23.01.20

Every carburetor know to man will have a way to adjust the air fuel ratio at idle. This adjustment will trim the mixture so that you can have a smooth running engine.The first step is to determine if you have an AIR screw that meters air, or a FUEL screw that meters fuel. They work opposite of each other so it is important to know that before performing this procedure.A carb will have an air screw or a fuel screw, never both. The GoalWhat you are after when adjusting this screw, is too reach peak idle RPM and smoothness. This is the engine telling you it is happy, and it runs best at this mixture.

How do you tell if your pilot jet is too lean, rich, or just right?The pilot jet is sized correctly if the fuel screw setting falls between 2.5-3 turns out.The pilot jet is sized correctly if the air screw setting falls between 1-2 turns out, with 1.5 being the sweet spot. What does a pilot air screw look like? What does it do?A pilot air screw will be made of brass, gold in color, and will have a blunt tip compared to a pilot fuel screw. The pilot air screw meters air, so turning it clockwise/in will richen the fuel/air mix, while counter-clockwise/out will lean the fuel/air mix.What does a pilot fuel screw look like? What does it do?A pilot fuel screw will also be made of brass, gold in color, and will have a pointy “needle tip”. That is why you always want to “lightly” seat these, the tips can easily break! I use the smallest diameter screwdriver so I generate the least amount of torque.

The pilot fuel screw meters fuel, so turning it clockwise/in will lean the fuel/air mix, while turning it counter-clockwise/out will richen the fuel air mix. Where is the pilot air/fuel screw located?Let’s define the carburetor butterfly or slide the dividing line between the air box side, and the engine side. A pilot air screw will be located on the air box side, while a fuel screw will be located on the engine side. This is true for most carburetors. I’ve seen the pilot air screw on the engine side on most Amal carburetors. When in doubt, remove the screw and see what kind of tip it has, that will determine what it meters.If you can’t find the screw, it may be hiding under a welch plug.

Hi,I have an 82 Susuki GS650G, Canadian Model. The previous owner replaced the old air cleaner with four individual cone filters letting through allot more air. He also put on quieter exhausts. When he sold it to me he mentioned that I needed to increase the Jet size, Didn’t tell me which jets, and that I needed to raise the pins.

Due to time problems I have yet to do either. Could these be the reason my Engine just randomly dies while on the Highway? Or could there be something more serious involved.Thanks in AdvanceMichael. Michael,The pod filters may or may not let more air in at idle.

They do let more air in at higher RPM’s.Let’s start with the pilot jet. If your fuel adjustment screw ends up in the 1-3 turns out range, and have no bogging or hesitation off the line, it’s good to go.The needle clip position and main jet is something you will have to test trial and error and read the plugs. Usually, I would expect the clip to be one position lower than stock and the mains 1-2 sizes bigger than stock. Get a hold of a service manual and note stock jetting. Open up your carbs and write down what you find.Randomly dies on highway. Jetting may or may not be related to this issue. All things being equal and no other apparent or obvious issues/problems, the most common reason a motorcycle inexplicably “dies” on the highway (and nowhere else) is fuel starvation due to empty float bowls caused by clogged filter screens above float bowl needle/seat intake valve assemblies.

Remove the needle/seat assembly and use a small screwdriver or pick to remove the tiny little filters and clean them. If you can park the bike on the shoulder for 10 minutes after it dies (giving fuel time to refill float bowls) and then start up and run fine again (for awhile) dimes to donuts, that’s your problem. It only shows up on the highway because the increased fuel consumption at highway speeds outpaces the fuel flow slowly dripping into the float bowl(s) through dirty filters and empties the bowls before they can refill.

The most common reason for this condition is rust in the gas tank. Tiny particles can get through the fuel filters above the petcock (which also needs to be cleaned) or other inline filters (replace). People park their motorcycles outside, especially in the winter, with untreated bare metal interior, part-empty fuel tank causing the inside surface of the tank to rust.

Warmer days and colder nights affects air pressure in the empty space above fuel in tank which expands and contracts drawing in cooler moister air in the evening and overnight through overflow/breather = rust. I watched the how to about adjusting fuel/air mixture screws. I am currently tuning my carb and would greatly appreciate some guidance. I’ll give some info first.2012 klr650 w/CV40 carb,just made changes from stock:drilled slide 7/64klx needle 2nd clip 1 #4 shim142 jet (from 145)new extended fuel mixture screwslip on mid pipe back Lexx exhaustrace tip (big unrestricted one)original tube spark arrestor swapped with free flow Spark arrestor1″X 4 holes in air boxnew uni filterWith all the changes, the bike transformed in attitudegreat throttle response, and it just sounds mean! Only thing is, I’d like to get it dialed in nice. It’s a little over poppy on the decel( I expect some ) and the fuel mixture screw can be all the way seated and it doesn’t slow down idle.It does run pretty good across the spectrum of adjustment, but better towards the closed side of adjustment.

I did not change the pilot jet.Any advice on getting this carb tuned in would be appreciated!Again, nice work on the videos, real clear information for people to understand. Hi, Michael.First off, thanks for the compliments, and the KLR is a cool bike BTW! I do quite a bit of work on a 87′.Popping is to be expected with an open “race” pipe. You can get rid of it by richening pilot/fuel screw, but sounds like you need to go the other way, leaner, based off your fuel screw position.Since your best idle is at almost closed fuel screw position, I would decrease pilot by 1-2 sizes and see if you can get fuel screw in the 1-3 turns out range.BTW, can you give me a link to the adjustable fuel screw?

It’s impossible to adjust stocker. Hi I am having trouble getting my quad to run and I’d like some advice.It is a 2000 Kawasaki Bayou 220, we cleaned the carb in a dip tank andmade sure everything’s cleaned well, when we start it we use fluid becauseit wont start on its own 50% of the time.once it starts it idles for maybe 2-5 seconds and it dies, we’ve triedtapping the throttle to keep it running white we fiddle with adjustmentsbut we have found no success, currently it sits at 2 turn out on the A/Fscrew, and the idle screw does nothing regardless of position.We’re at a complete loss as to what to do with it. I’m really hoping youcan help me because I’m losing my mind with this thing and I’ve got 3more quads that wont run to fix still. And those are:a 94′ bayou 220a 88′ Blaster YFS200a 87′ LT250R QuadRacerI’m sure I’ll have more questions for you since all of them are havingcarburetor related issues:/. Hi,The first time I cleaned out my carbs I didn’t know I would get them out of tune. I got it working really well afterwards, a lot of power, but I heard a clicking sound after about 2 months and I thought maybe it was something I did in the carbs.

Polaris Mikuni Tm38 Carburetor Air Screw

I opened up my carbs once again and cleaned them. When I put it back together, the pilot fuel screw wouldn’t make a difference when I would turn it and my motorcycle only runs max 55 miles now and has very little acceleration. I can’t seem to adjust it. My pilot fuel screw is different from the picture though, mine has a spring on it and it screeches when I turn it. Supposedly its normal, unless it’s not a fuel screw.

Should I keep playing around with it or throw it at the junkyard? I don’t know what to do. If I can’t fix it myself, I don’t want to keep it. I don’t trust mechanics, I took it to one and he charged me 400 dollars and there was no change to it.

He didn’t even give me a receipt. Supposedly he worked for Universal Studios fixing all kinds of machines anyway thank you for help.

Oh and I didn’t know about the air screw. I don’t think I ever saw that on my bike, is that inside the carburetor? Do we adjust that too??Thank you,Allan.

Hello, i have a kawasaki mule 2002 with the 620 motor. It never has run right at idle so after some time replaced carb with a new one from kawasaki.

The new carb seemed a little better it gave me some idle adjustmant for a while but still seems to be quite lean at tickover. Now it has the idle screw right in and strugles to idle and stalls.

The top end revs seem good and there is power. Also when punching the throtle it bogs. I have looked on various sites and they say there is only one size of pilot jet and there non adjustable due to some regulations?. I have a 97 gsxr 750 sport bike that has 4 CV carbs. Do the setting for them have to be the same or different. I know the bike is running rich for sure(throwing fuel out) and it has a off idle hesitation. Happens a lot while cruising around town.almost feels like I’m riding a horse.

I have tried every gear and all different rpms and its no better no worse. Seems like any gear or rpm if I hold the throttle at the same spot it hesitates. It also does it really bad if I pull in the clutch then get back on the throttle lightly.

My manual says crazy numbers for the factory setting on these carbs. Its something like 4 1/4 outer and 2 3/4 inner. Runs terrible at those settings. I’ve got mods and the carb was professionally jetted.

But it was done at a way different altitude then where im at now. So I’m hoping to just adjust the mixture to fix it. Thanks for the reply man I really appreciate it. The bike has a full Yoshi exhaust header and silencer. The silencer has been cut shorter as well. It has an ignition advancer +2 and k&n filter.

My main jets are 132.5 outer and 130 inner. Stock is 130 outer and 125 inner. My needle is on the middle clip with a shim under it. And my pilot jets are all 2 steps up from factory which is 12.5 so they are all 14.5s and my mixture screws are all messed up. Bike runs amazing if I’m flogging it. But off idle it has a stumble I guess.

How can you tell a stumble from a hesitation? I’m 100% sure its not a bog let’s just put it that way lol. And by throwing fuel out I mean if I rev it I get some black smoke just a little bit but then it puffs a lot more once it comes down to idle.

And I had it at the drag strip last night and right at every shift it puffed black is what my brother was telling me. I’ve tried every setting I can try with the mixtures besides going way in like between 1-3 because of what my manual says they should be at stock. Should I just disregard the manual? Thanks again for you response I really appreciate it!. OK so I did what you told me to do.

My highest and smoothest idle was between 1 1/2 and 1 3/4 the difference between those two settings was about nothing. Any lower than 1 it starts to idle rough and stumble I went further to 3/4 and it was almost gunna stall. And out if I go any further than 2 3/4 it idles rough and 3 or further it is about to stall. Hope this tells you something. Havent taken it around the block yet because at this setting the bike idling in the garage needs the fan to stay at 225 degrees and that seems higher then I feel comfortable with. Should that high of a temp worry me? It is liquid cooled with a fan switch if I needed it but that made me weary.

OK so I did what you told me to do. My highest and smoothest idle was between 1 1/2 and 1 3/4 the difference between those two settings was about nothing. Any lower than 1 it starts to idle rough and stumble I went further to 3/4 and it was almost gunna stall. And out if I go any further than 2 3/4 it idles rough and 3 or further it is about to stall.

Hope this tells you something. Havent taken it around the block yet because at this setting the bike idling in the garage needs the fan to stay at 225 degrees and that seems higher then I feel comfortable with. Should that high of a temp worry me? It is liquid cooled with a fan switch if I needed it but that made me weary.

I took a spin around the block and it is going in the right direction for sure. I had it at 1 3/4 and it still stumbled just a little bit here and there. So I took it for a long ride. Temp is 70 degrees here and the temp at 60 down the highway was about 170-180s and in town it would creep up to 200+ one thing I did notice is that I was able to slowly ease off the throttle without it bucking and kicking and it was much smoother at cruising speed in 6th gear at 60 giving like a 1/16th throttle. So I’m confident were heading in the right direction. I really appreciate your time and expertise!

I’m gunna go down to 1 1/2 and see if its better. I’ll keep you posted. Brent,OK, let’s talk about this stumble. We have to determine if it’s too rich or lean. If you add more throttle during stumble does it go away? If so, it might be a rich spot. If it is a lean bog, more throttle will not help it recover.You can also install new plugs and go ride it around and cause it to stumble over and over again and pull plugs to read them.It seems this stumble is occurring when on the pilot 100%.

The fuel screw setting at idle trims the pilot fuel, and considering the fuel screw ends up at 1.5/1.75, the 1/8 throttle position (all 100% pilot flow) will be rich. You might want to drop a pilot jet size. More throttle does make it go away. But different gears and different rpm doesn’t.Its a really weird kinda studder I guess really. It almost seems like a misfire in a car. If I hold the gas at the same spot while its doing it I can get it to buck and kick and carry on as long as I want. Sounds like wah wah wah wah wah all the way down the street until I give it a little more gas.

I have tried a plug chop before to no avail. But then again I was also like 4 full turns further out on the mixture screwes thanks to the Suzuki manual lol. Thanks for all your guidance. I’m gonna try 1 1/4 and see if its any better or worse. I’ll keep you updated. My idle was below 1000 rpm when I made the fuel screw adjustment.

But right when I was done I put it right back by habit lol. I’m thinking when my fuel screws were super rich I had to idle it up to keep it even running. Because now that I leaned them out I can idle smooth at way lower rpm. Before if I tried to idle lower it would surge the whole time. So I’m wondering if that would be opening that un-meatered hole and causing my stumble this whole time. Once it stops raining I will test my theory and let you know.

So the weather in Ohio sucks and its been raining or to cold to ride ever since my last comment. But finally I was able to go another 1/4 turn in and take it for a ride. So here are my findings. Idling lower like in the 1100-1200 rpm range it is almost perfect at 1 1/4 turns out on all 4 screws. Any higher and the hesitation comes back. I also noticed something else that is different then before. The tip of my pipe Is turning brownish tan instead of black the way its been since I got the bike.

So I’m thinking that is a good sign. I will be taking it to the track tomorrow to see if its any faster or easier to get off the line and will report back. Thanks so much for your help Matthew I really appreciate it!. Sounds like you are headed in the right direction. So the stumble was probably a sputter, not hesitation? I say that because you went leaner and things improved.Now that your fuel screws are nearing 1.25 out, you can get a leaner cruise (1/16-1/8) throttle by dropping pilot jet size.

You will have to readjust fuel screws and they will end up about 1/2 further out. If you decide to do this, your will be at the same fuel ratio and the bike will be getting a little less fuel when 100% on pilot.Does that make sense?.

I have a 1999 Honda Shadow Aero 1100 with Shotgun pipes with the baffles cut down from 12″ to 4″. About 2 months ago I had the carbs cleaned due to a drop in fuel mileage, from 120 miles to about 85 miles. Since the was done, my mileage has increased but, when starting from a stoplight, there is hesitation and an occasional stalling. Also it is now backfiring in the pipes on decal that wasn’t there before the clean up. I am not mechanically inclined and the mechanic that originally did the work in no longer in my area. What are your thoughts on this. Thanks in advance.

I have an 82′ Yamaha xs400 heritage specialIt hasn’t run since I got it, its got a brand new battery, the carb was recently cleaned, but before I got it he took out the air filters and put in foam air filter coneswe got it to start and run for a few seconds while holding the choke but then it would dieI’ve tried adjusting the pilot screw and it hasn’t done anythingthe other thing that concerns me is the rubber hoses connecting the carb to the engine are slightly cracked, could that be an issue as well?. 1985 honda shadow 700.I have k&n filter, and the chamber between muffler pipes has been removed and holes closed off.I have 135 mains, and 45 pilots i believethe popping is finally gone (about 3 turns out on pilot screw). Or would popping on decel be normal with that exhaust setup?Bike tops out at 95mph, when it could easily do 105 last year before i played with jets to get rid of popping.

And also the bike stalls when gearing down and coasting to a stop sign at times (never used to do that). Buddy said it smells rich when he rides behind me. Is this a pilot issure or high speed jet issue. And will pulling the baffles help to correct the problem by leaning the mixture out a bit and allowing me to turn a/f screws in without having popping. Or is popping something i have to learn to live with?Hope that didn’t break your brain. Matthew, first off, great site and really great and easy to undrestand articles.Secnodly, I have a problem.

I have a 2008 Yamaha XT125 with a 150 ccm big bore kit and an Arrow exhaust and I’ve recently fitted a Stage 6 R/T Dellorto VHST 28 caruburettor on the bike. The Dellorto carb doesn’t seem to use the same system as is described in the article as it has a metal Air mixture screw with a spring on it (I could link you a picture for a better idea, but I’m not sure if your site allows it).My problem is that I have this screw screwed all the way in and the bike behaves very weird in idle/low RPM. In idle, the bike is very loud, doesn’t run smoothly and sputters a bit, but when I give it a bit of throttle (around 1/8, it sputters quite a bit more, sometimes the RPM fall down a bit and after I let go off the throttle they hang for quite some time and decrease very slowly back to idle RPM.I’m guessing I’ll have to buy a smaller pilot jet (idle jets as they are called on the manufacturers website).Any ideas/suggestions?.

Polaris Mikuni Tm38 Carburetor Air Screw Setting

Hey John,You mentioned vacuum leaks. Are they all gone now? If not, you have to take care of them.You should be able to get carbs synched perfectly, since there are only 2 of them, it’s even easier. Hook both gauges up to one carb to see if gauges both read the same.

If carbs are hard to synch or won’t synch, then you have more vacuum leaks.The fuel screws should be responsive. They should both be turned out the same amount. If you can close them and engine still runs, the pilot jets are too big.An IR temp gun can be handy for aiming at pipes to read temps and help understand what is going on.

Hey Matthew, finally got the jets. Well it finally dies when I screw the mixture screw all the way in. I have it out by about 2 turns now and it runs quite smoothly. The only issue I’m still having is that the bike is quite prone to stalling when I add very little gas (as in from a standing start at a traffic light). Also when I’m riding it at about full gas and high revs, when I let go off the gas the bike is a bit nervous, makes a bit of a different sound and likes to kick forward a bit as if there is still some extra gas that ignites in the cylinder even though there shouldn’t be any (if that makes any sense written like this). Both of these symptoms were worse with jets that were too big. Do you have any idea what could this be?.

I’ve read your link and I think it makes sense, especially seeing as it almost always gets flooded when going from half to 3/4 gas even though the needle is in the lowest position and the main jet is the right size (no problems at full gas, high revs).The parts seller gave me info on which the 4 stroke parts should be (the atomiser with its bush, if I’m not mistaken, this is supposed to be the needle jet) although they aren’t sure whether the bike will run better with these 4-stroke parts.I think I’ll go for it, hopefully it will finally work as it should 🙂. Hello Matthew,It appears that I have solved the air leaks ( no engine rev rise spraying carb cleaner now), Carbs are synced perfectly, I put my exhaust gas analyser in each exhaust pipe in turn, I then put both fuel screws too two turns out and the front cylinder is ok ( the front one is popping/chuffing at idle ) (before I put the larger baffle in and baffle wrap the front exhaust popped/crackled and banged ) but the rear cylinder is showing way way too rich so I turned it in one full turn and the reading came down significantly but still richer than the front.

Turning the fuel screws in or out makes no difference to the engine revs at all, you cannot hear or see rise on the auxiliary tacho fitted for the purpose of tuning.I`m confused as to why the front cylinder is the only one chuffing/popping. Hi, Mathew it’s Brent again don’t know if you remember me but my gsxr is running weird again. After I talked to you it ran good for about a month and then it started to run bad again. But it was more of a lack of throttle response. I bought a emgo sync tool and sync’ed they were way off.

And now the throttle response is fixed. But the weird thing is I reset the mixture screws and at 2 1/2 turns in it dies and the idle keeps going up all the way up to 4+ turns out.

My stock jetting is 127.5-125 main jets 12.5 pilot. My jetting in it now is 1 step up on everything so 130-127.5 mains and 15 pilot. It don’t make any sense to me why I would need to go up 1 step on mains and 2 on pilot for it to run right. Any help is greatly appreciated!. Well here is a little update on the tuning progress i made.

I put in the next size bigger pilot jet (17.5) and it made it run quite a bit smoother. I reset mixture screws after the change it smoothest idle was atleast obtained. It was 3 1/2 though so im thinking i will be putting in 20s next because it rode smooth and nice for about 10 minutes then it went back to its figgity self and was hesitating again.

Still better then with the 15s but not fixed yet. I checked and rechecked synch and it has been spot on since i first did it and i still cant understand why this bike needs so much fuel at idle.

Im only half a step up on main and when i put in the 20s that will be 4 steps up on pilot. Thanks for all the help i would never be even this close without your help amd write up. I put in the 20’s and i put the mixtures at 2 1/2 to start off. It started and ran pretty good and i let it warm up to try to dial in mixture and my highest idle and smotthest idle was 2 3/4 which seemed alot more like where it should be. It was way to late to do a test ride so i shut it off and waited till the next day.

I tryed to start it the next day and was experimenting with a no choke start and it turned over 3 maybe 4 times and then hydro locked or atleast so it seemed. So i held the throttle wide open and it turned and fired atleast. Smoked for a few minutes. Wondering if maybe i had a float stuck and not all the way seated and filled the cylinder full of fuel or something. Started fine next morning and has been good ever since. But after that hard start i rode around the block and then re-tuned the mixtures ended up at 2 1/4 turns out and went for a half hour ride to really see how it would run.

Could tell it was alittle lean. Popping on decel real bad and had quite a bit of ping.

Plus end of pipe was turning light greyish white. If i reset mixture and its over 3 turns out im gunna cry lol. Brent,You probably had a stuck float. Have you pressure test the needle and seats? I like to take 3-5psi of air and blow into fuel inlet. All should seal if float is depressed. High float levels could cause headaches.Since you shortened the silencer, decel pop may be normal.

Don’t try to eliminate this by increasing pilot jet size. You’ll end up way too rich at idle and cruise.Has anything been done to CV slide and/or spring? Maybe the stumble you are trying to solve is the transition between pilot circuit and throttle valve. I know jet kits in some cases drill the CV slide and replace spring so they lift easier giving the engine more fuel/air. Something to think about. Yes i did do a pressure test and all was good after that one little hiccup. Has been starting fine ever since.

My float hight is factory which is 7mm from the gasket surface with the float just touching but not compressing the needle spring. As far as drilling the slide i know i havent done it but thats not saying someone else didnt. It was jetted but not with a jet kit by a shop i do know that. Im not looking to get rid of all the pop on decel but it was excessive in my book. And the tip of the pipe being white/grey was another lean indication i was thinking.

Thanks again for everything. You have been a huge help btw. I tryed for leaks again like you told me. I sprayed everywhere and it is leak free. Ive actually done that like 5 or 6 times before just to make sure i wasnt missing something. Because to me it dont make any sense to need 20 pilot jets when stock is 12.5 but all tests i do come back negative. And compression is good and everything keeps pointing me to pilot jetting.

Polaris Mikuni Tm38 Carburetor Air Screw Parts

I guess this bike just came mega lean from the get go but how didnt it hessitate when new? Or maybe it did who knows. But its only got 13000 miles on it so it should be able to run and ride smooth i would think. Brent,I don’t know if drilling the hole is a good idea or not. I’ve read both sides of the argument. I did however drill slides per instructions on some jet kits, which supply new springs as well.However, it would be a good idea to check yours to see if someone has drilled them out.

Most kits supply a 1/8″ drill bit for this, so stock should be smaller.A larger hole will cause the slide to lift slower.If your slides flutter, make sure they are perfectly clean and operate smoothly. Polish them with crocus cloth if needed. Ok this bike is insane. It sat for about a week because it was raining. So i went out and choked it and boom hydrolocked again.

So i held the throttle open and it started just like before. I checked needles and they hold pressure fine.

So next day it sits over night and i try no choke. It starts and runs fine starting cold with no choke. So i check my mixtures and they are at 2 1/4. Still from that last long unsucessful ride and i start to adjust. If i get down to 1 or less turns it stumbles and dies.

Anywhere 1 1/2 turns or more the idle stays the same till about 2 1/2 no change at all. Then above 3 it starts to stumble really rythmicly.

So i think ok awesome im sure i got the right pilot 100% so i then put it at 1 1/2 to aim tward the bottom side of that range and see what happens. I take it around the block and it had been idling in the garage so it was 200+ degrees reading the temp guage. I mean absolutly perfect.until i started moving and temp dropped to 160-170 where it normally runs and boom stumbling and studdering like crazy so bad a fe2 times off the take off i thought it was gunna die out completely. How can the engine temp make such a huge differnce? I mean enough to make it run perfect then like crap just like that.

This bike drives me insane. I could swear its a nitro rc car. Thats a good idea using the box fans. I always turn on the fan that i have wired on a switch bypass and it still creaps up to over 200 degrees. The fan dosnt come on till 225 the way the factory has it and drag racing it i always have that fan on to keep the temps down.

As far as the t.stat goes i my fsm says its a range of 168-176 that it opens completely. Also with the mixture down at 1 1/2 like it is now i did a cold start try with the choke and it fired up super easy and the choke worked better than it ever has. With choke fully on the idle was at like 3k and as i eased it off it went down to 1200 perfectly. Before with the other jets if i choked it all the way the idle would be kinda low like 1k ish then as i eased it off it would go up to say 3k then come back down to idle.

So im pretty sure i got the correct pilot size now. Its just tuning it in and i should be good to go. Thanks again for everything. Ok now this one has got me scratching my head. Since my last post it has been to cold to ride until today so i went out and went a half turn more fuel. Which puts me at 2 turns out. I put alittle bit of sea foam in the tank and topped it off with vp110 and went for a ride.

Mikuni Carb Air Screw Adjustment

It will no longer accept full throttle. It breaks up really bad at wide open throttle. Almost like its lean on the main jets. But the only thing i have changed is the mixture. But plan on trying 2 1/2 but cant figure out how that would make any difference at wide open throttle. Well heres the update.

I put my mitivac on that line and left it all noght and it never budged. So im thinking that diapragm is good.

Not sure how the gas got up in the line. So i was testing a few things here and there and find the throttle position s3nsor’s resistance tested way out of spec. So now im ordering a new one. This bike will be the death of me. Could that be the cause of everything i have been experiancing?

It says its supposed to read between 3.5-6.5 k ohms and mine was reading 16.28 ish. And when i losened the screws to adjust it anyway i went it made no difference at all. I didnt even know a carbed bike had a tps in the first place. Kinda seems like it really wouldnt control much or as much as a fi bike would. Just an update.

I will update again when i get the new one on and test ride it. Well that did not go as smoothly as planned. I tested the new tps and it tested good and way different then my old one. So i installed it per the manual. I put the 15 pilot jets in and also put the needle back in the middle as a base line starting point. I took it around the block and it ran terrible. Bog off the line so bad i had to rev it and slip the clutch to get it to even move.

Once i got above 1/4 throttle it ran smooth and clean tho. And the jerky throttle response is gone now. But this bog is what has me confused.

My mixture is at 2 3/4 and if i go leaner it seems like the idle drops and isnt as smooth. I do know for sure that the tps not working made the bike run way rich because i pulled the plugs just grasping at straws and wow were they fouled. Black covered in soot. But i changed them with new ones and it still didnt completely fix it. It almost sounds like the tone of the exhaust has changed also. Way deeper almost v-twin sounding until i get past 1/4 throttle then it hits a turbo and most of the time blows the rear tire off. Kinda scary the couple times it did that lol.

Well heres an update. I tryed to set the mixtures and it was all over the place 1/4 turn idle up. Next 1/4 turn idle down. So i was talking to my father in law about it and he looked at the fould plugs and said for it to be dumping this much fuel in and not burning it like it was there would more than likely be alot of gas in my oil.

Sure enough he was correct. I looked in the view window and it was way over full. I drained alot of gas mixed with a small amount of oil out of the crank case. It was probably 75% gas. How screwed am i? Would it ruin the clutch?

Blow the engine? Or can i let it drip out for a while then fill with oil and call it a day? From what my father in law said it would make everything seem rich burning it off in the engine like that. Thanks for your advice man. Your a life saver. Don’t sweat the fuel in the crankcase.

Give it 2 drain and fills and it will be OK.If you had that much fuel in the crankcase, the float became stuck open due to debris of worn float needle. Remember when you had to hold throttle wide open during cold start?

That was probably due to all the fuel flooding the cylinders.At this point just replace the float needles. They can give owners so many headaches with high float levels or flooding that you are experiencing. And yes, it will cause the bike to run super rich is the float needles do not seal properly and result in high float levels.Another thing, you checked the vacuum petcock? Do you have a prime “PRI” setting? That setting bypasses the vacuum actuated setting and always allows fuel flow. Don’t use this setting other than when the carb bowls are dry to quickly fill them.

Well Matthew I honestly don’t know what else I can do. Installed the 4 stroke needle jet size 268 (smallest one available). It has 8 side holes on the top end, whereas the 2 stroke version doesn’t have any (it was a size 266). The bike only starts with a bit of gas, quickly dies without it, and smokes very badly (white smoke in lower revs, gets a bit darker if I rev her up). I had also installed a 4 stroke pilot jet of the same size as before (32) which has bigger side holes than the 2 stroke one and the smoking was much worse, which is why I switched back to the 2 stroke one and she runs like I explained in the beginning. The only thing that makes sense to me now, is that gas is coming in through these holes on the 4 stroke needle jet rather than air, which would happen if the gas level is too high.

Maybe it’s related to the fact that the carb is in at a bit of an angle?So yeah, she runs worse now, and I really don’t know what else I can do. Yeah, I don’t get it either. Only the needle jet (atomizer with its bush) were changed, the idle jet is the same again. The only reason that would make sense to me why it dies would be that the excess fuel when I let go off the gas floods it to the point of dying alhough that’s weird because apart from the terrible smoke (very light gray if not white), it revs up without too much hesitation.Also what bugs me is that I can’t adjust the idle screws because it won’t idle, but if I adjust them with the minimum amount of gas needed to not die, the revs are the highest when the mixture screw is all the way in (it meters fuel). Well finally some good news (sort of).

My theory proved to be right as far as it seems. I took some tape and closed the side holes on the needle jet, and it runs like it did before, just some sputter from 0 to 1/4 gas but other than that quite normal and also it idles with no trouble again. Also now I’m certain that the fuel level indeed is high enough to pour into these side holes as the needle jet was soaking wet to the top when I took it out, aside from the fact that it runs pretty well with the holes closed. The smoke is appearantly due to the mixture being way too rich.So the next order of business for me is to lower the fuel level in the float chamber.I currently have a needle valve #300 and a float with the weight of 9.5 grams and the fuel goes almost to the top of the needle jet.I know that I’m streching the original post a bit, but do you happen to have any reccomendations as to how much smaller would be appropriate?. I have an update on the gsxr.

Its cold here so i have it on a battery tender and i went out after the oil filled with gas catastrophe to try to start it. I tested the needle valves with 5 psi and they held perfect. I have been watching oil level like a hawk to make sure it hasnt changed. But im starting to think i either have a battery issue or a starter issue.

I go to start it and it turns over 2 or 3 times slower than normal and then stops. If i hold the starter button i can hear the starter motor almost sizzling but i only do that for a second.

Instantly my battery tender goes from green light to charging and then it charges for about a half hour and green light comes back on. I go over and try to start it again and boom turns over fine and fires right up. Im not noticing any change in oil level so im not thinking its hydrolocking from filling a cylinder with fuel.

Any thoughts? I resurrected this 1985 Honda XL600R from the ruins for my nephew.

The previous owner tried to make an attempt to do the same thing, but failed miserably. Too many armature mistakes almost caused this bike to be offered up as a eBay parts sacrifice.

Some people should not even try to do something like without the correct knowledge and experience. Currently is hard starting and pops through the carburetors while trying to start it. I rebuild the carburetors, however the jetting is not stock due to the changes the previous owner tried to do, such as drilling holes in the stock muffler and modifications to the air box. Per the advice of my local Honda Dealer, I installed 122 & 188 main jets. I had a 62 and 70 slow jet. I decided to put in the 70 due the mods to both the air box and muffler.

I do have a 58 and I am considering installing that to see if it helps with making it start easier. Any advice regarding this would be appreciated. The tank is from my 87 XL600R.My bike has a 87 XR600R motor and carbs. I have been having issues with it running lean, white spark plug. I went from 122 mains to 125 mains and the color on the plug is headed in the right direction, light tan to pinkish. My next jump will be 128. I do believe the 45 slow jet is not enough to get the bike started reliably.

I have not replaced the Air Cutoff Valve (ACV) yet. I have been reading that a faulty diaphragm can cause havoc. That’s what I have been experiencing with this bike.Any advice will on this bike would also be appreciated. Hi, I own a 1986 Yamaha fazer. I paid 1,600 in carb work and when I got the bike back I messed with the fuel screw the one with the spring. Now bike seems messed up.

Reason I messed with it is because it idled to fast. Now, I have to have the choke pulled out slightly to keep idle. And idle is too low.

If I pull choke out more it dies. I’m a female who owned a Fazer years ago and my old one never sounded or idled like this one. I won it on ebay. It’s way loud and if the mechanic tuned to proper specs, somethings off cause my old one never idled like that. So I played with the screw. Only to find that supposedly this fazer has different pipes than my original one. At least that’s what mechanic says.

But they look like my old ones to me. Also I hear a strange tick in a hose on the right side that has a metal spring over it. It sounds like the fuel is not working right or air is not. I’m not sure I’m a girl.

And need help. There’s no motorcycle repair around my area. Anyone who can help, I will pay only if it’s fixed. I do have a few videos of my bike on YouTube.

Cheryl Jenkins, the one with two cats in the picture. I live in Danville CA.

Please Email me if you can set to specs. 1986 YAMAHA FZX 700 FAZER. Hi Matthew, I’ve been working on bikes and cars for 40 years and never before ran into problem that I wasn’t eventually able to resolve through some basic applied knowledge and some trial and error but I haven’t had a lot of experience with multi-cylinder CV carburetion and I’ve been through all the knowledge, common sense and trial and error with this problem and after more then 2 months I’m right back to where I stated with out any resolution and I’m out of time.

In reading through your postings I saw how you helped Brent Pfeiffer with his carburetor issues and I was wondering I provided you with all the information on the issue I’m dealing with along with the results of my trial and errors, if you felt up to analyzing the info to see if you might recognize a solution. If you don’t feel like doing it I’d understand, I imagine you are getting pretty burned out on people always asking for help, I know I would be.

Let me know,Thx. Matthew,Not sure if you can help but I think the principle is the same. I have a 2001 Yamaha f80 outboard motor. Straight in line 1600cc (97ci) with four (BCF) keihin carbs. Engine ran fine until cracked block etc etc. Replaced powerhead with full reconditioned unit and reused my existing carbs. Prior to fitting them I dismantled and cleaned them and found that they had different jetting although they performed fine on previous engine.Carb 1 (top) main 128, pilot 42.

Carb 2 – main 112, pilot 45. Carb 3 – main 128, pilot 42. Carb 4 – main 112, pilot 45.After fitting on new engine I cannot get Carb 4 to operate correctly, engine idles but only on top three cylinders. It will rev out and after 3000rpm all cylinders work but below and number 4 will not. Also (engine side) Fuel mixture screw does not seem to make any difference at all even fully seated or out 7 turns. Have proved by pulling plug lead on 4 and engine note still the same on idle.

Hi I have a 1987 gpz 500 custom, I’m running single pod filters and stock cut down headers with slash cut cans.I’ve dynojetted the carbs and almost got it running sweet!It’s a bit of guess work as I have no rev counter, but it idles nicely mid to high revs seem good but it’s a bit boggy about a quarter up the rev range I would say! I’ve adjusted the needle hight to several positions but I can’t seem to iron out that boggy spot!Would this point to a richer pilot jet being required?I would say it the transition from pilot to needle that needs attention!Any help would be greatly received.Thanks. Hi Matthew, I have a 1980 Yamaha XS1100 Special with pods and aftermarket exhausts. The timing is good and I changed the oil and spark plugs already. The bike wouldn’t stay on everytime I give it a little throttle.

The bike only stays on if the choke is on. Once I fully close it, the bike shuts off even when the engine is warm. The jet sizes for Carb 1: 110 (main), 130 (pilot air), 42.5 (pilot jet).

Carb 2: 110 (main). 185 (pilot air). 42.5 (pilot jet). Carb 3: same as Carb 2. Carb 4: 110 (main), 125 (pilot air), 42.5 (pilot jet). I’ve had my fuel mixture screws turned out about 2 -2.5 turns, but it would bog everytime I throttle it (eventually it dies). It seems that the bike is running lean from the spark plug readings.

I did try closing the fuel mixture screw all the way in, but the bike does not die. Should I buy larger pilot jets? Should the fuel mixture screws e turned out differently due to different pilot air jets sizes? Or what else could be the problem?.

A: There are two screws to adjust when setting the idle on a carburetor. One adjusts the slide height and is called the idle speed screw, while the other adjusts the air-to-fuel ratio and is called the idle mixture screw. The manual for the AT1 says start the idle mixture screw at 1-1/2 turns counterclockwise from fully screwed in (lightly seated; don’t screw it tight), then adjust either way in 1/2 turn increments to find the fastest, smoothest idle. If the fastest idle is too fast, turn the idle speed screw out to slow it down. Again turn the idle mix screw a 1/2 turn clockwise and counterclockwise seeking the fastest, smoothest speed.

Once you determine which direction helps, refine it to 1/4 turns either way to fine-tune the mixture. If you find that the bike stumbles a bit when you open the throttle, 1/4 to 1/2 turn tighter will richen up the mixture a bit and compensate for the added air in the mix from the opening slide. Motorcycle Classics is America's premier magazine for collectors and enthusiasts, dreamers and restorers, newcomers and life long motorheads who love the sound and the beauty of classic bikes.

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