Air screw adjustment Mikuni carbs only

Air screw are at the bottom back of the intake bell. Fuel screws are a total different animal and on the top in the front of the carb. Carbs will have one or the other, not both. This is JUST for Mikuni carbs with air screws. Use this information at your own discretion.

[@Nascar-zv9lh]

This is the video I needed. Thank you so much

[@raymondpoteet8920]

I love it, a nice, simple and clear how-to video without nasty loud music too! Thank you.

[@aylorengineering.5513]

Thanks, I'm just a guy doing my thing.

[@smiththers2]

glad to find this, i wasnt sure if screwed in was rich or lean. now i can tune for idle, thanks!

[@aretools]

Only at idle, that’s what I needed to know Thankyou!

[@MyOldSled]

Hi A. Great video thanks for sharing. Just wanted to mention the mikuni manual references 3.0 turns as max before the spring no longer works and the air screw can fall out in operation. My sled’s stock setting is 2.25 turns. Whats your process for setting the throttle stop screw n relation to the airscrew. Great post.

[@Kyle_gallegher]

Its not perfect advice but a good indicator appreciate it man ty

[@bmxracer2701]

Thank you. Great video

[@intrudervs]

I just installed a set of 38mm TMX carbs on my bike. They came with 3 tubes attached. One for the float bowls and the other 2 are just breathers? I can sync them by ear of course but can you use one of the breather inlets to sync a pair?

[@MW300]

I have a virago 1000 with two new mikuni tm 38 and i have changed the pilot jets to smallest available, when air screws at 1.5 turns out it runs wery rich when turning them out 3.5 turns it runs better but still rich, spark plugs gets wery black.

[@nafetstv8652]

Sir may I ask how manny turns for the mixture screw for VS400?

[@beastgirl43]

Good explanation here. How do you though determine the best slide cutaway? I have a 40 mm Mikuni VM on my 1984 Yamaha SS 440 and the jetting is very close but from idle to quarter throttle is sluggish. I was able to improve initial off idle response with less fuel screw and can close the air screw completely off with no effect on idling so I suspect my slide cutaway is too much.

[@aylorengineering.5513]

Slide cut-away is a bit difficult to assess. As it is about 1/2 of your off-idle throttle response. The other half is your needle taper. If you can't get a crisp response with the needle moved up and down, then you probably need a different cut away. On special cases, I get the richest slide available and start filing the cut out up, leaning things out until you get a crisp response.

[@DCM1234epic]

You need to go up a size on pilot jet.

[@user-eo6he7kx5h]

how do I determine if the main jet or pilot jet needs bigger or smaller.

[@sarahdell4042]

Great video!

[@cambo1200]

Great explanation, pretty simple once you understand it.

[@xxshiftlockxx]

I have a vm24-512 round slide carb. So what you're saying is if i have to back out the screw more than 2 turns i need a smaller pilot jet?

[@qwxs963]

Good information thanks!

[@ryanchenier-poulin8570]

Good information sir.

[@griffind262]

Great information

[@TheDromen69]

Got a question, the old flat slide 38 tmx have an idle screw, does the air screw have the same function as this one? (cause the idle tuning is already covered by the idle screw)

[@mephistoc9286]

No 2 fonction different

[@PetesClassicCycle]

thanks John!!

[@aylorengineering.5513]

You are more than welcome! That's why I do these vids, to help when I can.

[@asher2281]

Thank you!!! I subscribed

[@bobnewman9493]

What is the difference between a number 30 piolit ot the old ones that have a 1 in front of the number like 130

[@nybergsgarage]

thanks for the video!

[@zachattack745]

I just got a honda 230f with a mikuni swap and it lurches just off idle. I've tried a leaner and richer pilot jet and needle placement with no success. The engine rpms jump when you crack the throttle. Any thoughts on why that is? Thanks!

[@aylorengineering.5513]

Actually that is what you want, crisp throttle response. BUT, it might be you have a too aggressive throttle tube cam for the Mikuni, that is opening the carb too quickly. That would be my best guess.

[@Shadowban69]

Replace the o rings on the air screws. Torn or flattened or roached o-rings need replacing.

[@fourbyfourer]

If I have a bad o-ring in behind the fuel air mixture screw, will that cause the engine to not have quarter throttle? Will it suck air through there?

[@aylorengineering.5513]

I think you have another problem, but do get the o ring fixed, they are there for a reason.

[@jakemac1273]

Other 2 cylinders?

[@yourfactstory]

Thanks !! Same on the Mikuni vm22 clone ?

[@smokedadowg]

I wonder if you could help I have an Aprilia rs 125 2001 34mm carb bike will start and idle perfect as soon as go to ride it cuts out any help would be highly appreciated thank you

[@VOLVOb18ft]

So, how do i get the highest rpm on one cylinder? Shall I disconnect ht leads ti the other cylinders?

[@aylorengineering.5513]

No, just adjust one screw at a time. The rpm change is very small, usually'

[@CarlMont3]

My dr 650 only has one cylinder. Mikuni bst40 with 155 main jet, open airbox and I don’t know the size for pilot. While at idle I can turn the mixture screw all the way in and nothing happens to the idle and supposedly the bike should turn off if I turn it all the way in. Do you know what should I do?

[@CarlosGarcia-ls4hn]

Can you adjust the air screw with the air cleaner off and the engine off (after warm up)?

[@Shadowban69]

No

[@dominickmagoon5731]

Intake carb boot is cracked and i got a new carb but only moves when quarter choke. I open air box and cover 3/4 then it moves is it becouse of boot or do i need to adjustment on the air intake?

[@greghujing5692]

Please help Letting off my throttle downshifting in my DR 650 it backfires everything else is good but that that be my pilot jet do you small to big ?? Thanks

[@zachwood3670]

My genuine Mikuni 22mm carb wants to shoot fuel out the over flow line and the two brass things that stick out both sides with little holes on the bottom when it’s at full throttle the needle valve seals well and my float it bent upward to lower the float level when there is fuel and it still shoots fuel out of those 3 places? Do you know any fix?

[@aylorengineering.5513]

If you really think the fuel valve is sealing correctly, then it's probably one of 2 things. One is the brass tube that is for the fuel overflow in the bowl can crack and cause a leak. Second and most likely, is you have the float set incorrectly. Set by turning the carb upside down, and the bowl gasket removed, measure from the gasket surface to the top of the floats, (the actual bottom) they should be 24-25 mm.

[@pauloconnor7951]

Yep

[@juandunas2381]

I have a 1988 Suzuki savage 650 ,idle is perfect, my problem is that on low throttle is black smoke coming out from exhaust I replaced the spark plugs and still do it , I look info about it and some people say that is the main jet too big and is running rich, what do you think?

[@Randomandyprojects]

I have a 1980 Suzuki gs450L with mikuni bs34s carbs. It bogs down at high speeds/high rpms. I took the carbs off and cleaned them. Cleaned the air box and got new air filter. How do I set the carb? I’m cashing my tail when it comes to setting the air screw, I keep bogging down in 4th-6th gear. Any help/point in the right direction is appreciated!!!!

[@aylorengineering.5513]

I believe your carbs a diaphragm type, which are a different animal. But anyway, your air screws have very little to do with any high speed stuff. That would be your main jet. Or you might have a mid jet also.

[@hillbillymitchswhitetailst6215]

Probably your problem is your valve clearance.or maybe even your camshaft is worn

[@jakelang4387]

on my 1982 gs 750 ..they have them on the top front..and i had to turn then in to richiñ it ..if i turned them out it didnt give it enough fuel

[@aylorengineering.5513]

I think you need to look at your manual. It's possible, but I have never heard of a fuel screw adding more fuel by screwing them in.

[@timmberthy]

i have a 2001 polaris super sport 550 fan cooled how would i set the air screws again ?

[@aylorengineering.5513]

If it's a Mikuni carb, same as the video.

[@Ghost1ssp]

Kk I’m confused I see other videos explaining the full turn on air screw, is it a full turn from 12 o’clock-12 o’clock or is a full turn from 12-6 ?!?!

[@beginnerrider6681]

Hey can you tell me how to seat a air mixture screw on a Yamaha 2006 TTR 90 electronic start Mikuni carburetor because I remove a Welch plug to put in a new air screw and now it doesn't want to start

[@aylorengineering.5513]

Sorry buddy, no clue on the newer stuff. Vintage is my game.

[@kelumrupasinghe4051]

Pls help.I have a honda cd 90 similar engine to honda c 90 i think. Could you pls tell me how many turns do i have to turn anticlockwise this airfuel screw .also pls tell me when turning clockwise this screw what will increse ,air or fuel ? Thank you so much.

[@aylorengineering.5513]

I would Google your factory manual, or find a message board for your bike. I'm not really versed in those particular carbs. Sorry I couldn't be more help!

[@peeinginthewind]

Question I have that carb on my 95 polaris 500 indy .. it idles about 1000rpm cold when first started for the day but once its warmed up it stalls out everytime u try to idle .. so I can turn up both screws to bring up the idle ?

[@aylorengineering.5513]

You want to adjust your idle screws, not your air screws. Cold start ups should have a choke feature of some sort to raise the idle. So I would set the idle when the motor is at running temp.

[@peeinginthewind]

@@aylorengineering.5513 okay thanks ! I know this sounds dumb but the idle screw has the spring on it right ?

[@aylorengineering.5513]

Well, all adjustment screws have springs. Refer to the video for the placement of the air screws.

[@rooster68able]

Effects off idle also lanch

[@salvadoribarra4551]

Would that be the same for a hsr42?

[@allincamo]

What will cause a cough back fire . At low acceleration.

[@kevinandmel300]

How do you set the idle adjuster. What’s the factor settings?

[@aylorengineering.5513]

Depends on the specific carb and engine. You need to look up the spec for yours.

[@chuckspeer2163]

Thank you

[@StokesSibs]

Hi, my mini bike starts to run, without touching the throttle, it’s won’t stop running either, even when I press the kill switch and turn off the ignition. After a new minutes it eventually cuts out by itself, but I need to hold the breaks until it does so. Do you have any ideas what this issue could be please?

[@Platinumtag]

Idle too high...... Should have put in neutral.....

[@dillonwhite4444]

I’m having a big problem with my Suzuki 230s quadsport. I know it’s a carb problem for sure, My gas is going into the oil reservoir and making it unable to start. It smokes when it’s starting to fill I don’t know what could cause that.

[@aylorengineering.5513]

I don't know specifically about your motor, but I would take an educated guess that a check valve in the oiling system is stuck open, or faulty in some way.

[@Lucas-vk8fz]

Little late but could be a stuck float perhaps..I've seen that a few times

[@FreakinSteez]

thank you

[@gordwiesner3753]

Perfect!

[@Lakatancastle]

Help bro. Ihave fzr1000 1986 model and using mikuni air screw only. How many turn do i need to adjust. Thanks bro.

[@aylorengineering.5513]

Mikuni usually like to have their air screws in the 1 1/2 to 1 3/4 turns out. Now sometimes factory carb setting are different. So it would be best to find a manual online and find the spec. And remember, fuel screws are a different animal.

[@glen8447]

My makuni only idles correctly with the air screw 1 and 1/2 turns out and the choke pulled out. It will slowly quit if I push the choke off. I don't understand why I can't adjust it to idle smooth with the choke off. Any Ideas, it's a like new 2017 ktm 250xc, only ridden about 4 times.

[@aylorengineering.5513]

Your pilot jet is clogged.

[@glen8447]

Thank you, I drained old gas out of my float bowl, so I'm sure you right.

[@rondavis7980]

Is the what is the adjustment to the other screw supposed to be the one that's not the air screw

[@ryanchenier-poulin8570]

I have a carb that floods itself. Are my slides to high ?

[@aylorengineering.5513]

I would look for a worn out fuel needle or seat. Or something lodged in there. a sunk float, or stuck, float.

[@garrettpolzin7235]

I have an 82 Gs650g and it has these carbs but I don’t see this screw anywhere. This bike doesn’t run it has rebuilt carbs and I have ran through a very long check list on things. Anyone have any ideas???

[@aylorengineering.5513]

Well, unless someone adapted flatslides onto your engine (very unlikely) you have the stock carbs. The air screws will be hidden under a plug, and most likely they will be fuel screws in the front of the carbs.

[@LucianoFirst]

My cr125 2001 leaks oil from the exhaust tip and its running too rich smoking too much, will this air screw solve it?

[@aylorengineering.5513]

The air screw is really for setting the correct mixture at idle, and not much else. So your problem is with your oil mix ratio or your jetting.

[@skyb8687]

Leaky rings ,but thats pretty common with cr125,potentially you can change your pilot jet or needle clip.if you have aftermarket gear on there it effects the fuel ratio.if your bike is stock(stock pipe ,air filter ) then make sure your running the stock plug and stock jets in the carb.which u can look up.anyhow I hope this helped

[@AkinaGod]

Just got a new Mikuni for 2 bikes. One wouldn't' start and flooded right away. Tried resetting to factory, and still floods. I suspect this has to do with the idle. How do I adjust the idle back to a factory setting? The 2nd one worked fine when it first started but slowly started to bog down. I tried adjusting the idle, and it picked back up. Eventually it started to bog down again and I tried adjusting the air screw and it flooded. This bike also, before flooding, would bog down if you revved it at any small throttle amount. I'm currently waiting for it to clear out for the 5th time. What am I messing up?

[@aylorengineering.5513]

Sorry about the late reply, but most all Mikunis, have the idle screw right on the side of the carb by the bottom of the slide. Turn out to slow idle.

[@robhoward35]

Fouled sparkplug.

[@readylucas]

My adjustments are way off. Fidgeted with the carb and now bike wont even start. For the air screw can i screw it all the way in? And feul screw should i turn that in all the way and out 2 turns? Feul screw is my main issue but when messing with it i forgot where baseline was.

[@aylorengineering.5513]

You need to find your baseline for fuel screws, for your particular motor.

[@readylucas]

@@aylorengineering.5513 turns out my pilot jet was damaged. Idk if it happened when i cleaned it but i slapped a new 26mm mikuni and it fired up. Feul screw 1 1/2 turns out

[@readylucas]

Tried tuning it multiple times but it wouldnt run no matter what because the pilot jet was damaged. I think it happened when i cleaned carb but not sure..

[@highlightzilla9581]

Ty

[@erdalerdul]

is this Mikuni BSR36? Because mine is different. I have DRZ400SM 4 stroke.

[@aylorengineering.5513]

Then you probably have a fuel screw, not an air screw. I don't know all carbs.

[@rupturemetal5471]

If the bike backfires from the front cylinder while idle, does it require PMS screw adjustment? Currently both carbs PMS screws are set to 3 turns out. Can synchronize the carbs resolve the backfire issue? Thanks again for the video.

[@aylorengineering.5513]

The carb adjustment would have to be wayyyyy off to cause a backfire. I would look at the ignition first.

[@ahmeddanka4978]

what do you mean? ignition coil?

[@LucaMastrolacasa]

On my lt125 when i start or prime the fuel it leaks out of the overflow maybe a stuck float how do i fix pls help thx

[@aylorengineering.5513]

If it is a stuck float, you can first try to tap the carb bowl with a screw driver handle while running to see if that seats the fuel needle. If not, just pull the bowl take out the needle and clean it and the seat. make sure the float moves freely.

[@woodyWoodTimber]

@@aylorengineering.5513 sir is normal on yz 125 1986 my needle flot dont hold on those little metal wire ? Needle is just in the hole and and around that tube i have a pin that hold the flot first time i see that

[@aylorengineering.5513]

@@woodyWoodTimber That is probably specific to those carbs. And I have not dealt with them. So I can't help you there. I'm sure there is a breakdown on the internet somewhere....

[@DenzelWadeJohn-ej9gv]

thanks

[@joshsutcliffe6099]

Is this ONLY for idle. Im having an issue from 3/4 throttle to wide open. Bike just falls on its face. I believe she's starving for fuel.

[@derekfox6103]

That would be your main jet from 3/4 to full throttle

[@anthonywhiteaker8599]

That is not at all what it says in the mikuni HSR45 manual as far as your air screw being more than ONE full turn out or at which point you should main jet up or down so I'm STILL confused. And where should the idle be set when starting the procedure because not one person who supposedly knows how to tune these carbs ever even so much as mentions that.... Frustrating

[@ninoechavez1402]

Hi man. When i adjust the airscrew on a certain position, it seems to freeze out or hangs the idle. Is that normal or a lean condition? My best position is at 3/4 turns which erases the hanging idle. But it also means i have a small/lean pilot jet and have to go up the next number higher right as per your video? Carbs are really giving me headaches hahaha i got confused if the hanging idles are also from an air leak.

[@aylorengineering.5513]

You shouldn't get a hanging idle from the screw adjustment, just a idle speed change. You do sound lean. Maybe by 2 sizes.

[@trikelyfe6090]

My moto4 225 will start but runs away any ideas?

[@aylorengineering.5513]

Either an air leak, or stuck throttle cable or carb slide.

[@damacknificent151]

Hi, i have a 2000 yamaha road star 1600 with aftermarket exhaust and air filter. The bike is running lean. Do i need a rejetting kit or adjustment? It has a makuni carb. Thanks

[@aylorengineering.5513]

I would say you need a jetting kit if they are out there. You could spend a fortune and many hours figuring out what they already did.

[@ryanslapz]

Do you do anything with the idle screw?

[@aylorengineering.5513]

In what way? After getting everything in spec, you might need to readjust your idle. Just be careful to adjust all idle screws equally.

[@arloslab3469]

help!, my yz125 has a nasty bog to it (mikuni tmx 38 carb) if im riding in high gear just cruising along, if a slam the throttle all the way down fast the bike bogs, but when i gradually increase the throttle bike pulls fine. Please HELP

[@aylorengineering.5513]

2 things, first of all has it always done it, or have you changed something? And second, and most likely, the MX 125s are highly tuned and really only make power in the higher rpm band. What you are doing is trying to get the motor to go, out of the powerband, as most tuned 2 strokes just go flat if the rpm is too low. If you are just crusin and want to hit it, downshift once or twice to the get rpm in the area the motor is making good power.

[@arloslab3469]

@@aylorengineering.5513 Ok thanks, no the bike has not always done it, but the top end has 90 hours on it

[@aylorengineering.5513]

Have you done a compression test?

[@arloslab3469]

@@aylorengineering.5513 yes, bike has 110 psi of compression, why?

[@aylorengineering.5513]

90 hours can cause a lot of wear. Is 110 psi close to stock? Low compression will make a motor doggy.

[@sevinkadler]

my mikuni only has adjustment screw and it turns in like 6 times before setting ... how do i adjust that?

[@aylorengineering.5513]

What kind of Mikuni?

[@DarkIzo]

6 full turns or half turns ? any chance of trying while the bike runs ?

[@sevinkadler]

Mikuni 22 for predator 212cc. I found the fuel screw I think near the block under the inlet. But the afr screw turns 6full turns into the slide

[@aylorengineering.5513]

@@sevinkadler Into the slide? I "think you are turning the idle screw, not the air screw. The air screw is behind the slide, they do not go into the slide.

[@williamedwards7315]

I feel like this guy would tell me to stop turning air screws I wish he could set my air screws on my sled but I think I know how to do it now

[@2dollars833]

How many turns for 1 and half from closed 5 turns

[@aylorengineering.5513]

I'm not clear on your question. But one and half turns is with the air screw lightly screwed all the way in. Then backed out one and a half turns out.

[@2dollars833]

Thanks

[@eliking7391]

@@2dollars833 22.5 turns.

[@NouriGains]

What about if you can't find the idle, but the engine starts every time when you crack the throttle?? I have my idle screw all the way in, and still can't get it to idle.... 2 screws on a/f tried different pilot jets, but I think I'm still rich??

[@Jaxson8157]

Do you have an independent idle adjuster? That would most likely be your issue.

[@HaveUSeenMyBaseball86]

At the bottom of the bowl. What I call a drain screw. What is the actual name? Because I cant find the replacement for mine.

[@aylorengineering.5513]

I'm not sure the actual name, any Sudco dealer can figure it out for you. It is actually the main jet access plug.

[@HaveUSeenMyBaseball86]

@@aylorengineering.5513 it's called the Air Screw set. Yamaha changed the name from 2005 to 2007. Before it was called drain plug. Not sure why.

[@HaveUSeenMyBaseball86]

I'm very correct. Lol not sure why they call it an Air screw. When they already have something similar also called an Air screw.

[@aylorengineering.5513]

Well, that makes no sense, but you have your answer!

[@khadeersongs8208]

Yamaha fz 2009 model how many turns

[@aylorengineering.5513]

That would have to be a shop manual question. I'm sure you can fine it online my friend.

[@jamiebouchard9862]

Tough to hear a high idle per cylinder on a triple.

[@Trent960]

You have to know which carb is effectively hitting which cylinder listen to the cylinders individually if you can use hearing protection it helps.

[@jamesa6272]

This confuses me. If you set the idle and get it idling good then you go to rev it and it’s not clean obviously you still have to mess with the air screw. Why do people say it’s only for idle.

[@roccodimauro3116]

I’m having the same confusion the guy said it’s only for idle but for your situation it might be the main jet

[@jamesa6272]

@@roccodimauro3116 yeah I gave up on the mikunis and put the stock carbs back on

[@RickyRisnandar]

so clockwise=lean?

[@Shirley-hr8tx]

Clockwise (in) to enrich out leans

[@jacob14163]

Is one turn=180 degrees?

[@Ghost1ssp]

Broooo right I’ve been trying to find out cause I see them start 12 o’clock - 6 and other videos 12-12 is a full turn…….IM CONFUSED AS HELL

[@jacob14163]

@@Ghost1ssp 180 is equal to 1/2 turn. 360 degrees will be 1 turn. I reached out to Murray's carbs. He's an encyclopedia on mikunis

[@ngm3673]

My specs say 1 7/8 turns

[@LosVRC]

Bought a used Rm125. Put in a new Wiseco piston, and ruined it during break in....my fault. I didn't check the carburetor. It was way too lean.. .lesson learned.

[@aylorengineering.5513]

Sorry to hear. that sucks!

[@phoenixvids]

Mooooooo!

[@kamarulazizi4733]

I have mikuni vm26-9504 . I don't know it fuel screw or air screw . How to know it ?

[@aylorengineering.5513]

fuel is in the front of the carb, air is on the back of the carb.

[@Whitemale69]

Yep

[@stovenhd9665]

Yep

[@YorHighness]

Yep