Carburetor Overview, Keihin


Carburetor Overview (Keihin)

Although intended for Keihin non-pumper type carb, this info may be helpful for other carburetors as well.

Carburetor Overview

The carburetor on your dirt bike should perform suitably with the standard recommended settings under average load, climatic, and barometric conditions. However, to fine tune the engine’s power output, the carburetor may require adjustments for specific competition needs. Optional main jets and slow jets are available for your dirt bike. Any engine or air-box modifications or the use of an aftermarket exhaust system may require jetting changes. The function of the carburetor is to atomize fuel and mix it with air in proper proportions to suit engine operating conditions. In operation, the carburetor meters gas into the fast moving air passing through it. The atomized gas (a mist of liquid fuel) is then vaporized (changed from a liquid to a gas) by engine heat and the heat of compression to provide a uniform and efficiently combustible air/fuel mixture.

Air/Fuel Mixture

Carburetor Circuits


– Main Jet


– Jet Needle, Tapered Section


– Jet Needle, Straight Section


– Slow Jet & Pilot Screw

In theory, the perfect air/fuel ratio is 14.7 parts of air to one part of gas, by weight. A uniform air/fuel ratio of this proportion allows the mixture to burn completely without leaving an excess of either fuel or air. Rich (excessive fuel) or lean (excessive air) mixtures both result in loss of power. An excessively lean mixture can also cause engine damage. An intentionally rich mixture (from applying the choke lever) is used for starting because a cold engine reduces vaporization. A throttle valve (carburetor slide) controls the amount of air/fuel mixture delivered to the engine, regulating the engine’s power output. When the throttle valve opening is increased, engine speed (rpm) also increases and air rushes through the carburetor bore at a greater rate. Unfortunately, the rate of fuel flow through a fixed jet does not increase proportionately with an increase in air speed through a fixed venturi. At high speeds, the air/fuel mixture tends to become richer. For this reason, it is desirable to vary the venturi size and meter the fuel flow to maintain correct air/fuel mixture ratios over a wide range of operating speeds. This is achieved by using compensating jets and air-bleeds (air jets). Each of the carburetor circuits affects the delivery of the air/fuel mixture over a given portion of the throttle valve opening. These circuits overlap as shown on the graph.


Idle & Low Speed System

An adjustable pilot screw controls the idle mixture. The pilot screw is located in the passage between the low speed jet and the idle fuel discharge orifice to control the rate of flow of aerated fuel delivered to the carburetor bore. An adjustable throttle stop screw controls the idle speed by raising or lowering the slide position when the throttle is closed. A replaceable slow jet, located next to the main jet, controls the amount of fuel entering the idle and low speed system.

Intermediate System

Opening the throttle valve (carburetor slide) permits a transition from the low speed system to the intermediate system which meters fuel from the main fuel discharge (needle) jet. A tapered fuel metering rod (jet needle) connected to the throttle slide, extends down into the main (needle) jet. The jet needle position, which is adjustable, maintains the correct air/fuel mixture ratio through most of the carburetor’s operating range, just short of fully open throttle. At that point the jet needle is fully raised, and fuel flow will be controlled primarily by the main jet.

High Speed System

Fuel delivery is controlled by the size of the replaceable main (needle) jet and the thickness of the jet needle.

Float System

The float system is designed to maintain a constant and correct level of fuel in the carburetor’s float bowl. A float rises or falls with the fuel level in the float bowl. The correct float bowl fuel level is established by the carburetor manufacturer.

Float System Vents

Float system vents are necessary to ensure a smooth flow of fuel through the carburetor. The externally vented float bowl has its vent tubes routed to atmosphere so that atmospheric pressure can maintain pressure on the fuel inside the float bowl. Vent tube routing is critical. Any change in the stock routing of the tubes may pinch the tubes. Improperly routed tubes may also be exposed to low pressure when the motorcycle is in motion, which could change the pressure in the float bowl and alter fuel delivery.

Air Metering Systems

Air under atmospheric pressure is bled, into the carburetor fuel passages to improve fuel atomization, to stabilize fuel height in the jets, and to provide corrections in the air/fuel mixture ratio. Air jets and/or air-bleed adjustment screws control the relative amount of atmospheric air drawn into the fuel systems. This system is factory pre-set and should not be altered.

Main Jet Air-Bleed System

Low venturi pressure, which causes fuel to rise through the main fuel jet, also causes atmospheric air to flow through the air jet. Air and fuel meet and mix together in a perforated (emulsion) tube above the main fuel jet. The aerated fuel released into the venturi is more easily atomized than a dense un-aerated stream of fuel. Aerated fuel also has less tendency to fall back down the jet tube between intake strokes, thus stabilizing fuel height in the jet tube. The same effect can be observed when drinking beverages through a straw. When you remove your mouth from the straw, a frothy beverage tends to remain in the straw, but an un-aerated beverage will fall back down the straw into the glass.

Air-Cut Valve

An air-cut valve is used to prevent popping in the exhaust system during deceleration. The valve enriches the air/fuel mixture during deceleration. A diaphragm in the air cut valve is activated whenever high manifold vacuum is present, such as during deceleration. The movement of the diaphragm causes a partial blockage of the air bleed system in the low speed circuit. This reduces the aeration to the low speed jet which creates a richer mixture. The air-cut valve is factory pre-set and should not be altered. It may have to be replaced eventually, because the rubber diaphragm in the valve may deteriorate over time. If the diaphragm is deteriorating, you may notice leanness or a popping in the exhaust during deceleration.

Cold Starting System

Fuel does not vaporize well in a cold engine. For this reason, the carburetor must deliver a richer mixture. The mixture must not be excessively en-richened, however, or the combustion chamber can become flooded with liquid fuel. Your motorcycle uses a choke valve that en-richens the mixture by obstructing the carburetor bore. When the choke lever is closed (choke lever ON), it reduces the volume of air that can flow through the carburetor bore to fill the vacuum created in the engine cylinder. Atmospheric pressure in the float bowl then forces more fuel into the carburetor bore.


4-Stroke Engine Break-In

4-Stroke Engine Break-In Procedure

It is extremely important to constantly vary the RPM’s during break in. Do not let the motor run in one RPM range for longer than 5 seconds and do not let engine idle for first half-hour of operation.

Did you install a new cam?
During cam break-in, you need to maintain an engine RPM of 2500 or more. This is to keep good oil pressure to the new cam.



Warm the motor up thoroughly constantly varying the RPM’s until at normal operating temperature, approx 5-10 minutes.  Then shut off and let the motor cool completely (probably at least 3 hours).

Start the bike back up and warm the motor up thoroughly again.  It is now ok to mildly ride your ATV or motorcycle.  Ride about 15-20 minutes and then let the engine cool off completely again (at least 3 hours).

And yes, you’re doing the whole warm up/cool completely process TWICE.  It is important to properly break in your new piston/cam.


From then on, every time you ride, be sure that you warm the motor up thoroughly before riding.  The heat cycling of the engine tempers the piston.


For the first tank of fuel, do not ride wide open for an extended period of time.



What is Jetting

Carburetor jetting can be easily understood if we understand the basic principles of carburetor and engine operation. A carburetor mixes fuel with air before it goes into the engine. When the mixture is correct the engine runs well. The bottom line is a carburetor must be adjusted to deliver fuel and air to the engine at a precise ratio. This precise ratio can be affected by a number of outside and inside influences. If you are aware of these influences you can re-jet your carburetor to compensate for the changes. I’m going to show you some examples of how you can change your jetting for better performance and in some cases increased engine life. As with any engine work be sure you have good tools the correct parts and a good manual before you get your hands dirty!

Altitude Compensation

For our first example let’s say we find a new riding area WAY up in the mountains. Our jetting is dialed in for our usual riding area which ranges from sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from there. Going to a higher elevation will require will require a jetting change but which way? Like our fuel density, air density can also change. Higher elevations have less air density then lower ones. At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down one main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 160 main jet at sea level you would drop down to a 140 at 4000 feet. Something else goes down as you go up in elevation is horsepower. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted! For our second example let’s say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there’s a foot of snow on the ground the skies are clear and it’s COLD! Aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 140 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder than it was the day before you can actually go back to your sea level jetting, a 160 main jet! If you don’t rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference!

Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there’s a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny, sometimes “pinging” and running HOT. What happened?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually “leaned out” our mixture! One to two sizes bigger main jet will get us back into the hunt. If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and gray hair in the long run!

Pilots, Needles & Mains

So far we have only talked about main jet changes to compensate for altitude, temperature and fuel density. As most of you know there is a pile of jets in a carburetor. While main jets are the most critical for ensuring full power operation and engine longevity, the other jets are equally as important for a good running engine. Let’s run through them quickly.

Pilot Jets: Pilot jets control the low-speed and idle mixtures. Many times an adjustable jet is used in conjunction with the pilot jet. The adjustable jet allows a precise setting of the idle mixture. If the adjustable jet is located to the rear of the carburetor and usually on one side it is a AIR adjustment. It controls the amount of air that mixes with the fuel coming from the pilot jet. If the adjustable jet is to the front of the carburetor, on the side or bottom, it controls the amount of air/fuel mixture going into the engine. In either case if adjusting the mixture screw won’t improve the low-end running speed it’s time for a different pilot jet.

Slide: Throttle valves (the slide) control the off idle, to one-quarter open, mixture. Some aftermarket carbs have replacement slides available with different “cutaways”. Changing the cutaway changes the mixture. More cutaway is lean, less cutaway is rich. Some carbs do not have different slides available, so you have to compensate by changing the mixture on the idle circuit or needle circuit. Partial throttle hesitation or rough running can be caused by the slide cutaway.

Needle Jets: Needle jets control the amount of fuel going by the needle and into the engine at low to mid throttle. There are 2 types of needle jets used in a carburetor. One is a primary type that has a very precise hole hole drilled through the middle of it, along it’s length. The size of the hole relative to the size of the needle determines how much fuel goes into the engine. The other type of needle jet is constructed essentially the same except for a bunch of holes drilled into the side of the jet. These holes allow air to mix with the fuel before it’s metered into the engine. Either type of needle jet works well in most cases but there is power to be gained on high performance four-strokes by going to the needle with the holes in the side. These are called “bleed” type needle jets and produce more midrange power in a four-stroke. In any engine going to a leaner (smaller) needle jet is the easiest way to rejet the midrange running when going to higher elevations. Changing the needle jet leans out the mixture evenly at all the midrange throttle settings moving the needle clip doesn’t.

Needle: Jet needles more commonly know as the “needle” control the fuel mixture throughout the midrange. The shape or taper of the needle dictates how much fuel goes into the engine at a given throttle opening. The needle must work in conjunction with the fueling requirements of the engine relative to slide position. If you have an engine with a strong hit in the midrange the needle will probable have a noticeable reduction in size the the slide is half open. Remember it takes fuel to make power and when the engine makes power it needs fuel NOW! If it doesn’t get the right amount of fuel it pings or misses. You many have cleared up a little midrange pinging by moving the needle up a notch but at the same time you may have over richened some other areas. If the problem isn’t too bad you won’t even notice the rich condition. If the machine stutters before it comes on the power that part of the needle’s taper is too small and the only way to cure it is to get a needle with a different taper. Finding the right needle can be difficult so hopefully moving the clip will do the job.

Main Jet: Finally the good old main jet comes into play at three-quarters open to full throttle conditions. Most of you already know a bigger main jet has a bigger hole so it lets more gas into the engine! Pretty simple! As simple as it is the minuet is absolutely CRITICAL to high-speed engine operation. Not only does it meter the gas into the engine, it can aid in cooling the engine as well. A properly sized main jet will let the engine make good power for a long time. One size smaller main jet may make greater power for awhile. A slightly rich mixture burns cooler than a lean one so be sure the main jet is big enough!

One final note on jets. All of them and the carburetion functions then perform tend to overlap into some other jet’s territory. If you mess with one jet, you may have to mess with a few of the others. My best advice is to not change more than one jet at a time. Slowly work out the correct jetting and keep notes on what you are doing. If you get totally fouled up at least you can go back to where you started.

Sign, Symptoms & Causes

How would you know if there was something wrong with your jetting? If you listen, your engine will tell you! All you need is an interpreter. Since I speak and understand several different engine dialects, I will give you a hand. Let’s start with lean conditions because they can cause the most damage. In a lean condition the engine will surge and sometimes ping under acceleration. The engine will also be “cold-blooded” (hard to start and keep running) but will run better when hot. The spark plug will look bone white or burned in extreme cases. The engine may spit back or sneeze through the carburetor once in awhile too.. If the engine is running rich the throttle response will be fuzzy and not too quick. The engine will burble, miss and blow black smoke. It will start easy but will run funny when fully warmed up. The plug will be dark, wet or fouled (possible all three!).

Ok so what do you do first to cure the problem? The very first thing is to check and adjust the float level. If it’s off one way or another it can throw the jetting off too. Set the float to the specs and retest the running. The next item is to determine a rich or lean condition. Let’s say the engine gets hot and doesn’t pull well. This is a lean condition so the engine wants more fuel. Stick in at least a two size bigger main jet and try it again. If it’s better but still not right go even bigger on the jet. and try it again. Bear in mind that drastic or sudden changes in jetting usually mean an air leak has developed somewhere in the engine. Find it and FIX IT! When the engine burbles on the top end come down one jet size at a time until it winds all the way down. Don’t drop and more sizes! If the engine seems sluggish and lumpy or want to load up on the bottom end the mixture is TOO RICH. Adjusting the low speed mixture screw helps a little but doesn’t cure the problem completely. What you need now is a new pilot jet. Go one size smaller and try the adjustment again. When the engine runs smooth with the adjustment screw about one and a half turns out from the seat you have it!

Final Thought

There’s a lot more to jetting than just stuffing jets in holes and hoping the problem goes away. If you can understand what your engine is trying to tell you when it runs funny you will have a better chance or correcting the problem than someone who doesn’t have a clue. When you rejet, go slowly and carefully until the problem is solved. As a final thought let me remind you that jetting is a lot like life, if you have a choice it’s always better to be a little rich!


Credits: Article posted in our forums by qadsan, info from December ’96 issue of Dirt Wheels, and editing by

Do I Need to Run Race Fuel?


That’s right – fuel is a performance part, as critical to your vehicle performance as a camshaft or piston. When we talk about “Race Fuel”, in general we mean a fuel with a motor octane level of at least 100 octane.   Motor octane and Leaded or Unleaded are not the only variables to consider when choosing a fuel, but for most of us, that’s just exactly all we need to care about, so we’ll keep it simple.

In general and for most of our builds, we recommend VP-110 Race Fuel.   VP-110 is a LEADED racing fuel designed for normally aspirated engines with compression ratios up to 13:1 and works well in both 2-stroke and 4-stroke engines.  VP-110 has a motor octane rating of 107.  Higher-octane fuel burns cleaner, reduces detonation and improves engine life — and although race fuel is expensive, it’s a lot cheaper than having to rebuild your engine!!


100% RACE FUEL – Run 100% race fuel if the piston compression ratio is 12.5:1 or higher, or with a stroker crank or our full porting.

50% RACE FUEL – Run a mix of 50% race fuel and 50% premium pump gas when your piston compression ratio is between 11:1 and 12.5:1.

PREMIUM PUMP GAS (91 OCTANE) – In a stock atv or motorcycle, you can run premium pump gas (in our area, the highest octane available at the pump is 91 octane).  But the new stock 450’s have both a higher piston compression ratio and a better combustion chamber design, and they typically run just fine on premium pump gas.

After installing any performance upgrades, you should always check your jetting and rejet if necessary to ensure proper air/fuel mixture.  Unless we do the install, the customer is responsible for rejetting.

NOTE – once you choose a fuel and jet your motorcycle or atv for that particular fuel, don’t switch.   Fuel affects your jetting just like air quality, air temp and performance upgrades do.  Proper jetting strikes the balance between air and fuel delivery for your motor and keeps an engine running at its optimum – not too hot and not detonating or backfiring.

Contact us if you have questions.