Carburetor, mystery revealed!

[P.O.L. Mafia]

Carburetor, mystery revealed!

I have had a lot of people ask questions on this subject. Most don’t understand how a carburetor works, nor do they understand that, in a sense, all carburetors are the same. The first thing you need to understand is that, if there is a fuel jet, there is a corresponding air jet. Regardless of who makes the carburetor, if there is a fuel jet, there IS an air jet that goes with it. From Holley to Carter, to Rochester, to Mikuni, to Delorto, say it with me, if there is a fuel jet, there IS a corresponding Air jet. So, lets look at these jets and how they work together, shall we?

Fuel Jets and Air jets what’s the difference?

To answer this, we need to get a little history first. Back a long time ago, when the internal combustion engine was first made, carburetors were much simpler and inefficient. Many of the first engines ran very rich and blew black smoke all the time. After some research, and a lot of headache, it was found that no matter how they metered the fuel the engine ran rich. It was discovered that different engine combinations needed different fuel requirements, thus leading to different Fuel jets for different needs. An engine at sea level needed more fuel than the same engine at 3000 ft so it came to be realized that making removable jets, with different sizes you could tune an engine for its environment.

Fuel jets meter just what they say, fuel. They allow a certain amount of fuel to go through them for a given vacuum signal on the port they are attached to. They have a max and a minimum flow rate and are normally machined from a solid piece of brass. The only difference in the actual jet is where it is located in the carburetor. A jet is a jet, this is a broad statement, but accurate none the less. If you only have fuel jets in a carburetor then you would get a steady stream of fuel into the air charge leading to the cylinder. This causes very pour atomization of the fuel and air leading to fuel puddling and will give a very poor burn leading you to believe that you are running rich. Many of the first carburetors were plagued with this. Fuel must be mixed with air to burn properly. it is a misnomer that gas will burn. Gasoline in its liquid state will not burn; it is its vapor, or aromatics, that burn, not the liquid. In order for a carburetor to do its job better it must pre-mix the fuel charge. This is done with the Air jet.

The air jet meters just what it says, air. It allows a given amount of air into the fuel before it makes it to the venturi . This is normally done in what is called the “emulsion tube”. This is where the pre-mix begins with the air and fuel. The fuel and air jet go hand in hand with each other however a change in one does not necessitate a change in the other. Normally an air jet will never need to be changed, however, when cleaning a carburetor, you need to insure this passage is clean and clear. A blocked air jet will cause what appears to be an overly rich mix.

You can effectively change the fuel ratio, by changing the air jet. A smaller air will richen while a larger will lean. The opposite is true with the fuel jet, a larger richens, while a smaller leans. Another way to look at it is, the air jet regulates the vacuum signal to the fuel jet. A smaller air jet raises the signal; a larger air jet lowers the signal effectively changing the air fuel ratio.

Hopefully you can now see how the two jets work together to make a good fuel mix and make your vehicle run correctly. Lets move on to some more areas.

Main, Pilot, Idle, it makes my head hurt!

Let’s get into the nuts and bolts now. We will start with the Pilot or Idle circuit.

The pilot or for you car guys “idle” circuit operates from idle to about one quarter throttle. After that, it just sits there. All it does is regulate fuel during idle and very low throttle position. If the butterflies are open to far you will be running on the pilot and the main circuit. One way to tell is when you open and release the throttle quickly, the idle should come down quickly. If there is a lag then you need to close the throttle stop and get it running on the idle or pilot circuit alone.

Attached to the pilot jet circuit is the pilot screw. The pilot screw is your rough idle adjustment. Like on any car carburetor, it is the screw that sets the fuel amount coming from the pilot jet to the hole just below the butterfly in the throat of the carb. This “screw” is more of an adjustable jet than a screw. It does have threads but the end is machined into a tip. The tapered tip closes the hole it screws into as you screw it in. When you tighten this screw in you only go to soft seat. This means that you screw in (righty tighty-lefty loosy) till you feel it seat and stop. Going any farther will damage the seat. To far in and your bike (or car) wont run, to far out and you will have to open the butterflies so far that you will almost bypass this circuit. The base setting for this is 2 ½ turns from soft seat. It has been my experience that this setting works on any carb be it on a small engine all the way up to big blocks. The thumb screw on the side of the carb is for your fine idle adjustment.

The main runs from ¼ throttle on up. At ¼ throttle you will be running on a mix of pilot and main until the main can take over. On most car, carburetors this area is supplemented by the accelerator pump pushing extra fuel into the mix. On a variable venturi carburetor this is handled by the slide diaphragm and the tapered needle going down the main emulsion tube. If you have stumble when you rap on the throttle and your plugs look good at idle, you need to shim the needle in the slide to allow more fuel in during exceleration.

Choke

The choke on most cars, is simply a butterfly that closes off eliminating air and effectively richening the mix until the engine is warm and can support better fuel combustion. On most bikes the choke is handled a slightly different way. First let me say that it is a misnomer that the choke “creates a vacuum leak” which makes the engine rev. if you simple add air behind the main butterfly you will lean the fuel mix causing a poor burn cycle to become worse. Opening the choke plunger on the variable carb does allow air behind the butterfly, however, as that air comes around the passage it pics up fuel from the bowl via the choke circuit. If you open the choke and the bike dies immediately, 90% of the time the choke port in the bowl is clogged.

What about this slide diaphragm thingy?

This is the toughest part to describe on a variable ventri carb. Below the diaphragm you have atmospheric pressure, above is a signal from the throat of the carb. As air passes through he carb it creates a low pressure signal to the top of the diaphragm. When you open the throttle you allow more air to pass through the carb creating a vacuum on the port leading to the top of the diaphragm. If pressure is greater on the bottom than the top, then the diaphragm rises, raising the slide and allowing more air and fuel into the engine. When you close the throttle, you lessen the vacuum on the port making the pressure more equal dropping the slide, and allowing less air and fuel.

With this, you should begin to see how the different parts of a carburetor work hand in hand to deliver the proper amount of fuel to each cylinder.

Hope this helps clear some confusion and not create more.

[grazingazer]

excellent...to the point...and informative...well done mike

[P.O.L. Mafia]

Thank you Sir.

[club_c]

wow...

[eaglebeak]

Bravo, Mike - very well written and comprehensive. Certainly needs to be parked in our tech library.

EB

[yamaguzzi]

mike thank you for going to the trouble of writing the article...so let me get this straight.....the pilot air screw,,the one we have to access from underneath on the outside of the carb body...not the screw that is underneath the cap which is often times removed and left off....please explain if the pilot air screw is screwed in,,what does it do,,if it is screwed out what does it do??? my apologies for this question,, but as i have said before there are too many jets and screws each with their technical name and each with a full range of nicknames...i believe many of us suffer from jetscrewdigo,,,,and combination of,,,,jet,,,,screw,,,,ID,,,,vertigo,,,you are one great guy..yg

[P.O.L. Mafia]

mike thank you for going to the trouble of writing the article...so let me get this straight.....the pilot air screw,,the one we have to access from underneath on the outside of the carb body...not the screw that is underneath the cap which is often times removed and left off....please explain if the pilot air screw is screwed in,,what does it do,,if it is screwed out what does it do??? my apologies for this question,, but as i have said before there are too many jets and screws each with their technical name and each with a full range of nicknames...i believe many of us suffer from jetscrewdigo,,,,and combination of,,,,jet,,,,screw,,,,ID,,,,vertigo,,,you are one great guy..yg

Yo Guzzie check the "main, pilot, idle" section i added a piece just for you m8

[ms14796]

Actually they are one and the same. On the earlier Hitachi carbs the idle mix adjust screw was open so one could tweek it if need be. The needle is mounted under the carb next to the intake boot. On later models Yamaha installed a plug to prevent those of us that like to tinker from getting into the Idle Mix circuit. I call it a Idle mix because in essence that is what you are doing; adjusting the fuel/air mix during idle for optimum mix. When you screw the needle in you are leaning out the mix which is why the engine starts to run rough. Conversly when you turn the screw out the mix gets richer to the point where the engine is getting too much fuel and it will, again, stumble. So by turning the screw in and out you will eventually find the sweet spot where the engine will run at it's optimum mix.
The screw mounted on the side of the carb along with the trottle linkage is the Idle Speed adjust screw. This is only to set the idle speed to 1000rpm during idle. There is a set screw mounted on the rear cylinder with a nut that, in a perfect world, is supposed to be factory set and should never need adj. RIGHT...

[grazingazer]

i've always thought of it as the idle mixture control screw...cause that's what it is and that's what it does

[yamaguzzi]

the older i get the more tolerant when it comes to the vgo and the wiser than me..i understand the line " do as i say, you dont have to know why,,just trust me." thanks mike for the adjustment and edit.

ms: right,,i discovered that adjustment screw and its factory set lock nut early on..quickly became an advocate for its double chek and slight tweeking,,made a huge difference in my idle.. take care yg

[Callan]

Great post. Helps the non-mechanical like myself get a bit more of a handle on the way that these mysterious things called "carbs" work.

[Bad_Neko]

Is there a way to adjust the main air/gas mix? I replaced the old, worn, cracked and very obviously leaking intake boots on my XV750, which it sounded terrible but ran fine. Once I replaced those intakes, it now runs like poo. The gent I bought it off of had left it to sit, but was nice enough to have tuned it up before I bought it, I'm ASSUMING the carbs were adjusted to run with the leaky intakes, and now he's passed away so I'm kind of...stuck. It just dogs out, terribly, there's no acceleration or top end. It idles so-so, but you give it the throttle, and it wants to die immediately. I'm not completely mechanically illiterate, but the service manual diagram makes my head hurt, and I'm not sure what I'm looking for.

[ms14796]

http://www.viragotech.com/fixit/carbfix.html
http://viragotech.com/fixit/techtips.html

Please open and read the first link. It will get you started on tuning the carb. Then come back and to this posting.

If the carb was tuned with leaking manifolds then it was tuned too rich. Once the leaks were sealed you began feeding too much fuel and not enough air into the intake. I would start with re-adjusting the Idle Mix screws on the bottom of the carbs to lean out the idle and then go back and tweak the Idle speed and re-sync the carbs. Unless the Main Needle valve was raised up you should be fine with these adjustments.

[funkamongus]

concise and clear. Thank you very much for broadening my understanding.

[ßullet™]

Great read here I recently ordered some K & L repair kits from an ebay seller, it was listed as " Yamaha,XV 920 Virago,fits 81-83,two carb rebuild kits,*" part number 18-2578. Upon receiving and after disassembly of carbs i find it's not the right kits. My carbs have the pressed in pilot jet tubes, the bowl gaskets and idle mix screws are different. So basiclly all i'm able to use are the bowl drains and the main jets. I emailed the seller and informed him of this. His reply was..."you have 83 carbs same as the 84's, I worked for Yamaha from 1980-1985, carbs on the 920 shaft drive modele were the same excepting jetting from 81 until Sept of 83" ...Does this sound right? My bike had no title when i bought it from the salvage yard, so a lein sale purchase title was made. The bike was inspected by the California Highway Patrol and was said to be a 1983 model 920. There seems to be a lot of conflict, depending on which article you read on these carbs, when and where they were used.
Thanks for your input.

[P.O.L. Mafia]

He may have worked at Yamaha, but he is only half right. You have second Generation Hitachi and his kit is for first Generation Hitachi. There are a few parts that interchange but not much. The good thing is 99% of the parts don’t ware out. A simple cleaning will get them back in good order. Even the floats are a composite that cant leak so float sink is virtually imposible.

[seren]

Good read, thanx for the info.