Pilot Air/Fuel Screw Adjustment Explained - Multiple Carbs - Part 2

[Music] alright guys so in this video I want to cover on how to adjust pilot fuel mixture screws on a multiple carb setup okay and quite simply the goal is to adjust the pilot screws to achieve the highest and smoothest idle rpm now hopefully your fuel screws will end up at two to three turns out then that tells you the pilot jets are correct the correct size and this will provide good acceleration when you rev up the bike and the RPMs will don't will come down quickly so that is the ultimate goal right there summarized in two sentences and I got my notes here because I'm half asleep right now so I need to just ramble off of this sheet I wrote it all down ok so I already have a youtube video on this subject however it was like five years ago and I like to dive a little deeper into this subject that video has over two million views and I also have a blog post on pilot jet explained and that is like my most visited or viewed page on my website so definitely check that out and then watch this video as well and hopefully it should answer all your questions on fuel screw tuning ok so some people call this the fuel air mixture screw or fuel screw I like to call it the pilot fuel screw reason is it's the pilot circuit and then you even you put fuel or air in front of it there's two types there's a fuel screw and an air screw a carburetor will have one or the other never both and it's important to know which type of screw you have because an air screw meters air and a fuel screw meters fuel and you they work opposite of each other in terms of turning it and whatnot but in any case we're covering fuel screws on a multiple carb engine on inline four in this video ok so I feel like the pilot jet is the most ignored jet in the carburetor everyone is so quick to swap needles clip positions and main but I'm here to tell you that the pilot jet and the fuel or air screw fuel screw in this video adjustment will make the bike idle and accelerate way better and after all unless you own a hundred percent race bike you'll spend most of the time idling around and running down the road at a throttle cruising which is all pilot jet I also feel that it's the easiest circuit to jet because the answer lies in the fuel screw adjustment the number of turns out or in will tell you whether you're too lean or rich on that circuit okay also here are some observations I see on bikes newer than 1980 the pilot jet is usually lean by one to three sizes and here's my theory on why bikes in the 70s really didn't have this problem so what happened I think the EPA and emissions came down hard on OMS and as a result they started supplying bikes to meet these standards and as a result they're super lean from the factory and some bikes are worse than others okay and these are observations I found living here in Chicago temps range from 60 to 90 degrees Fahrenheit humidity 50 to 100 percent an altitude is 600 feet above sea level also consider that the OEM supplies one bike that is supposed to run good across the entire US from high and low altitude along with temp and humidity they need to supply a bike that will run okay for everyone and meet standards that are kind of you know out of their control so I think everyone can make a few tweaks that will tailor the fuel mix for your conditions the service manual is a good starting point but I would go through the procedure anyway and see what your bike likes and then if you have any modifications like air box or air filter modifications and exhaust well then you know you obviously have to go through this okay if you're shopping around for a jet kit make sure to find one that comes with pilot jets most do not you know even before buying a jet kit I would run through this procedure and see if you need larger or smaller pilot jets because you can always buy them separately even if the jet kit does not come with it but but really good jet kits will come with pilot jets as well ok let's talk about the bike in this video it's a 90 Yamaha Radian with good compression 150 psi across the board no vacuum leaks and the carbs are clean I took the carbs completely apart to get rid of the vacuum leaks on the throttle shaft seals I saw it'll blasted and ultrasonic clean the carbs and has a jet kit in it and the stock pilots were 30 I went two sizes larger to 35 and the fuel screws ended up at three and a quarter turns out so really it could could have gone one size larger on the pilot to say 37 and a half to get those fuel screws right at three turns which would have been perfect but it ran good okay so we're gonna head over to the white board I'm gonna do a little bit of theory and then we'll get to the bike with adjusting the carbs and the footage of me adjusting it on the bike as well over two years old I never got around to posting this video and also there's some audio popping I have no idea what that is so I apologize for that in advance it might have been right around the time I got my new camera equipment and audio and I think the the mic might have been clipping or something so sorry about that okay a couple more tips so if you can't find the fuel screw it's because they are hiding it from you it could be under a Welch plug so I have a video on how to remove that up here or sometimes they put these tamper proof caps on the fuel screw that you can only turn it like quarter turn one way or another I recommend removing those and don't even replace it I never put the Welch plugs back in and the tamper screws I if you apply a little heat to it they come right off they're just glued on and I just throw them out I don't even bother with those things anymore okay make sure to lightly seat the fuel screws it is a little screw and it's got a tiny pointy end to it it's so easy to snap them and have the point stuck in there and then you're really screwed you're in for some fun to extract those and I don't want to go there I hope that doesn't happen to you so really light pressure when you seat that it's a very small screw okay so the goal here is to have a responsive fuel screw and it should die when it's screwed all the way in because essentially you're cutting off all the fuel it should begin to stumble as you turn it in also when you rev it up at idle the RPMs should not dip below idle speed if it dips and almost stumbles then it's too rich if the RPMs hang and take a while to cut or take a while to come down the bike is lean the fuel screw ends up at three turns or more the pilot jet needs to be increased a good rule of thumb for that is about half turn per jet size so if you're at two if you're a three and a half and you want to get down to three bump it up one size also if it's under one turn it's too rich maybe even under two turns it's too rich I would decrease the pilot jet size alright white board time let me draw a carburetor really quick okay so we got the air side air box got the engine let's just do a butterfly instead of a CV just for simplicity here okay we got our fuel bowl okay so this is just the car body it's very generic drawing just to illustrate my point here all right so we have some fuel in here and here is our pilot jet okay so here's an orifice and the larger the number on the pilot jet then the larger this hole will be okay so this is your your metering for this circuit okay so fuel can gum up two ways it goes here in front of the butterfly and it also goes here right behind it okay and I'll explain what both of these do in a second here I don't have I ran out of colors here so what we're gonna do is draw a fuel screw in green okay now the fuel screw is typically found on the bottom that is the most common place it could also be right here or on top but it is always in front of the butterfly or the slide on the engine side of the carb so this is we're gonna call this the centerline of the carb okay and it's always ahead of that on the engine side okay so at idle you have a very small gap right here okay this is an idle you have barely any air coming in so what happens is as air gets drawn past this blade it's gonna pull fuel out of here out of this tiny hole and this is what your fuel screw will trim so if you want less to come out then you screw this in right this thing moves up and down okay so when you turn this guy all the way in in theory it should stall the engine if it does install the engine that means the pilot jet is too rich and it's actually pulling fuel out of here okay and I just said that it only pulls fuel from here but it's sometimes can pull a little bit from here but just think of this is the only spot that fuel can come out of and idle and this here is the trim okay so that's at idle now let's bump it up to eighth inch I'm sorry now let's bump it up to eighth throttle position okay so that's that's considered your cruise okay so what's gonna happen is more air is gonna come here and it's gonna start pulling fuel out of this hole here so now your your technically a hundred percent on the pilot jet your your flowing the maximum this thing can flow and that is your cruise air fuel ratio okay and this hole is actually closer to the throttle blade but in any case I'm just you know trying to draw here okay so that's that's what happens at a throttle okay so these two are important typically when this is two to three turns out trimming for maximum rpm and engine smoothness then you also are gonna have a good air fuel ratio at cruise okay so let me let me do a case study really quick if you had a thirty pilot jet and that Nets you three turns on the fuel screw okay and what I mean by that is if you adjust for peak rpm you and you get three turns out let's just say that's a 13 to one AFR okay I'm just making that number up okay now let's say you go to a 35 but let me back up for a second okay so let's say that Nets you a 13 2 1 AFR at idle and let's say at Nets you a 13 2 1 AFR at cruise cruise and idle okay so let's say you bumped it to a 35 pilot jet and that Nets you two turns on the fuel screw your AFR it idles still the same why because you trimmed it out you turning that screw in to reach the peak rpm so it's still the same AFR at idle the difference is now you're at 12 to 1 AFR at cruise so it's going to be a richer cruise ok or if I went to a 25 pilot jet let's say it's 4 turns on the fuel screw still 13 to 1 AFR maybe maybe you can't even get there because you're 4 turns out but then this would be like a 1401 AFR at cruise ok so the pilot jet and the fuel screw it has to be set up properly to get your idle correct and also your cruise ok so consider that ok so I mentioned 4 turns out well what happens at 4 turns out this screw is so far out that your taper is no longer effective on the screw okay and when that happens the engine wants more fuel but you can't give it anymore because this thing is all the way open and it just won't flow anymore ok so that's when you lose the effectiveness of the screw ok one last thing every jet change is about a 3 to 5 percent change approximately and I also made that number up alright so here we are on the inline 4 cylinder and we're going to talk a little bit about the fuel screw adjustment now here's the carburetor and I want to define a few things first here is where the throttle shaft is and this is where the butterfly opens and this is a CV carb and here is where the constant velocity slide moves up and down so if we put an imaginary line right here we're gonna call this side the airbox side and this side here the engine side whenever the screw is located on the engine side it's considered a fuel screw if it's located on the airbox side it is a air screw and that's what it meters so if it's on this side it's a meters fuel on this side air if it means fuel you turn it in to lean it out out to rich in it if it's located on the airbox side you turn it in to rich in out to lean okay now that is in most cases there are a few carburetors the screw is located right here and it is a pilot air screw I've never seen a fuel screw locate on the airbox side so on some key hand carburetors there are air screws here as well as some amal carbs on Triumph's they are located in front of the slide but in any case if you're in doubt pull the screw out and compare the tip if it's blunt it meters air if it's pointy and meters fuel all right guys so I have the tank off and I have my auxiliary fuel tank hooked up now if you're interested in buying one of these tanks I'll have a link to it in my video description I bought it for about 30 bucks and I use it a ton any time I'm working on a bike I just get the tank off the bike and I strictly work with this so I can you know check for vacuum leaks just the pilot screws and so forth it's just much easier with the tank out of the way all right so what I'm gonna do is this bike is already set up I spent you know past few weeks working on this and they're all set at three turns out now this fuel screw is located up here sometimes they can be on the side most common are on the bottom now these oftentimes are covered with a Welch plug and basically what that is is it's a tamper proof plug that they put in here and they don't want you to mess with these you know due to emissions that really showed up in the 80s and the 90s and they still use it today so I have a video on where you drill it out and pull it out now not only do you want to adjust these but if there's a plug in there anytime you go to clean the carbs that's an often overlooked item the plug is in there people don't take this out therefore you can't clean the passages very well and as a result people put their carbs back together and it pretty much runs the same and that's because that passage has not been clean their idle circuit it's very important to clean that up all right so what this bike is fully warmed up I warmed it up it's it's nice and warm rotifer a good ten minutes came in here pulled the tank off and what I'm gonna do is I'm gonna start to close these screws down from three till whenever it starts to stumble I'll go half a turn at each carb and you'll notice that the RPM dies down and then I'll bring it back out and you'll see how the RPM increases you basically want the highest smoothest running rpm also I'll blip the throttle f3 turns and you'll notice how quickly and smoothly the RPM is return back to idle I'll do the same at two turns and four turns and just this give you a lean enriched condition just so you can see how the engine behaves with less fuel and more fuel and a perfect mix it turns out okay so won't even run it - turns out let's set them back to three let's go Richard alright guys as I mentioned before this bike has pilots which are two sizes larger than stock I'm at three maybe a little more than three turns out maybe three and a corridor three and a half and I think this bike would really benefit to go into another size and you know since it wants to be a three three and a quarter three and a half right now I think with another size up that'll add about five percent more fuel and it would get the screws to three turns out and it would probably be really happy with that now I do want to say you know at three and a quarter or anywhere from three to three-and-a-half it runs great I going here on the stand and it does run really good on the street no bogging no issues so you know I'm gonna leave it at this for now there's other issues I have to dress with this bike and it also is about sixty degrees right now so as it warms up to seventy eighty degrees it may be perfect at three turns out so just keep that in mind that temperature does affect everything and currently it's spring here in Chicago one day it's 50 degrees and other dates seventy Aries so the temperature is kind of the weather is playing tricks on me you know so in any case this is a huge improvement over stock or what it came in is so that's pretty much how you tune multiple cards just jump half a turn at each tire time each carb and you can definitely see how the auto rpm changes as you turn these things in now it was running really rough at two turns and as I increase that to three it's smooth out and it even went smooth out even further as I went to three and a half four turns out four turns obviously the the taper of the the screw losses effectiveness so you know three to three-and-a-half is where this bike is happy right now and I'm really debating on throwing another pilot in here one large size up so with any hand of 30 as it came in it's a head of 35 next would be I believe thirty seven and a half and then forty or whatever the sizes are in mikuni here [Music] alright guys a huge thanks to my patrons who support videos like this if you'd like to support me on patreon there'll be the link right there you get some perks like premium videos and t-shirts and stuff like that so make sure to check it out alright that's it for this video make sure to hit that like button and share this with your ridin buddies don't forget to subscribe and hit that Bell icon to get notified right away when new videos come out if you'd like to support me in creating more videos such as this one check out my patreon page and see if you can help me out you can also buy me an ice cold beer or two by clicking on the link thanks for watching and I'll see you in the next video you [Music]

Channel: MatthewMCRepair

Views: 397,361

Keywords: MatthewMCRepair, motorcycle repair, Matthew Bochnak, idle mixture, pilot air, pilot fuel, pilot jet, welch plug, multiple carburetors

Id: DLldmBzqR_s

Length: 27min 2sec (1622 seconds)

Published: Tue Feb 05 2019