Jetting theory

From Quad Hub Wiki

Carburetors look very complex, but with a little theory, you can tune your quad for maximum performance. This article is a general overview of the concepts that factor into achieving the correct jetting for your quad. Included is a basic guide to apply this theory.

Contents 1 General 2 Air Flow 3 Circuits 3.1 Pilot System 3.1.1 Air Screw 3.1.2 Pilot Jet 3.2 Slide Valve 3.3 Jet Needle and Needle Jet 3.3.1 Jet Needle 3.3.2 Needle Jet 3.4 Main Jet 3.5 Choke System 4 Air/Fuel Mixture 5 Troubleshooting 6 General Guide 7 Altitude, Humidity, and Air Temperature 8 References

General

All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow.

Air Flow

Atmospheric pressure will force high pressure to low pressure. As the piston on a four-stroke engine goes down (or goes up on a two-stroke engine), a low pressure is formed inside the crankcase. This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and mix with the air.

Inside a carburetor is a venturi. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.

Circuits

Most ATV carburetor circuits are governed by throttle position and not by engine speed. There are five main metering systems inside most ATV carburetors. These metering circuits overlap each other and they are:

• Pilot system
• Slide valve
• Jet needle and needle jet
• Main jet
• Choke system

Pilot System

The pilot system has two adjustable parts. The pilot air screw and pilot jet.

Air Screw

The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulates fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next larger size pilot jet will be needed.

Pilot Jet

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

Slide Valve

The slide valve affects carburetion between 1/8 through 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Slide valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).

Jet Needle and Needle Jet

The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle.

Jet Needle

The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.

Needle Jet

The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 through 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet.

Main Jet

The main jet controls fuel flow from 3/4 through full throttle. Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the main jet, the more fuel that can flow through it and the richer the mixture.

Choke System

The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed.

Air/Fuel Mixture

The air/fuel mixture must be changes to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer.

Troubleshooting

Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.

General Guide

Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a slight uphill is the best place for this). If you hear pinging or if full throttle causes gasping and poor pulling at mid RPMs, this is a good sign that the engine is running lean. If the quad runs clean select a larger main jet until you find the jet that causes a blurbbing (four-cycling) sound. When you experience that sound, you have found the jet that causes the engine to run too rich. So back off one size to a smaller jet. This is the safe main jet to use. You could go another size leaner; but you need to be careful to avoid running too lean which causes the engine to run hot and could seize the piston. It's better to jet on the rich side.

An alternate way to figuring out if you're too lean or rich is to examine the spark plug. After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan color. If it's white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run.

After the main jet has been set, run the bike at half throttle. Any pinging means too lean. Blurbbing means too rich. Excessive smoking is also an indication of running too rich. As before, you can check the plug color. If it's white, lower the clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the air/fuel mixture. Next go along slowly in one of the lower gears at less than 1/8 throttle and move the throttle quickly to 1/2 throttle. If the engine does not pull strongly. Sort of gasps for breath and only runs well after the RPMs build up, it is an indication that the needle is too lean.

The pilot circuit can be adjusted while the bike is idling and then test run. Get the bike idling by adjusting the idle stop up where it will idle slowly on its own. Try to find the air screw adjustment where you get maximum idle RPM. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite. As the idle RPM increases turn down the idle stop to return the idle RPM to a slow correct idle. You objective is to find the pilot jet that will give you maximum idle with the air screw set at 1.5 turns out. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the ATV runs cleanly off of idle with no hesitations or bogs.

Now when you find the correct pilot jet size you will want to do some final air screw adjusting to improve throttle response. Let the engine idle for 5 seconds then open the throttle abruptly and be aware of how the engine responds. If it almost dies, then you need a slightly richer idle mixture so that as that mixture is gulped at abrupt throttle openings it will be just a little rich and therefore give good response.

You can test this as you ride say slowly in 2nd gear with the throttle closed so that the engine is drawing only from the pilot. Open the throttle quickly. If you find yourself doing a wheelie, your air correction screw is set right. If instead the quad slowly accelerates, then your idle mixture is probably too lean and you need to richen it up a little.

Altitude, Humidity, and Air Temperature

Once the jetting is set and the bike is running good, there are many factors that will change the performance of the engine. Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temperature drops, the engine will run leaner and more fuel will have to be added to compensate. When the air temperature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32° F may run poorly when the temperature reaches 90° F.

Altitude affects jetting since there are less air molecules as altitude increases. A quad that runs good at sea level will run rich at 10,000 ft due to the thinner air.

Humidity is how much moister is in the air. As humidity increases, jetting will be richer. A bike that runs fine in the morning's dry air may run rich as the day goes on and the humidity increases.

References

• Original source