at full throttle. It could best be described as a rapid misfire. Note: a rich condition is not the only possible source of this type of problem.

Bogging: Again, not a technical term but commonly used. Bogging can be a result of a lean setting where not enough fuel is entering the combustion chamber for the motor to run properly. In some instances this can cause the machine to "bog" as if it were actually running out of gas. Note: a lean condition is not the only possible source of this type of problem.

WOT: This an acronym that stands for Wide Open Throttle.

Plug Chop: Plug chops are the most accurate and easiest way to check your jetting. A proper plug chop should be done with a clean spark plug. Example; to check your main jet, put in a clean spark plug and run the trike at Wide Open Throttle (WOT) for several seconds, then in one motion, pull in the clutch (if applicable), kill the engine and let off the throttle. Remove the spark plug and check the color (refer to chart in this thread).

Float: The float controls fuel level in the float bowl at the bottom of the carburetor. It has no effect on jetting but can cause some symptoms that can be easily confused with a jetting problem. If the fuel level is too low for example, it can cause a bog similar to a lean condition.

Float Seat/Valve: Basically the valve that stops fuel from flowing into the float bowl when the float reaches a specified level. If worn it can cause the float bowl to overflow.

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Nobody can tell you exactly what jets to use!

This is especially true with these 20 year old trikes. No two are exactly alike. Different mods, various states of mechanical well being and different locations (altitude, humidity etc) all play a part. Something as simple as removing your airbox lid can have an effect on your jetting. This is why it's critical that you understand how jetting works so you can get it dialed in perfectly for YOUR machine and riding conditions.


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Proper Jetting Procedure

The first step is to get your Main Jet dialed in. This circuit should be done first because it has a trickle down effect.

The Main Jet effects your performance most noticably at WOT. You do not fix a low throttle problem by changing your main jet. When the main is correct, there should be no sputtering or bogging at WOT.

Other than performance, the way to test your main jet is by doing WOT Plug Chops. The color of the plug will tell you if you need to do any fine tuning. DO NOT assume it is correct just because the engine appears to be running good. Remember, white and light gray is LEAN which means you need a larger main jet. Black and wet is rich which requires a smaller main jet. The target is a nice tan color. For a much more in depth explanation of plug reading and pictures, click here!

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After your main jet is dialed in, it's time to move on to the needle.

The Needle effects performance from approximately 1/4 throttle to 2/3 throttle. If you're experiencing performance issues in this throttle range but your main jet is correct, this is where you fix it.

To adjust the needle richer(more fuel), you need to raise it up out of the main jet. You accomplish this by lowering your needle clip. This allows more fuel to flow from the main jet by raising it out of that circuit earlier in the throttle's range of motion. It will make sense once you take a good look at it.

To adjust the needle leaner (less fuel), you need to raise your needle clip. This leaves the needle in the main jet circuit longer, allowing less fuel to flow past.

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Now it's time to adjust the pilot jet and air screw.

This is how I adjust my pilot jet. If anyone has a better technique, chime in.

If the engine does not quickly return to idle after a good rev, it's too lean and needs a bigger pilot jet. If there's bogging when taking off or reving from idle, it's too rich. These problems can often be tuned out with the air screw.

There's a couple techniques for the pilot/air screw. One that I've heard is to set the idle somewhat high, tighten the air screw until the motor starts to die, then loosen it until it starts to die again, then set the screw right in the middle between these two points, then adjust the idle. I haven't tried that technique, personally.

The way I do it is basically by checking off idle throttle response. Adjust it both ways until you get a clean response when you stab the throttle.

When adjusting the air screw, tightening the screw is richer and loosening it is leaner. This is because tightening the screw allows less air to pass (resulting in more fuel in the mixture which = richer), loosening it lets more air through. Edit: depending on the carb, it's possible that this adjustment might be the opposite. Some experimentation will show you which way does what. The type above is the most common with 3 wheelers.


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Symptoms and Solutions

My bike is breaking up on the top end. What's wrong?
That could be a sign of too rich of a main jet. You need to do a WOT plug chop to be sure because an electrical problem can cause a similar symptom. If the plug is black and/or wet, you're too rich. Drop a couple sizes down and try again.

I have no or very little power on the bottom end. It's bogging out a little but once I get past mid-throttle and over it seems fine.
Check your needle setting. If it's bogging a little until about 2/3 throttle, richen it up a little by lowering your needle clip. If the problem gets worse, go two clips in the opposite direction and see if it improves.

My bike is running fine but it won't idle no matter what I do with the idle screw. Should I change the pilot?
Try adjusting the air screw before you go tearing into the carb to change the pilot jet. The stock pilot size should get you close enough to fine tune it with the air screw for most applications. The pilot jet can also become clogged. In which case it needs to be removed and thoroughly cleaned out or simply replaced.

My trike is cutting out every couple of seconds under throttle. The faster I go, the shorter the intervals between drop outs.
This is most likely a float adjustment issue and has nothing to do with your jetting. The float level could be low, causing the engine to die out for a second while the bowl fills back up. Adjust your float to attain a higher fuel level in the float bowl.

How do I know where to start with my main jet?!
When in doubt, go rich! You can make an educated guess by looking at what the stock jet size is, then taking into account what kind of mods you have. If you have an aftermarket pipe and filter, you're going to be flowing a lot more air through the engine than if those items were stock so you have to richen up the jetting to match that increased oxygen. In my opinion it's always best to go richer than you think is really needed, then jet down. Jet it rich until it sputters at WOT, then go down until it stops sputtering, then check the plug for piece of mind. This technique is especially useful in situations where you do not get good readings from your plug (which can be caused by a number of factors). This is my primary method of getting my main jet dialed in.

When I'm climbing a steep hill or dune, I hear a little bit of a pinging sound. What is that?
That is called detonation. This happens when you need to run a higher octane fuel. Under that kind of load, your engine is generating more heat which can cause lower octane fuel to ignite earlier than it's supposed to. It is not a jetting issue.

[3Razors]

Good info for anyone out there. Careful with checking the plug sometimes you can get a false reading depending on the fuel or few minutes with a warm motor at idle or slow speed will darken a plug and the mixture can still be dangerously lean at the torque peak. There is only one way to know what is rich: that is make it too rich! Jet it up to the point where it blubbers and misfires at wide open throttle (say 3rd or 4th gear). Then you can lean it down a little at a time until it runs clean. Do the same at half throttle, with the needle position.

Also stick with genuine mikuni or keihin jets. The aftermarket jets do not have the same precision tolerences, and can really make a jetting nightmare.

[ditchmud]

Good topic, carb tuning can be a trick, but when you get it right it'll make all the difference in the world. I have some real good info that was discussed in another web group that I belong to for my street bike. I posted a question for some help jetting my carbs, and recieved a lot of good intruction including some diagrams that made everything click and I wish I knew how to send it to this thread but I do not, sorry! BUt if you want to check this info out it is at the following web group address http://autos.group.yahoo.com/group/KAWASAKI_KZmessages the conversation started with my post on june 5 of this year,and continues with the other members replies. I hope you go and check it out it's some awesome information I just don't know how to get it sent here, maybe someone else has the know how to get it attached to this thread

[Dammit!]

Good topic, carb tuning can be a trick, but when you get it right it'll make all the difference in the world. I have some real good info that was discussed in another web group that I belong to for my street bike. I posted a question for some help jetting my carbs, and recieved a lot of good intruction including some diagrams that made everything click and I wish I knew how to send it to this thread but I do not, sorry! BUt if you want to check this info out it is at the following web group address http://autos.group.yahoo.com/group/KAWASAKI_KZmessages the conversation started with my post on june 5 of this year,and continues with the other members replies. I hope you go and check it out it's some awesome information I just don't know how to get it sent here, maybe someone else has the know how to get it attached to this thread

That link doesn't work for me.

[smokinp]

Good read up

[crackshot]

yep good read. Worth making it a sticky.

[85hondaatc125m]

I have a question about jetting that I didnt see up there. About the cold wether, would replacing your jet for cold weather help it run. My trike is always dieing in -10 temperatures and I was wondering if a bigger jet would let me ride in the coldness?And another question too. In my manual it says that stock my trike came with a no.92 jet, it then says for going 5000feet+ you need a no. 88 jet. Are those numbers the size of the jet? or are they just a factory product number. If they are the sizes of the jet would a 92 jet be bigger than an 88 jet or the other way around. Sorry for all of the questions Thanks

[team-red-rider]

92 is the size and it is also bigger!

[Dammit!]

I have a question about jetting that I didnt see up there. About the cold wether, would replacing your jet for cold weather help it run. My trike is always dieing in -10 temperatures and I was wondering if a bigger jet would let me ride in the coldness?And another question too. In my manual it says that stock my trike came with a no.92 jet, it then says for going 5000feet+ you need a no. 88 jet. Are those numbers the size of the jet? or are they just a factory product number. If they are the sizes of the jet would a 92 jet be bigger than an 88 jet or the other way around. Sorry for all of the questions Thanks

You should jet richer in extreme cold temperatures, yes.

Yes, those are the actual jet sizes and the bigger the number, the richer the jet.

[Frankencelery]

Wow, awesome info! I put an aftermarket carb from Beatrice Cycle on my 200s, and it's always been horrible at low throttle. Admittedly, I've never adjusted the pilot screw, because its in such a terrible place I haven't been able to get to it. But I was thinking I needed a bigger main jet until I read 's tips. Now I know it's probably the pilot screw or the clip position that will get me power at the bottom end. I'll let you know.

[Dammit!]

Could be a clogged up pilot jet too. The opening is very small and doesn't take much to block it.

[Frankencelery]

I don't think so, because it was that way from the time it was brand new. I'll mess with it tonight and we'll find out.

[Billy Golightly]

I just got done leaning out the main in my new 250R. It had a 175 in it and blubbered badly. I installed a 168 and it runs much better now. However it has a little bit of an oddity. When I short shift the engine and lug it where it has more of a load it runs "cleaner" then it does say I put it in first gear and hold the throttle wide open or during a 1st/2nd gear wheelie after the engine gets up to higher RPMs. Still a little rich?

[Dammit!]

I just got done leaning out the main in my new 250R. It had a 175 in it and blubbered badly. I installed a 168 and it runs much better now. However it has a little bit of an oddity. When I short shift the engine and lug it where it has more of a load it runs "cleaner" then it does say I put it in first gear and hold the throttle wide open or during a 1st/2nd gear wheelie after the engine gets up to higher RPMs. Still a little rich?

Could be. Mine used to do something a little similar after the last top end rebuild. All I had to do was ditch the airbox lid (which I don't need out here anyway) and it cleared it up.

How's your plug looking?

[Billy Golightly]

I pulled the plug after a full throttle run, turned it off and coasted into the shop. Porcelin was still dark, the bridge looked a little on the lean side (had some white on it). The porcelin was probably dark from me riding it earlier because I did not change the plug for just the chop run. So I put a 170 in it, and went down one notch on the needle. The mid range feels about perfect now but the top is still really boggy. I'll probably try putting the 168 back in tomorrow.