Carburetor Jetting throughout the RPM range - Throttle openings explained!

In this video you will learn what adjustments to make to the #carburetor for each throttle opening throughout the rev-range. This can help in fine tuning the #jetting for specific conditions or to diagnose a fuel #mixture problem in the carb.
Check out these supplemental videos for more!
Is your #carburetor to big?
• Is BIGGER always better?
Rich or Lean? How to know.
• Rich or Lean, how to f...
Idle mixture adjustment explained
• Idle mixture adjustmen...
Got Jetting problems? www.thejunkmanadv.com/motorcy...
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0:00 - Intro
0:15 - Idle speed
0:59 - Idle speed mixture problems
2:27 - Slow speed 1/4 throttle
3:44 - Medium speed 1/2 throttle
4:17 - Full speed wide open throttle
4:53 - Symptoms of carb jetting problems
7:24 - Further videos & information

[@thejunkman]

If this video helped you considering giving "THANKS" ❤ it goes to help making more videos like this.

[@bjusticeforever]

Junkman, I'm working on an EX30 Subaru Robin motor on a Powermate generator. under a load, it runs FANTASTIC. When the load is removed, however, the engine runs rich, sputtering through the exhaust. I've filled in the pilot jet (JB Weld) and reduced the pilot jet from .026" to .0225" and it has helped. Should I continue to reduce the size or do you have any other ideas? I know that generators don't 'idle' per se, and thought you might have some thoughts. Thanks.

[@SteveHacker]

YAY! Made my day! Thanks for getting back to motorcycles! 😊😊😊

[@johnpillinger7220]

Great explanation. Many people don't really understand what affects what at certain throttle positions. Even old timers commonly tell me to adjust the idle air/fuel screw because it bogs at WOT lol

[@brucemcginnis6141]

Thank you. Your videos help me out a ton.

[@vivekkakkat]

Brother..thank u very much..you solved my headache...my bike missfire on lower rpm...now only i understood its the pilot jet...great help..keep on going....👍👍

[@Larry-vm9jh]

Thank you junkman great job on your presentation I'm a retired master technician I'm glad to have found your site. It is absolutely correct in every way the information that your sharing with others and that is something not common to.all sites on the internet there's a lot of videos out that if you are not a master technician you might not catch the fact that some of them are lacking in some important details that an average d.i.y. needs to make use of the information successfully

[@louispeters2105]

Thank you. Well done. I'm writing "logie" into my dictionary just for you.

[@adrianfarhan2659]

Perfect explanation

[@chasingwaves11]

Excellent work man !

[@mexicanbandito]

Best explanation of tuning carbs I've come across thanks for the visual as well sure earned my sub

[@frerecampbell9291]

Thanks for good explanation, very easy to understand.

[@damonjones9270]

Excellent video

[@makelsevilla]

Very nice and helpful 👍👌

[@gster0101]

Excellent info... appreciate your videos.

[@MOSTAFIZURRAHMAN-ht2qz]

keep up the good work

[@ricnegrete1991]

You're the sh*t broooooo. Suuuupeer well explained, by far the best carb vids!!!! Thxxxxx all the way from Mexico!!

[@aaronjeffries9617]

Very helpful! Thank you. I've owned a few carb'd bikes and don't understand how that system works, so I'm learning up as I'm looking to maintain them better, or make adjustments with different intake/exhaust changes

[@keithparady2594]

This video is awesome

[@Ryan_Carder]

Hoping you can help. I'm using a wideband O2 sensor to take any guess work out of determining the stoic nature of my bike. The chart I'm following says that a richer mixture provides better power, and cooler engine temps. I tuned the idle circuit to a target of 13:1. That was easy. Now, when I took the bike out I was INCREDIBLY way too lean along nearly the entire throttle range. I raised the needle, yet it changed little. So, I bumped up my main jet and it fixed the issue for 1/4 - 3/4. However, I am running entirely way too rich at wide open throttle (WOT.) About 10:1 and less AFR. My needle is at its highest position, and by decreasing the size of the main jet to solve the WOT issue, wouldn't that also lean out the needle jet throttle position throwing it off my desired AFR? Not a whole lot of people I've tried asking help from can give me a solid answer. Thanks for the video by the way. Incredible animations, and I love the truly scientific approach!

[@jean-nicolaslemire894]

Hey merci Yvon😊

[@BobbyBasketballl]

Great video, gave me the rxact information i needed to fix my lean issue. I noticed i cant get the bike to run rich at all and now i see its likely because of the aftermarket exhaust and smaller stock jet.

[@user-rz8su6dk4e]

Fuel filter does wonders, I would never run any of my motorcycles without one

[@thedaydreamer7972]

Nice video,HI all I have taking a liking to motorcycle and scooters and really like tuning them and playing with settings to get more performance, better milage, I really enjoy it and feel I have a natural talent in doing so Id like to talk about tuning a CV carb, specifically one bike I own a Honda Helix, its a 250cc 4T, simple design and seems to respond to changes This scooter has a CVT and will go 75MPH on top end OEM, but people complain you cant tune it, and I think its said becasue they dont know how Typically this bike has a 110MJ and 38PJ fuel screw 2.5 turns out, There is a fella who posted vidoes of him tuning this bike on a dyno and by simple adding a K&N free flow filter and taking the MJ to 130 he got 1.5more HP out of a 15HP bike I decided to try this with his setup and sure enough its faster, but the very low end throttle crack there is a slight bog and Im thinking a shim will resolve this, in fact he was using a 1.5mm shim I think I can tune further it further to improve Here are some questions 1. would it be better with a CV carb to NOT run the K&N directly on the carb body and use say a few inch pipe in between? 2. this video this guy is using a lollypop to meter air flow, what you think about that? 3. Any other suggestions for tuning a CV carb in this instance? TY all Heres his video kzbin.info/www/bejne/hJKZlZV-brKjqMU

[@whydahell3816]

Perfect

[@AngelCervantes405]

Thanks sir

[@scottcates]

Good 👍👍👍

[@mathster]

Awsome video man, great content

[@tonza3086]

Anyone know what i have to change on my 26mm molkt on a lifan 125 to get rid of bogging? I have tried from 100 to 122 main jets (102 is the stock one) and nothing is helping, i have also tried 10 sizes bigger pilot jet, im not sure if it helped, but maybe it did a little. I tried a cloth around the air cleaner that removed the bog but it doesnt work well with it on. So do I have to get bigger or smaller main jets and/or pilot jets? I appreciate if someone could help

[@harrybrown4815]

Best Carb animation and explanation ive seen to date. brilliant have shared.....

[@williamfoster4268]

If you have fuel traveling up a needle jet at idle what does that indicate? The motorcycle starts fine, idles like it should and drives fine but there is a slight and I do mean slight hesitation at 5-6k rpm. I have checked the float levels four different times, new floats, new float needles and I have bench tested the float needles too. Everything checks out.

[@DDBmaster]

I m wondering if you know about: Black exhaust smoke when needle is at middle setting, but its revving on it's own. If I lower its idle it stalls. When the needle is at top slot (close), its jerky and wanna stall at low revs (1/4th), and ok mid/high, but still on swift gas open, engine stalls.. Its 125-130cc which I use at work, and really bothers me why its behaving like that. Ant clues?

[@mac2k2020]

what would cause a bike to backfire at normal 1300-1500 rpm or lower but run ok at around 2k ? and after a dah or 2 sitting it just doesn't start and floods the intake i played with the fuel mix screws and nearly closed them to 1 turn out vs the 1 /78 -2 recommended in the manual .. i'm assuming too rich i read even lean conditions can cause backfiring at low rpm

[@1N786K3]

i tried to tune my atv but every time i am in 4 and 5 gear at half throttle it bogs out i was riding and ran out of gas and it ran great so i no is getting too much gas but win i adjusted it it still ran the same what do you think i should do

[@mozezcen-valley7555]

I feel like my bike wen I go fast for a while it cuts off turns off. Like no gas or its drinking all it gas it starts up fine after and idels what can it get the jet???? Need bigger jet to not finished gas right away? Or what can it be

[@vdubplate]

Would love to see one of these animated videos on how elevation effects the carburator

[@thejunkman]

I'll have to think about that.

[@useribp]

I want to put my carburetor head down because its not an original one can it work what u think?

[@agacezarvlogs5225]

New subscriber from phillipines god bless your channel AGA CEZAR VLOGS shout out next vlogs

[@ACommenterOnYouTube]

So my carb was cleaned about 3 yrs ago, paid like $374 ... she ran like a champ, no sputter, no bogging ... i ran her for about 3 months and had to park her. I have not ridden the bike for about 2.5 yrs but i do crank it up every month and let her idle for about 20 min ... She has always had ethanol free fuel and just today i want to take a ride and she is sputtering. Idle is perfect but once i get on throttle and get about 1/4 in she sputters and looses power. Gave 1/2 throttle and she gave some power but again started to sputter with hesistation. Could low fuel level in the tank cause this ??

[@oskarsmeri3073]

Absolutely amazing videos, I have a question, I'm converting honda cbr1000f to café racer and I need to get rid of the air box for space purposes. I was wondering if there are any cone air filters that would mimic the airbox resistance and I wouldn't need to re jet the carbs. Thank you

[@thejunkman]

You will need to rejet. No way arpund it.

[@sc2dave1]

cover up some of the carb's openings under the filter

[@zainulabideenzain3]

Recently I installed a new carburetor in my car. The bigger main jet that it came with was of size 180. With 180 jet, the acceleration was jerky from half to full throttle, but it accelerated pretty quickly. I swapped the larger main jet from 180 to 135. Now with 135, the car bogs down under wide open throttle (especially when the pedal is quickly depressed), and then picks up after around 2 seconds. I've also noticed that the acceleration isn't as good with jet 135 as it was with 180. It idles perfectly fine and acceleration is smooth and crisp for half pedal; only problem is that with WOT, it bogs down and isn't accelerating as it should. Should I go for larger jet size of 150?

[@mohammadahmad9551]

I'm not an expert but i think yea the 135 was too small for a Main jet so the car is starving for gas on WOT (Lean condition). Lean condition is harmful and can kill your engine really quick, However, Rich condition doesn't although the engine isn't running properly. From what you said the 180 Jet gave you the Rich condition symptom (sluggish acceleration but not a bog). Just wanted to say ALWAYS start with Bigger Jet (Richer) then go down, not the opposite. You could also try to keep the 180 and lower the needle (Make it leaner) if you have an adjustable needle, but since you don't have any issues until 1/2 throttle, main jet is your best bet.

[@jdmtechllc6589]

Great video, but I would like to add, If the float height ain’t right your just spinning your wheels

[@thejunkman]

Yes, correct. That is dealt with in a different video

[@TimPaddy]

Great video. Maybe I need to peruse more of your videos. My kid's 50cc has a very simple carb. Low hours, stock everything. It bogs and/or dies if you rip the throttle off idle. I've cleaned it, raised the float, cleaned it, cleaned it, and this one symptom remains. I'll be trying to adjust the needle height soon, but I'm out of ideas. Seems odd for a basically new bike. Are slides adjustable?

[@thejunkman]

Try that, if it improves you might need a larger main jet.

[@chasingwaves11]

I having a little problem with my kawee 250F at low throttle prob 1/8 to 1/4 throttle only happens when I wanna cruise slow and be stealthy lol I’m getting a popping and backfire .. other then that the bike performs good no lag in power starts up decent all the time .. just a low throttle when I’m sneaking by someone’s house .. but if I wack the throttle from low to high it’s fine ... is it getting to much fuel at low cruise ??

[@Jboost412]

Slightly too much fuel…I mean slightly so for example if you’re on a 40 pilot jet go down to a 38…if you only have say a 35 as your next option then just simply raise the needle jets clip 1 position effectively leaning your mixture ever so slightly

[@Zalagar619]

Opposite of what the other guy said. Popping/ Backfiring/ Decel Pops are caused by lean conditions. Try going up one size on the pilot. if that doesn't help you may need to lower the needle clip in the slide down one notch to richen the needle.

[@TheMisteraska]

Hey man, you may be the one that can help me solve my mistery. So, i have a '95 zx6r ninja, the previous owner tuned the hell out of it badly so i am trying to bring the bike back to stock. I put all the stock parts back and reset the carb on the factory settings, but the bike struggles to go between 7 and 9k rpm. Goes just fine before and after that range. I noticed that the problem increases when the bike is hot. Could it be the jet needle? Should i raise or lower the shim? Thanks

[@thejunkman]

I think you are on the right track. Just experiment.

[@mopedmessiah]

I have a little 125cc scooter that i got in a wreck on. The header pipe is only about a foot long, as the muffler broke off, so i threw a 125 main jet in it (i think it came with a 105 stock?) And just have the back half of the airbox on with no restriction besides the open to outside tube running to the carb, and the needle is in its highest setting. It's spitting blue flames, and in the low range, or if i quickly turn the throttle back while im moving, itll miss and hesitate before going faster. At full throttle, itll sometimes cut out for a full few seconds, then take off really hard, then miss again. Can ya help me out with this? Its a Vino 125 with a cv style carb

[@thejunkman]

You are going to need to fix those other problems before you can even try and tune the carburetor. With no air filter you probably have dirt and debris in some of the passages as well.

[@petermarkowsky5716]

Hi, I think you answered my question in this video but here goes... When I'm at full throttle top gear, after a few minutes at high revs the bike totally cuts out like it's not getting any fuel at all. I have to back off the throttle to about 3/4's to then hold it at its top speed, which is now obviously now less. Is this too lean? Brand new carbie with needle in position 1 at top. Cheers

[@thejunkman]

You answered your own question in your response. Short answer is yes.

[@cHAOs9]

Im not a pro but if its running fine at wot for a couple minutes before theres a problem, it sounds like your carb is using up the fuel in the bowls faster than the pump can supply it. I would replace your fuel filter. Also check your fuel pressure while at wot and revving and see if your pump is keeping up.

[@Gogetter0118]

when my bike did that, the main jet was too rich You should start in the middle clip on the needle, tune your main and pilot jet then use the clip for fine-tuning at the end You need to get in the habit of looking at your spark plug, I do after every ride for tuning. You can even hold the throttle position where you're having trouble, ride around a little bit right there and then look at your spark plug it will tell you what's wrong

[@norwichscott]

Hi, I've recently replaced my stock non adjustable needles with some stock adjustables... my bike idles ok, but it bogs down midrange and clears itself the wider I open the throttle. The plugs are also black. Would moving the clip up the needle help this?

[@thejunkman]

Yes it should help.

[@barcafilms2000]

I have this problem with my Rmz 250. What would I have to do?

[@GamerGeneral99]

Hey man! While cruising at slow speed in a higher gear, the bike bogs if I fully open the throttle, like a lean condition. However, the bike misfires/pops under full throttle at top gear especially with a pillion, sort of behaves like rich. Any suggestions?

[@camgearmoto2260]

Diaphragm issue

[@randomvideosman938]

I bought a brand new Chinese pz20 carb it idles perfects but Boggs at higher rpm and when quickly pulling throttle.

[@thejunkman]

Richen the overall jetting

[@randomvideosman938]

@@thejunkman I’ve tried the air fuel mixture. It didn’t seem to help much. Could it be the needle needs to be moved one notch?

[@thejunkman]

What do you mean you have tried the air fuel mixture? If you mean the screw, that doesn't effect the jetting past idle speed. You need to change jets.

[@donay3773]

My carb does not have a vacuum it a mikuni vm22 22mm will it be the same?

[@thejunkman]

What do you mean does not have a "vacuum"? The VM22 is a piston type. This video applies to all types of motorcycle carburetors. The basis of this video is to explain how the fuel and air are flowing through the carburetor at different throttle openings. Thus with this knowledge you can pinpoint the adjustments that need to be made.

[@dedeabsrn8044]

Automatic choke does not activate in cold engine. What is the reason? How do I set it.?

[@thejunkman]

Usually that is an electrical problem as there is a wire/heating element involved (on automotive type) Otherwise there is a bit of a linkage that might be stuck that operates the butterfly(s).

[@kaylaimey9298]

So the pilot jet is keep open all the time? At low, med, high rpm?

[@thejunkman]

It is, however once the main jet opens the fuel level starts to dip below the pickup of the pilot jet making it not a factor in jetting much above idle speed.

[@t-g4m1ng]

Hey Junkman, I own a Kawasaki Vulcan 400 Classic. It recently had a problem with front cylinder. We got it fixed, but in the doing, my mechanic said that my old carburetor needs to be replaced. It was an old school manual choke carburetor. Now I have an "electronic carburetor" which supposed to work but does not since it's electronic choke is not connected. Here is the real question. Recently I ended up opening the air filter box and saw that my carburetor and air filter is not compatible with the air filter box. The end of carburetor at the air box has gap in between and the air filter itself does not fit and is cut to make it somewhat fit. So, does that gap between the air filter hose and carburetor affect how the motorcycle run? (Not even mentioning the air filter)

[@thejunkman]

Yes, and not to mention that unfiltered air has been ruining your carburetor and cylinder.

[@t-g4m1ng]

@@thejunkman Hey man thanks for the reply. I ended up removing the current carburetor. As far as I have researched, it is a "Keihin" carburetor for probably 200cc. I see it has a electric choke but it is not connected either. Is it possible to connect the choke to ignition so I can fire up the motorcycle? Since the choke is not working, the motorcycle cannot start due to mixture.

[@thejunkman]

The electric choke doesn't work off the ignition. It is just a small solinoid that opens that closes based on engine temp, on when the engine is cold and off when the engine is warm. So what you could do is just wire a switch to power it on and off when you want the enrichment for cold start. It would be completely manual. Here is a video that shows testing, but it will help you understand how it works. kzbin.info/www/bejne/f4eveZSMhctqias

[@t-g4m1ng]

@@thejunkman Hey man, sorry for another late reply. The video link you sent me gave me the idea and I had connected my electric choke to the ignition actually. It got hot within less than a minute and it had fired up. I managed to run the motorcycle on idle for a while but then it died out. I think I forgot to connect the vacuum lines at some places. I have one more question right now. I noticed that the current carburetor is way smaller than the original one, since it has a plastic "adapter" looking part to fit it to so called intake manifold. Do you have any recommendations for replacement carburetor that I can buy off from Aliexpress / Amazon / ebay? My bike's old carburetor was "Keihin CVK32" according to some websites. Thank you :D

[@athensWEB]

my bike consuming too much fuel for little distances what should i do....

[@thejunkman]

Alter your riding style.

[@WILLY52368]

What if the carb does not have as fuel mixture screw and the bike is snappy responds good throughout throttle but on deceleration it pops and backfires and my plugs are not sooty

[@thejunkman]

It might have an air screw. See my video on that specific adjustment.

[@WILLY52368]

@@thejunkman it doesn't have an air screw but I just noticed there's a nozzle for the fuel screw but I can't get a screw driver in there. After many unsuccessful tries at getting to that fuel screw it was damage causing a pinhole in the bowl of the carb. With a little JB weld its patched and running again but I can't get it to specs. I follow recommendations but results differ. I realize tuning is a complex and time consuming process when you're a novice with no garage and limited to the work allowed on property, it's hard.

[@rmoran9999]

Where does the air come from on the air screw when you screw it in or screw it out

[@thejunkman]

From an oriface on the intake side off the carburetor

[@rmoran9999]

@@thejunkman ty

[@isidrosevier1125]

If there is some fuel coming out the exhaust and it's kinda sluggish does that mean it's too rich?

[@thejunkman]

Yes

[@isidrosevier1125]

@@thejunkman so would you recommend going lower on the main jet and lowering the needle to the first slot

[@damnthiccassboye7138]

Does the main jet affect the fuel of jet needle??

[@thejunkman]

What do you mean? The jet needle effects the operation of the main jet until 3/4 to full throttle.

[@Savage-lx5yj]

What is the purpose of the extra blue fuel line? The black fitting on the left side appears to be the fuel inlet.

[@thejunkman]

Can you be more specific?

[@Savage-lx5yj]

@@thejunkman Please pause video at 58 seconds, note the blue fuel line. What is the purpose of this extra fuel line ? It does not appear to attach to the fuel inlet. Thanks

[@Savage-lx5yj]

@@thejunkman Could it simply be a vent? If so, why don't all carbs have this style vent? How does it effect the performance? Thanks

[@thejunkman]

That is for the enrichment circuit on that particular carburetor. Not the spray bar on the inlet. There is a plunger that would open and close that only used in cold starts. It doesn't pertain to the concepts presented in this video.

[@thejunkman]

All carbs need a vent to atmosphere, just like the fuel tank, otherwise eventually fuel will not flow because of a vacuum that is created. Think of why you can hold water in a straw by putting your finger on the end. A carburetor at is core works entirely on basic physics of pressure differentials. I have another video where I build a crude carburetor out of common plumbing parts.

[@Khs_9]

i have a question. my motorcycle when start up it starts smoothly good idle. but when it reaches in 1.5k rpm it kinda coughs. and sometimes in high speeds sometimes the throttle response is delay. please help me. its a CV carburetor

[@thejunkman]

Check the rubber diaphragms for cracks or tears. How old is the motorcycle?

[@Khs_9]

@@thejunkman 9yrs

[@Khs_9]

@@thejunkman i checked it there's no cracks nor tears. but when i manually upping the piston it stacks up a bit

[@DerrickWoods-sq8fs]

Biggest problem is vacuum slide carburetors and air cutoff valves

[@johnmcgarry4191]

Waw

[@jacek1235]

I have a problem with the carburetor adjustment, the candle is clean all the time, the air adjustment does not help, the dose on the needle does not help either, the nozzles are clean

[@thejunkman]

Never heard of a candle? What is the symptom.

[@jacek1235]

@@thejunkman The spark plug is still clean even with the needle up. The nozzles are also clean

[@thejunkman]

Sounds like it is running fine then.

[@aliqadri4672]

You mean pilot mixture doesn't effect when pilot jet was flowing full fuel mixter..???

[@thejunkman]

I am guessing you are asking does the pilot effect the mixture at full throttle, correct? No it doesn't' because the fuel in the float bowl level is too low to be sucked up, as well as the pressure drop is going to force the fuel up the main jet as the path of least resistance.

[@aliqadri4672]

@@thejunkman i mean what happen to pilot jet when wide open throttle does it giving same af ratio according to what af mixture we set at idle..??

[@thejunkman]

I answered that question in the response above. No it has no effect at wide open throttle. That is all the main jet.

[@aliqadri4672]

@@thejunkman thanks dude..

[@aliqadri4672]

@@thejunkman so why company keep pilot jet up then the main jet in float bowl..??

[@ironhead320]

What causes soon as you give it gas to take off it dies doesn't bog it just dies and cuts out 150cc

[@thejunkman]

Many possibilities. Its lean for some reason is the cause, you need to find out why.

[@ironhead320]

@@thejunkman found it seems this carb has a welch plug over the mixture screw closest to intake.

[@thejunkman]

That means it is pre-set and not intended to be adjustable. First make and sure the pilot jet is clear and clean, float height is set correctly. If that all checks out do your adjustments with different pilot jet sizes.

[@ironhead320]

@@thejunkman ok thanks

[@ironhead320]

@@thejunkman I did get that welch plug out factory set was 2 turns so I played with it went to 3 turns and hell of alot better.It's more crisp and idles better so is it wise to let it there or go up on the idle jet.

[@2217Video]

Why are all the graphics blurry???

[@thejunkman]

This was made before KZbin had HD encoding.

[@franksavignac2788]

Well. Full Throttle. does not mean.. Hight rpm.... you can be WOT at 2000rpm...

[@splak_5624]

needs more jpeg

[@angusmethane3802]

Instructions unclear. Cowbell stuck inside airbox.

[@jimbosavage]

14.7 is not optimal for the engine. 12.5 is more optimal.

[@jonnnyonion]

Nope 14.7 is optimal 12.5 is on the rich side.

[@paradiselost9946]

the problem with all of these tutorials is they assume the air speed through the venturi is constant. that the RPM is directly related to the throttle setting. it isnt. thats what the emulsion tubes are for... i forgive them for being completely ignored as the idea behind them is slightly confusing, yet simple. the suction on the jet drawing fuel up is not linear with air speed, yet the air speed is linear with RPM. venturis tend to create an overly rich condition as RPM builds. by bleeding in extra air through the emulsion tube, you can regulate the suction to air speed to be "relatively" constant, and that means the fuel delivery is relatively constant. get a lil 50... do the jets... then pop the emulsion tube and solder all the holes up... now start riding it around. oh my. really good if you can stick them on a brake dyno, hold the RPM steady at any throttle setting. do plug chops... smell the exhaust... then you can start drilling new holes, at new diameters, see how it affects the curve... a wideband is handy ;) you do it on a 50 as its got so little power you actually notice these things rather than it being simply overwhelmed by using say, an R1... and yeah, little bikes are easier to work on.