Category Archives: Carburetor
Achieving Better Fuel Economy and High Performance with Gasoline Direct Injection
Back in the Day
How Direct Injection Works
Like an atomizer bottle spray, the fine mist generated by each solenoid-controlled injector’s tiny outlet holes creates a well-atomized air/fuel mixture. Injectors spray fuel into the cylinders at pressures of up to 2,150 psi, about 35 times more intense than port fuel injection. (Courtesy of Bosch.)
Each bank of cylinders has a high-pressure fuel rail that feeds the individual injectors, and a high-pressure pump with a rail pressure sensor that helps the vehicle powertrain control module precisely control fuel pressure. (Courtesy of Bosch.)
To control injection valves, new common-rail injectors use a rapid-action actuator made of piezo crystals. Piezo package movement is transmitted non-mechanically and without friction to the rapidly switching nozzle needle. This doubles the injector’s switching speed, allowing a more precise measurement of fuel injected, and thus reducing harmful combustion by-products. (Courtesy of Bosch.)
How Working Together Improves Performance and Economy
Servicing
How to Repair Small Engines part1-Small Engine Basics
Fuel and Exhaust
Ignition
Combustion
The Benefits of Regular Small-Engine Maintenance
Benefits of Regular Service
- By establishing a service schedule, you will gain confidence that whenever you need the unit it will be ready for use.
- By performing a number of service functions together, you will save time. You can pick up all needed parts and lubricants in one trip to the parts store. Then you need to disassemble a component only once to perform numerous service procedures rather than taking it apart many times.
- Regular service gives you a chance to visually inspect the entire engine and related components for damage, wear, and other potential problems.
Four-Stroke Carburetors – Jetting
- Pilot too lean
- Idle set too high
- Improper starting procedure
- Bike wants hot-start button (KTMs and 400Fs)
- Idle set too high
- Air leak in intake or engine
- Pilot too rich (when bike is hot)
- Pilot jet too lean
- Air filter over-oiled
- Motor oil too thick for temperature
- Main jet too rich
- Air filter over-oiled
- Spark plug has debris on electrode
- Pilot jet too lean
- Idle set too low
- Valves set too tight
- Decompressor is set too tight, so turning the bars engages release slightly
- Float level too low
- Carb vent tubes blocked
- Main jet splash shield not installed
- Float level too high, gas is trapped in vent tunes (install T-vents)
- Pilot jet too rich
- Water in fuel
- Debris in main jet
- Stuck float check valve
- Debris in gas or carb
- Main jet too lean
- Fuel octane too low, causing detonation
- Needle too lean
- Slide cutaway too lean
- Pumper circuit blocked or too lean
Understanding Carburetion – By: Canadian Dave
The air screw is most effective between idle through 1/8 throttle.
The pilot (slow) jet is most effective between 1/8 through 1/4 throttle.
The slide valve is most effective between 1/8 through 1/2 throttle
The jet needle is most effective between 1/4 through 3/4 throttle.
The main jet is most effective between 3/4 through wide-open throttle.
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I’ll talk about jet needles in greater detail in the Tuning section.
A Helping Hand
Stock -Run the stock R1174K jet needle in the second from the top clip position, 45 pilot jet, 155 main jet and fine-tune the pilot circuit using the air screw.
With a performance pipe/expansion chamber, the air box lid removed and the stock or a performance silencer run a 42/45 pilot, R1174K jet needle in the mid clip position, a 152/155 main jet fine tune the pilot circuit using the air screw.1997 to 2001 KDX220Stock run a 42 pilot jet,the stock R1173L jet needle in the second from the top clip position, a 142/145 main jet and fine tune the pilot circuit using the air screw.
With a performance pipe/expansion chamber, air box mods , the factory or after-market silencer and the stock 33mm carburetor run a 42 pilot, the stock R1173L jet needle in the second from the top clip position, a 145/148 main jet and fine tune the pilot circuit using the air screw.
Same as above but with your carburetor bored between 35 and 36mm or running a 1988 to 2000 KDX200 35mm carb jet according to 95 to 2000 KDX200 requirements.
Addition of Boyesen reeds or a Boyesen RAD Valve will require you to lean the pilot and main jet one size and readjust your air screw.1989 to 1994 KDX200Stock -Run the stock R1172N jet needle in the second from the top clip position, 48 pilot jet, 155 main jet and fine-tune the pilot circuit using the air screw.
With a performance pipe/expansion chamber, the air box lid removed and the stock or a performance silencer run a 45/48 pilot, R1173N jet needle in the mid clip position, a 152/155 main jet fine tune the pilot circuit using the air screw.
Addition of Boyesen reeds or a Boyesen RAD Valve will require you to lean the pilot and main jet one size and readjust your air screw.
how to clean your carburetor the proper way
Carbs come in many shapes and sizes. Single carbs, dual carbs, racks of 3, 4 or 6, V racks, carbs with ticklers, carbs with accelerators, carbs with asynchronous designs, and carbs that operate vertically. While working on some carburetors is more difficult (due to design) than others, they all share the same basic components, and the process of cleaning those components is generally indentical.
BEFORE YOU START
Make sure that dirty carbs are actually your problem. Lots of things can make a bike run poorly or not start. Weak battery, corroded electrics, old spark plugs, bad timing, low compression, mis-adjusted valves, dirty air filter, and plugged exhausts can all cause poor running. I’ll write an article on how to diagnose poor running conditions shortly, but for now – lets just deal with the carbs.
OK, SO YOUR CARBS ARE DIRTY
Once it has been determined that the carbs are the problem it’s time to get to it. Some racks of carbs are easier to remove than others. If you’re working on a newer model bike the rubber boots from the airbox to the carbs and the manifold boots from the carbs to the motor should be relatively soft and pliable. On older bikes however this is rarely the case.
First remove the fuel tank, seat, and side covers. Depending on your model of bike other parts may need to be removed too. Â For many single cylinder bikes the carb can often be removed without removing any body work at all.
The bike below is a 1983 Yamaha XJ750 Seca with 4 inline Hitachi carburetors
You’ll want to loosen the circle clamps on all of the rubber boots. Sometimes I’ll even take them all right off (carefully, without bending them too badly) so that they aren’t in the way.
Inspect the airbox. On many bikes it is bolted in place to tabs on the frame. Remove those bolts and try to create as much space as possible for the airbox to pull backwards.
Next, put the bike on it’s centerstand and straddle it facing forward. Put your right hand on the right-most carburetor and your left hand on the left-most carburetor and get ready to sweat. Sometimes you’ll be able to pull the carbs straight backwards nice and easy, but that is pretty rare. I usually end up rotating them up then rotating them down as best I can while pulling backwards furiously. This can really take some work and time, especially if you’ve never done it before. In real extreme cases where you simple can’t get the carbs to pull backwards out of the manifold boots I have a couple tips. These tips should only be used in extremely difficult cases when you have been struggling for an hour and simple can’t get the carbs to pull backwards out of the manifold boots.
Tip 1: Ratchet Straps – This is sort of a last resort, but it has worked without fail for me when I’m pooling sweat on the garage floor and the carbs aren’t budging. Wrap a ratchet strap around one of the outer carbs and put the hooks somewhere on the rear of the frame. Then slowly ratchet the carbs right out of the boots. Be careful not to pull them too cockeye’d or you could damage the boots. Attach a second ratchet strap to the other side if necessary. (Note: you can do this in the opposite direction to force carbs back into the boots once they are clean.)
Tip 2: Full Pull! – You should do this before you do the ratchet strap method above. Sit down on your butt along one side of the bike. Wedge one of your feet up between the forks and the front fender, then put both hands on the same outermost carb and PULL PULL PULL! This might not work so well if you’re short! Ha.
Ok, So The Carbs are pulled back
Chances are the airbox boots are all crammed up now. Do your best to rotate the carbs up and out from the boots and pull them out one side of the bike. Sometimes it’s easier to pull the carbs out one side than the other, so have a look to see if there are frame elements, motor elements, or hoses that may block the carbs from coming out on one side.
Also keep note of the throttle cable(s) and choke cable (if there is one). Now may be a good time to loosen the nuts that hold them in place and disconnect them.
Struggle just a couple more minutes wriggling the carbs out the side.
Ok, You have the carbs off the bike
Make sure you brush off any loose dirt or grime, then flip the carbs over and remove the screws from the corners of the carburetor. Some carbs won’t have bolts in the corner and instead have a wire latch over top which can just be forced over.
Remove the bowls.
If the carbs are real gummed up the insides might look like this:
It’s obvious that these carbs are all clogged up. Some carbs might not look so bad, some might be a lot worse. It’s always a mystery what will be inside the bowls.
Now it’s time to remove the floats. It’s generally a good idea to drench everything in carb cleaner (available at any autoparts store). Sometimes the pins will practically fall right out, sometimes they’ll be so stubborn you won’t think they’ll ever come free. But they will! Carefully push on the pin from either side. Sometimes a nail and a gentle tap from a hammer is helpful. **BE CAREFUL**, using force to remove a stuck float pin can break off the pin tower. If they are really stuck and you can’t seem to work them free here are a couple tips.
Tip 1: Heat – Adding a little flame to the float pin towers can help. **Don’t Burn Down Your Garage!!**
Tip 2: Pliers – Using pliers to gently clamp the end of the pin and push it through has worked well for me in the past. **Don’t break the towers!!**
Once the float pin is out you can remove the floats, the float needle, and unscrew the float jet screen.
Set everything aside. Next remove the main jet, pilot jet, and idle jet (if there is one). They should come out easily with a flathead screw driver.
Set them aside.
Next flip the carburetors back over and remove the caps. Underneath the caps is a rubber diaphram with a spring. Sometimes the caps have a tendency to shoot off the top, so be very methodical when removing the screws. Other times the cap tends to stick down until you start to pry at it, then it shoots off, again, just be cautious and don’t loose any parts.
Next you’ll want to gently pull the slides up out of the carburetor body. You can gently pull on the rubber diaphrams, but be very careful not to tear them. If they don’t come up easily stick your finger into the carb intake and push the slide up with your finger. You can also gently pry it with a screw driver (gently). If it doesn’t want to budge don’t force it. Instead finish reading this article and pay attention to the boiling tips further down.
Now your carbs should be pretty well emptied out. If the throttle on the bike moved fluidly and smooth there is little reason to do much to the carb bodies themselves. However, if the throttle was real sticky or frozen there are a few things you can do to free it up. Sometimes just drenching all the throttle components on the carbs and letting it soak is enough, other times it is not. I generally try not to break racks of carbs apart. It isn’t often necessary and can be confusing to put everything back together in the right places. Also, the little rubber connector hoses and o-rings have a tendency to crack or leak if you mess with them. If you can’t work the throttle back and forth until its smooth have a look at the boiling tips further down.
Keep it Neat
Organization pays off.
Clean the Main, Idle and Pilot Jets
Hold each jet up to the light and see if you can look through it. The idle and/or pilot jets have extremely small holes so make sure you are looking through them straight. If you can see through the jet it isn’t clogged. There could be a little gunk built up around the edges so spray them down with carb cleaner and let them sit a bit.
If you can’t see through the jet it is clogged and needs to be cleaned. Always try the easiest things first. Here’s an ordered list of a few things you can do to clean the jet.
Blow through it. – Rarely works, but hey, who knows.
Compressed air. – Force 100 pounds into it. Works occassionally. Make sure to hold the jet tightly so it doesn’t go flying across the garage. You might put the jet back into the carb body to hold it in place for this.
Soak it in cleaner. – When I first started cleaning carbs I thought carb cleaner would be the magic answer. It isn’t. In fact, I hardly ever use carb cleaner any more, because it simply doesn’t do a very good job of anything but removing varnish from the bowl and slide. But try this.
Poking it through. – Collect a few different diameters of needle like objects. A wire from a steel bristle brush works well, a bristle from a broom works well, a baby pin, small sewing needle, etc. Very gently try to poke it through the jet. If you are using a metal needle use caution, brass jets can scratch and deform easily.
Boiling! – This works better than anything. Toss the jets into a pot of boiling water and let them bounce around for a couple minutes. When you pull them out blow some compressed air through them and you’ll most likely be good to go.
Some idle jets can be real tricky and never seem like they’ll be cleaned out . . . Just keep working at it, I’ve never met a jet that couldn’t be cleaned.
Cleaning the Choke and Air Mixture Screw
Air mixture screws have a tendency to strip or break. If the carbs were real gummed up you might find that the air screws are stuck. Don’t force them, if they don’t want to come out, just leave them for now. It is fairly rare that these screws will need to be cleaned because they are above the float level. If you can get them out just wipe them down with carb cleaner and spray some through the jet.
Cleaning the Slide and Needle
These are easy to clean. Squirt them with a bit of carb cleaner, wd-40, or anything similar, then wipe them down with a rag. Once the varnish is gone they’re good to go. Sometimes they get heavy varnish on them which I will scratch off carefully with a piece of plastic. Scratching the slide and needle is a BAD thing, use caution.
Cleaning the Carb Bodies
Use the same squirt and wipe method noted above. Most of the time the other pressed jets and passages in the carburetors won’t be clogged. But if the bike has been sitting a real long time with squirrels in the airbox it is certainly possible. Us a compressor to blow some air into every passage you can see. Listen for the air coming out the other side. If no air compressor is available use a can of WD-40 with a straw attachment.
If some of the pressed jets are clogged it can be difficult to open them up. There are a few things you can do.
Carb Dip – Most autoparts stores sell carb dip. It comes in a can similar to a paint can and is a VERY harsh cleaning agent. Soak the entire carbs in this dip. This dip can eat at rubber and plastics if they are submerged for too long, so try and remove everything you can from the carb bodies before soaking them. Once you pull them out swish the carbs around in a bucket of water to clean off the excess dip, then hose them down with WD-40 to get rid of the water.
Boiling in Water – Not many people do this but it is by far the best way of cleaning carburetors. Dropping the carbs into a pot of boiling water will instantly free up stuck slides, throttle plates, and other frozen parts. It will also loosen the dirt and grime clogging up pressed jets and other passages. Just make sure to dry the carbs thoroughly with compressed air or the sun afterwards.
Boiling in Lemon Juice – There is NOTHING BETTER at cleaning carbs than a giant pot of boiling lemon juice. The acidity from the lemons eats through everything; gas varnish, oil build up, dirt, grime, etc. Sometimes I won’t even bother doing anything but this – I’ll just remove the bowls, remove the caps, then drop everything into the pot and let it sit for 20 minutes (rotate them a few times). The one caveat to doing this is that you’ll want to wash the lemon juice off the carbs as soon as you pull them out. So have a bucket of water ready, or a can of WD-40 to hose them down. Also note that the acidity has a tendency to put a dull finish on the aluminum bodies of the carbs. This isn’t a problem in most cases, but if you must have everything shiny be prepared to do a little scrubbing and polishing afterwards. It may sound weird, but trust me, I just saved you LOTS of time. (Most dollar stores sell 1/2 gallon jugs of lemon juice, so buying a few gallons will only cost you $6. Plus you can put it back into the bottles afterwards and save it for next time.)
Cleaning the Bowls
This is pretty straight forward. Use any of the methods above to tranform your varnished bowls.
Most carb bowls are simple, just clean them up and they are good to go. But I picked this Hitachi’s for photos because they have a jet built into the bowl. You can see the ‘fifth’ hole along the edge of the bowl, that is actually a thin passage that extends to the bottom of the bowl. This is for the idle jet and is extremely important. If these passages are clogged, the bike won’t stay running, period. Use the same poke, soak, and boiling methods outlined elsewhere in this article. Not all bowls have these passages, only some, if your’s don’t – good for you!
Once Everything is Clean
Now that everything is clean it’s time to put it all back together. Take your time and make sure you put everything back where it came from. WD-40 is your friend. When screwing in the jets don’t over do it, they only need to be seated and snug, do NOT use any force putting the carbs back together.
If the bowl gaskets got goobered up you can put a little RVT on them. So long as the float needles are still in good condition leaky gaskets shouldn’t be an issue. However, prudent carb tinkerers may want to order replacements if necessary.
Once the carbs are back together stuff them back into the bike!
Extra Notes
Rebuild Kits – This guide did not mention rebuild kits until now. Rebuild kits (consisting of new gaskets, jets, needles, etc) can be purchased for nearly any bike, both old and new. 95% of the time these are NOT needed. I have rebuilt enough carbs to block off main street, and only once have I used new parts. ONCE!
Carburetor Adjustment – Carburetor adjustment, setup, jetting, and synchronizing is a whole encyclopedia waiting to happen. Those topics are not covered in this article, but I will address them in future articles.
Carburetor Polishing – External carb asthetics will be important to some, and not to others. Cleaning is all I am covering here, this will be addressed in the future.
Work Space – Make sure you have lots of space to keep organized. I also like to work on a wooden surface because it absorbs the spilled gas and cleaners rather than pooling.
That’s it! You’re Done!
Fuel intake system – Basics
Dear Biker,
Are you tired of cold starts? Need to raise the throttle everyday to get the cold engine running?
Does someone have the same bike as yours and are you tired of losing to him in drag races?
Suddenly, the fuel efficiency has reduced after a service?
Has the mechanic has screwed up your tuning?
Engine gives hiccups when you open the throttle
Or your bike is just missing that smooth acceleration whenever you want?
Or your engine overheats even if you did not race too much.
May be the following will help you in tuning your mean machine for optimal power delivery and best fuel economy. This guide aims at tuning the fuel/air screw to give you an optimal air-fuel mixture. An optimal mixture gets maximum performance from your machine.
Getting The Basics
I am assuming that you know driving very well and have completed at least 1000kms on road. If not, then you are too amateur to read this guide.
Every bike has a engine (motor/mill/muscle) and a carburetor (breathing system/heart) – Yeah I know that. My point is, I assume that your engine is in perfect condition. That is, there is no oil leaking from it etc etc.. The next assumption is that your engine is not DEAD. You know where your engine is and where your carb is and you have a screw driver/blunt knife with you. Your machine must have finished at least one service ~500kms or more. Also make sure you do not change the fuel type while tuning i.e. Power, Speed, Extra premium, normal unleaded etc.
This Guide Applies To
All the single cylinder bikes, 2 stroke or 4 stroke having a carburetor. Tachometer is very handy and amust for a newbie or the tuning process will be very slow or painful or imperfect.
Disclaimer: Though whatever I have written here wont do any damage to your machine, finally you must not blame me for whatever happens. I am here to help if anything goes wrong.
So Lets Get Started
Warm the engine nicely. Go get a ride around the town. Speed nicely at least upto half the top speed of the bike. Drive at least for 10-15 minutes so the engine system warms up nicely or you’ll get a bad tune. Do not just leave the engine idle and start with process. I mean it. Ride the bike.
Now that the engine is adequately heated up, you are ready to begin with the process. You can select a place far away from the city or your residential area so that you do not disturb the tired, sick, disgusted, old people or babes (babies) living in your colony. You are likely to be shooed away from them when you are at a critical point. You may choose a place near your girl friend’s house just to impress her! Put the bike in main stand and get hold of your screw driver. That is all you need, together with your eyes, brains and ears.
Searching the fuel/air screw… The above mentioned screws are responsible to adjust the air-fuel ratio which forms ‘food substance’ for your engine. You can relate it to our human body, where proteins, carbs, fats, water etc all are required in appropriate proportions with respect to each other for good strong body.
There is another screw which sets the idle speed of the machine. This screw is not related to pickup or mileage. It just sets the engine rpm at ’’idle’’ run. 2stroke machines have an Air screw and 4 stroke machines have a Fuel screw. I hope you note this VERY well. Air screw is located on the carb away from the engine. [ Engine – Carburettor – Air screw]
Fuel screw is located on the carb but its near the engine. [Engine – Fuel screw – Carburetor]. If the make of your carb is Mikuni (Pulsar, Yamaha, Fiero) the fuel/air screw probably will be of brass (golden colour). If you fully unscrew this screw and take it on your hand, you will see a needle like tip.
Idle screw is closely linked with the throttle cable. Idle screw can be turned by the hand. Screw driver is not essential for it. Finding these screws are very simple. If you still cannot find the air/fuel screw, ask your mechanic or I can help.
Ok now what? Apart from the above difference I gave between Air and Fuel screw, there is one more major difference. Fuel screw turned in (clockwise) gives a lean mixture and turned out (anti-clockwise) gives a rich mixture. Air screw turned in gives a rich mixture and turned out (anti-clockwise) gives a lean mixture. Lean means more air, less fuel. Rich means more fuel, less air. This ends the basics.
Getting Things In Action
Turn the idle settings screw so that rpm reaches about 3000 rpm. Now tune the air/fuel screw to make the mixture leanest as possible. Please refer to the above whether you must turn the screw clockwise or anti clockwise.
As you make the mixture leaner, slowly the engine RPM decreases… Go on doing this until you have put the fuel screw to the leanest possible point. At the same time, ensure that the engine does not stall, by turning the idle-screw.
Tough huh? You’ll get used to it
OK the engine is running and the mixture is lean. If you notice from the engine sound OR if you see the tachometer, the engine RPM will not be steady at this point. Now very slowly start turning the fuel screw anti-clockwise, quarter to 1/8th turn at a time. You will notice that the RPM increases slowly and steadily. Again, do this very very slowly. Also count the total number of turns as you wind out the screw.
You’ll notice that when you have turned it to about 3-4 full revolutions, the engine RPM slowly becomes constant. It is this point that you must stop screwing more. This probably is the optimal setting for your engine.
Further on, try turning the screw even more and more to 5-7 revolutions and you’ll notice that the engine RPM will slowly decrease. When this happens, you are just putting in a too rich mixture in your engine. At this point the engine loses all the fuel efficiency and the mixture is not optimal. Repeat the process about 2-3 times. Count the revolutions each time and get the setting which you feel is correct.
OK the engine is humming perfectly, now what? Decrease the idle setting screw to about 1000rpm in the tachometer. When the engine slows down, just twist the throttle. The response should be crisp and quick. It should not give any hiccups! Try shutting off the engine and restarting. The engine MUST start in a single kick or self with out giving throttle. If this happens, the setting is ok. Now get a ride and you’ll notice the difference for good or bad
You’ll immediately notice change in the engine sound and the throttle response. Your engine can become more smooth or harsh. Another important point is, Ride and Feel. Always take a ride and get the feel of the bike in each gears, check the response and the engine sound. You’ll quickly come to know once you get the feel of the bike that you want to make the mixture rich or lean.
For some reason everyone seems to think tuning a carb is just real easy. Change a jet or two and boom, your there. Yeah, right ! There are quite literally millions and millions of jet combinations. A rough check on Bing carbs shows there are at least 13,860,000 different combinations of jets. If you are going to change carbs you’d better be prepared to spend some time and money on the job.
If you look at a carburetor, you will notice a rather large hole going from one side to the other. This is called a Venturi. Air passes into the engine through this hole (Venturi). As the velocity of the air entering the carb (and then the engine) increases, it’s pressure decreases, creating a low pressure or vacuum in the venturi. This vacuum moves around in the venturi, as the throttle is opened, and sucks gasoline through the different jets in the carb. The gas then mixes with the air going through the venturi. The way the jets are made causes the fuel to vaporize as it goes into the venturi. Where the jets are placed in the carb and where the jet’s outlet is located in the venturi, determines what part of the throttle opening that jet controls. The idle jet system (comprised of pilot air jet, pilot fuel jet and pilot fuel screw) controls from 0% to about 25% of the throttle opening. The throttle valve controls 0% to 35% of the throttle opening. The needle jet and jet needle control from 15% to 80% of the throttle opening and the main jet controls 60% to 100%. This means that when you open the throttle about one eighth of the way open, all of the gas/air mixture going into your engine is controlled by the idle jet. As you can see, the different jets over lap the operating range of each other. That is, the jet needle starts to effect things before the effect of the idle jet ends. This is something to remember when working on carbs… everything is interconnected. Change one thing and it will effect other things.
OK, let’s go over the different systems in the carb and see what they do.
Fuel level. The fuel level is controlled by the fuel floats and the fuel float valve. The floats are hollow or made of something that will float on gasoline, such as cork. Part of the float presses against the float valve, sometimes called a needle and seat. Most times the part of the float that touches the float valve needle is bendable so you can adjust the level of the fuel in the floatbowel. All plastic floats are not adjustable. If this level is way too high, gas can leak out the carb overflow tube or into the engine. If fuel gets into the engine it will thin out the engine oil, ruining it’s ability to lubricate. This will, sooner or later, blow up your engine ! If a full tank of gas in the evening turns into a half tank by morning, check your oil. If it’s thin and smells like gas, change it and replace your float valve and/or check your fuel level. If the oil is OK, check under the overflow tube. If it’s OK, then check where you are parking your bike ‘cuse someone is walking away with your gas !If your fuel level is just a bit high, the mixture will tend to be a bit rich. If it’s low, the mixture will tend to be a bit lean. This is because a high level takes less vacuum to suck fuel into the engine and a low level takes more vacuum to do the same.
Pilot or idle jet system. The idle jet controls the idle and on up to quarter throttle, give or take a bit. On some carbs, like Mikuni there is an air jet too. In conjunction with the idle jet there is an idle jet air screw. This screw leans or richens the fuel mixture for a smooth idle and on up to one quarter throttle. From the idle jet, there are little passages cast into the carb that lead to holes just in front of the throttle valve or plate. There can be just one hole or there can be several, depending on the carb design. They effect the mixture as long as the vacuum, in the venturi, is over them. As the throttle opens further, the vacuum moves to the needle jet and jet needle.
The Throttle Valve. The big slide that opens and closes your throttle has a bevel angle cut in one side of the big round (can be flat, too) slide, toward the air cleaner. This angle comes in several sizes and helps control the fuel mixture from idle to about 35% open throttle.
Needle Jet. This jet doesn’t really even look like a jet, but it is ! It controls the fuel mixture from 15% to 60% open throttle. It sets in the center of the carb, right over the main jet.
Jet Needle. This is the needle that rides in the throttle slide and goes into the needle jet. This needle controls the fuel mixture from 20% to 80% open throttle. It can come in many different sized tapers. Sometimes, one needle can have several tapers on it. The top end of the needle has grooves cut in it, usually five, and you can move the little clip on the end up or down to lean (down) or richen (up) the mixture. Most late model bikes have needles with only one groove cut in them. This is so you can’t richen the mixture, thereby keeping the EPA happy.
Main Jet. This jet controls the fuel mixture from 60% to 100% open throttle.
We want nice clean acceleration from idle to full throttle, with no stumbling or flat spots. This can be quite a tall order if we are starting with a new carb. Actually, it can be a real challenge to get things to carburate right after something as simple as an exhaust pipe change.
Now, I wish I could tell I’m the great carb man, but, well… no one has ever been dumb enough to hire me to really work over a carb. Well, there was that one time with that Kaw 650 and aftermarket pipes. It had some kind of weird stock carbs that looked like Mikunis but really were not. It had TDK or KDT or DTK, something like that, carbs. It had aftermarket exhaust pipes and was running too lean, and stumbled at one point under acceleration. Worthless pig ! The jet needles where not adjustable, so I put little washers under the needle clip, to raise the needles. The main jet only came in one size, so I drilled it out with ity-bity, expensive, jet drills. I could move the miss around, but I could not get rid of it. From the beginning I told the guy it wouldn’t work and that he was wasting his money, and that at the least we needed carbs we could get parts for, but nooo. Just rise the needles, drill the jets he said… $200 later he finally gave up. I guess I shouldn’t complain, I did get paid… but !
But you want to try it, don’t you ? OK, the drill really isn’t that hard. Simply run the engine at whatever throttle opening you want to test, for a mile or so, and look at the spark plug. Is the spark plug reading lean or rich ? Now look for the jet that controls that particular throttle opening and exchange it for a richer or leaner one. Now that doesn’t sound very hard, does it ? Oh yes, the throttle transition from one jet to the next must be smooth too ! Go back over the areas that each jet controls. They overlap each other. Some a little, some a lot. Make sure you have a good selection of jets ! Most carb manufacturers have tables of specifications on the jet needles and needle jets, and other jets that you will find very useful. With these specs you can make a better guess as to what jet will work best. Some places use motorcycle dynamometers for testing. These can be a big help to get real close to the best jet setting. Working out the best main jet for a 170 MPH bike can be quite unhealthy if you only have a freeway to test on ! Just remember one thing. A dynamometer is not the real world. A fact more then one factory has found out the hard way when their super hot, dyno tested, race machines didn’t run so fast in the real world, on real pavement, in real air with real bugs on the windscreen !
Anyway, what I’m trying to get over to you is that just because your buddy said he got new carb, changed a jet or two and now his bike gets 100 miles per gallon and has double the horse power, doesn’t mean you can too ! It just might require a lot more work than you bargained for.
Look on the bright side. Carbs used to be real simple at the turn of the century, but they didn’t work as good as today’s carbs.
Oh, one last thing, seeing how we are talking carb theory. When an engine is cold, like when you first start it up. It doesn’t evaporate the gas well. Liquid gas does not burn, so you have to put in lots of gas, because a lot of it does not vaporize. The choke helps the carb to put into the engine a very rich mixture, and at least some of that mixture will vaporize and burn.
I had one guy tell me that the reason for a rich mixture when starting was so the pistons would be lubed by the raw gas and spin the engine over easier so it would start ! He felt very strongly about this, so I didn’t say a thing.