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Inside A Motorcycle Carburetor - Main Jet Tuning | MC Garage
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- Опубліковано 17 сер 2024
- “I’m on the main jet!” Have you ever heard that expression while bench racing with your riding buddies? What the heck does that even mean? Well, after this episode of MC Garage, you’ll know. Today we talk about the main jet.
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Modern fuel injection is amazing, but before injectors, ECUs, and fuel pumps motorcycles used carburetors. Some smaller bikes and two-strokes still do. Today on MC Garage we talk about the carburetor.
For those of you who have an older motorcycle model or a modern two-stroke, one of the most confusing and intimidating tasks is probably jetting the carburetor correctly. To some, it amounts to black magic. But if you know the theory and reasons for each circuit in your Mikuni, Keihin, or Lectron and you take it step by step, it’s really not too difficult. Today we are going to start a multipart series on how to tune or “jet” a carb. Step one is to understand how it works and what all the parts do.
So how does the carb mix that fuel and air? In simplest terms the air comes through the venturi and mixes with fuel supplied by the carb in a specific ratio. This is called the stoichiometric ratio. That ratio is theoretically 14.7 parts air to one part fuel. In reality, your machine probably runs better at a richer ratio. Some tuners say 13.2, some say 13.7-each motorcycle has its own happy ratio for proper combustion. This mixture is achieved using small orifices or jets to mix the fuel with the air.
First and foremost, there is the place the fuel is pulled from: the float bowl. The float sets the level of the fuel from which the jets pull. The float operates the needle valve, letting fuel flow in when the level drops and closes when the correct level is reached.
On the bottom for the carburetor you typically have two jets. The pilot, the smaller of the two, and the main jet. Let’s start with the pilot. The pilot jet handles the mixture from idle to 15 to 20 percent throttle. Air comes in through the front of the carburetor drawing fuel up through the pilot with a vacuum that is created as the air flows through the pilot circuit.
When your engine is cold, a richer idle mixture is needed to make starting and running easier. This is the job of the choke. It adds more fuel into the mix to assist the idle circuit or pilot when the plunger is moved to open an additional path for that extra fuel. Once the engine is working, closing the circuit returns the carburetor to normal operation, relying only on the pilot for fuel.
On the side of the carb you have the mixture screw. Generally, if the screw is located on the engine side of the slide or butterfly, it is a fuel screw. Or if the screw is located on the airbox side of the slide, it’s an air mixture screw. Here you can fine-tune the idle and compensate for moderate changes in temperature and altitude without changing the pilot jet.
The larger jet next to the pilot is the main jet. This is screwed into the needle jet, sometimes this is referred to as the nozzle. More on the needle jet later. The main jet provides the fuel at 80 percent to wide-open throttle. The fuel flows up and out through the needle jet into the throat of the carburetor. When changes in air density are significant the main jet will need to be swapped.
Handling the mixture between the pilot and main is the needle jet and needle. This is your midrange, or about 20 percent to 80 percent throttle. Within the needle jet the needle moves up and down to vary the orifice size to meter the fuel. The tapered shape of the needle controls the amount of fuel coming up through the main jet and through the needle jet. As it goes up, the opening gets larger, letting more fuel into the mix. You can tune this function by moving the static position of the needle up or down or by changing the taper of the needle.
The needle sits in the slide that moves up and down with throttle position. That can be controlled by a cable the flat-slide we are using for this video or by airflow passing around the throttle butterfly on a CV, or constant velocity carb. In CV, a vacuum is created that sucks the slide up as airflow increases.
Those are the most important pieces of a carb and what they do. On the next episode we cover the tuning the idle circuit.
Find out more: www.motorcycli...
The basic layout of the carb, the float bowl - www.motorcycli...
Pilot jet - www.motorcycli...
Check out Ari’s video on how to clean a carb - www.motorcycli...
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I know people miss Ari but it's ridiculous Motor Trend charge for the videos. This guy rocks. We need more videos though on a weekly basis too.
"Ridiculous" is a strong word. Vids don't appear from thin air for free, and despite what everyone thinks, nobody is making a fortune on YT (unless they're a makeup channel with 10M subs, that's a different story) or even really covering the expense of making feature-length content like On Two Wheels or Throttle Out, which is a longer format and uses a full-on production team.
Justin is doing a great job with MC Garage and I'm happy to see the show continue. It's not an "either or" scenario with Motor Trend and Motorcyclist/other YT channels, it's in addition to. MT is paying to create more motorcycle content. In order to watch it you have to pay a subscription fee of $5/month. No it's not free, but it's not expensive, either.
@@AriH211 I'll look into this subscription. MC garage definitely has great production quality. I figure you got a better gig with MT which is great but UA-cam is just an easier way to view videos on the go. By "ridiculous" I mean interesting lol
Wow, these videos helped me out so much! Thank you so much for helping me to understand carburetors. So much appreciated! Great content and a treat to watch.
Just the stuff I need to learn about.
Its been years since anything with a carb has been inside my garage, nice nostalgic video.
That was true for me from 2004 until last year when I bought a KLR650. I wish it was FI, but it is a great bike for local fun riding.
@@LTVoyager KLR great bike, nice one mate.
Wiremu “
hey MC garage .. the for the talking about this issue.. however, I'm one of those who learn by seeing how this thing is done.. it would've been a lot more useful if there would be more of doing it, instead of talking about it !!
thanks
Nice n clearly explained, I’ve recently seized my 1952 BSA A10 , suspect it was running lean on either the main or needle jet. Like the man said, it’s difficult to hear it pinking on a 4 stroke especially an air cooled one as there’s a fair bit of mechanical noise anyway. Going to experiment with both main and needle jets. Great video.
I want a keihin carb video
I miss this series. Wish you'll were more consistent
Hi, Im just wondering how to find choke on carburetor ? The new carburetor I've bought from my friend don't have choke on it.
Thanks for share!
please excuse my wrong english.
I need some help now for jetting.
I'm crazy looking jetting my Honda XR150L, Keihin PD 20mm bore carburetor. I'm not technician just researching.
Local service does not have a O² sensor and spark plug reading knoweledge seems to be not good.
I made 2 times plug chop for main jetting and it goes wrong without vissible carbon deposits making me to ask for myself if jets that I have been testing are out of range or they are still in lean range.
Stock jets are 38 and 102, 150cc, 20mm bore carburetor, stock airbox and filters, 1500f ASL, 82~95°F. Chevron "97 RON".
Spark plug reading for stock jets shows white dots, white deposits, so jets are very lean.
I step up for #112 main jet but plug chop (run almost 6 miles on main jet), plug chop shown just a little brown mark but Not a complete ring, and it spot in the upper part of insulator and oriented to the exhaust valve, not more!
This question is for your knoweledge, for this engine, carburetor and environments, what numbers of jets could be the range to test?
I just like to protect my engine from overheating and knoking or some else failure...
Very Thanks!
Hi i wanna ask you something and i think you are the right person to explain for all this this people witch are watching you . For example if you have 145 main jet and your needle is set on the midle setting ( needle has 3 settings, 3 clips) and you upgrade for 150 main but you drop the needle in on the first clip you gonna get the same ammount of fuel??
What is moving the needle worth? A fifth of a main size or more or less? If its a 5 position needle of course.
Seems like my bike runs fine until 7000 rpm, then if I go wide open throttle it dies until I let off the throttle again, OR if say I'm below 7000 rpms I can go wide open throttle all day long. The issue is my bike is set up with the power band between 8000-9000rpm is the sweet spot so I need it to hit that range to get the full 30hp... below 7000 rpms is not ideal. (no poping or backfiring noises are present.)
Same here let me know if u get a solution
My ktm 125sx before powerband is making a weird thing, i don’t know how to called is not like a 100% bogging but yeah kinda like that when I go middle to full throttle, but when I hit the throttle 100% the powerband active normaly. But this thing get me frustrating maybe I have to put a bigger main jet?
23 yz125 keihin carb
Can a pilot be lean and be safe?
My bike is getting spooge from silencer
Stock settings are
Main 160
Pilot 75
I’m looking at jd jetting kit but
Jd jetting comes with a 50 pilot
And is telling me to use a 170 main
On 4 position clip of the needle.
Not sure if that pilot is too lean for my 2 stroke
Video for mechanics. Great!
Never thought I'd see Tom Segura teaching people about carburetor jetting! :P
i swapped from a stock 30 main jet to 32.5 and put the jet needle on its leanest, ran great up until a certain high rpm at high speed, but the same rpm at low speed didnt have a a lean condition problem, so ive adjusted the needle twice, back to stock, since then and it still seems lean and dies at that high speed high rpm range. of course i always go to full throttle every time but past 3/4 only goes lean at highway speeds. its a mikuni bsw27 btw. cc double barrel gsx250f across carby.
3 washers under needle, 60 main jet. Bam bam perfect.
MrCokemaster05 nah
4th out of 5 carb vid
Great tutorial !!👍
THIS DOESNT TELL ME HOW TO TUNE THE MAIN JET. how do I know whether to go up or down a size? whats the process?
YES
btw the only way to tune the main jet is to change it
@@38Fanda No shit, but whats the process that dictates whether I need to go up or down in size.
@@ParadiseArcadia u need to fell it how it runs cheak sparkplugs 2
So my harley is running little rich I was thinking it was the pilot jet....should I be looking more at the main jet and needle I do alot of round town riding fyi??
I have installed bigger pilot jet and needle lowered to 2nd position but there is light sputtering apart from that performance is very good. So due to sputtering I lowered needle to 1st position and sputtering stoped but less power in middle throttle position.should I change the larger main jet to gain more power or the larger needle to be installed with the circlip positioned to 2nd or 3rd depending on the performance
Thanks
You could adjust the float high and see what that does, could be a lack of fuel off the pitch that’s causing the stuttering
Is there a video on tuning my mikuni carburetor?
I wanna ask for 4stroke 150cc bike better use nozzle with more hole or less hole?
what?
I have a 06 vtx1300 I have a stage one dj kit I do some drag on my bike and looking to get the most out my carb I'm running straight long pipes and a stock airbox and a K&N filter I live in Florida I have the 195 jet can I put the 200 in it to gane more power at wide open throttle and were should my needle be at
Will a shop let me bring in the carb alone (Bike runs with starter fluid so I KNOW its a carb issue)? It's not even dirty but has a clog somewhere and after cleaning it 4 times I have given up. I don't have a means to transport the entire bike and the fee is well over $200 alone for one way.
Of course they will, you just need to specify which bike it is from. Also a phone call beforehand doesnt hurt. They even take just the wheels to mount the tires on them. If I were you though I would just ride my bike to the nearest shop, and let them do a carb sync(if you have more than one carb) mixture and idle speed adjustment. (which they can only do when its in ur bike, since factory specs seem to not hold up after a couple of years and carburetors do not self adjust)
Nice
carbs, carbs, CARBS!
So i dont have any reason to care about carbs since all of my bikes are modern but I still watch, that must mean something. Im not feeling the new "commute" rider and the group thing wasnt as good but this segment hasnt dropped off with the change in host. Now they just gotta get this one a friend to help with the other stuff.
Can the engine ping happen also at lower throttle openings? Like 20-30%?
my 30% throttle already go to 110km per hour, mine is a chopper
Great videos don’t change a thing. I wish some of these negative Aholes would take a long walk off a short plank.
If you want a little more power out of your carb can you just change the main jet
my 250 polaris atv . like the old skidoo, just move the Needle Clip.
and adjust air screw. ( round slide vm series.MIK..)
Let me ask you this. My wifes Kawasaki Vuocan 750 is acting lean at idle, rich at a cruise but runs great when getting on the throttle. It doesn't idle consistently, when you rev it, it hesitates then revs and takes a second to go back down to normal idle sometimes. When cruising at steady throttle it has a slight hesitation but when when you crack the throttle it pulls good and smooth. Also I pulled her plugs out and they had a yellow glaze like a rich condition. What do you think that is?
Idle screw. Adjust it
@@resaleking14 The float was sticking and basically flooding the carbs sometimes and not giving it enough gas others. Cleaned the carbs.
My bike feels low on 1 to 4rpm but from 5 to 9rpm it really feels strong except this everything is fine
Plz tell me what should i do for low rev range power
2 stroke?
@@magnumllama400 4 stroke Yamaha ybr125cc cv carb
Can you make a video about dual sport tires? Thanks.
How can I get 10 more horsepower out of my PW50?
t u r b o
on a serious note,better flowing exhaust,different sprockets,etc.
Couldnt come at a better time......My shits fucked
Every video: inside a motorbike carburator🤦♂️
But still hella good videos
I was hoping he would show how to remove main jet instead of trying to prove how smart he is.
More examples, please. We know the theory, but most of us lack the practice. So when it comes to talk about bogs, sputter and etc don't just talk but show us.
Can we have videos other than carburator? Come on guys..
Take the shirt out back and burn it......ditch the gloves. Real mechanics don't wear gloves
Lean fuel mixture makes engine cool not hot...😎
is it fair to speak for the majority and say fuck carb tuning
never change the main jet without changing others thing like high air intake or out take, you need to change the main jet only you have change the racing pipe, cold air intake, if not your bike will run rich
Mc garage is dying, bro let's move on from talking about carbs. Most bikes don't have carbs nowadays, why spend 2 damn episodes talking about it.
most people don't ride brand new 2019 bikes.