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Jetting 125 shifters

Jetting of 2 cycle shifter karts is part science and part art. There are many variables to consider when selecting a jet that the experts all recommend that we follow the process of the professional moto cross tuners. The process starts with a detailed log of inputs that are considered to make the jet decision. You should log altitude, temperature, air density, relative humidity and barometric pressure as well as all of the temperatures recorded on the kart, head temperature, exhaust gas temperature and water temperature.

Most 125 shifter carburetors are governed by throttle position and not by rpm or engine speed. To understand shifter jetting you have to understand what aspect of the carburetor is operational at what part of the track.

There are five metering systems in most shifter carburetors:
· Pilot circuit Just off idle through 1/4 throttle is affected.
· Throttle Slide Valve 1/8 throttle through 1/2 throttle is affected
· Needle jet and jet needle 1/4 throttle through 3/4 throttle is affected
· Main Jet 3/4 throttle through full throttle is affected
· Choke circuit Used to start cold engines

The good and bad news is that there are likely two or more systems operating at any moment and they all have to be correct to get optimum performance. Do not be dismayed most racers only adjust two of the systems. Most only change the needle clip position and the pilot/main jet. We will discuss each of the five systems and tell you the impact of each so that you can decide which adjustments you will use to tune your performance.

Before beginning a jetting exercise, there are some basics to be reviewed. First, be sure that the fuel/oil mixture in the gas tank is the same as what will be use for the race. To often, racers wind up with the fuel left over from the last event or which someone donated to the racer for practice. The fuel oil mixture has a big effect on the jetting and must be consistent when attempting to set the proper jetting. After we do the setup. Other concerns are: clean the air cleaner and keep it clean, know where the timing is set if someone had been tuning the motor, clean and maintain the pipe and the silencer, the must be plug gapped correctly and free of excessive deposits.

Now we can tune using the carburetor systems. This tuning is independent of carburetor brand. We all have our favorite brand of carburetor. I even have a favorite model of a favorite brand. The principals when applied will work for all of you that are using carburetors from the manufacturer of 125CC shifter motors.

Addressing the High End

Main Jet
The main jet is the mixture at full throttle. With the throttle at full open the fuel flow is under the control of the main jet the balance of the systems are passive at this throttle position. Please note that while the throttle position is a full open you must recognize that you may not be at full rpm. On shifter karts we discuss the position of the throttle not the rpm’s.

At full throttle if there is any perceptible ping, or if the motor is gasping and pulling poorly this is a symptom of a lean condition. Read your spark plug immediately. An air/fuel mixture that yields maximum power is only slightly richer than the one that causes detonation. The plug will tell you when there has been even slight detonation inside your engine. The signs to look for are pepper-like black specks on the insulator nose, and tiny balls of aluminum concentrated mostly around the center electrode's tip.

Severe detonation will blast a lot of aluminum off the top of the piston and give the plug a gray coating. If you find examples of detonation get you engine builder involved an determine if the motor can be run again or if it is in need of a rebuild.

If the kart is (what is called 4 cycling) or not running up crisply and blubbering then the symptom is that you are likely to rich. We all see plugs which are just wet and know that the mixture is far to rich but the real analysis takes place by reading the insulator deep inside the plug body. The insulator is coolest where it contacts the metal shell, which is where you "read" your mixture stetting. Look far inside the plug, where the insulator joins the shell, if your engine's mixture is too rich is a colored ring will be present. If this ring continues outward along the insulator to a width of even a millimeter you can be sure the mixture is rich enough to be safe, and too rich for maximum performance.

Addressing the Mid Range

Needle Jet Clip Position and Needle Size
The needle jet controls the mixture from ¼ to ¾ throttle. The jet needle is a long tapered rod that controls how much fuel can drawn into the carburetor venturi. The thinner the taper the richer the mixture. The thicker the needle the leaner the mixture. The tapers are designed to give different mixtures at different throttle openings. Jet needles have slots or grooves cut into their top. A clip in inserted into one of the groves holds the clips position in relationship to the slide. Changing the clip position will cause the motor to run leaner or richer. To get a leaner mixture the clip is raised. This causes the needle to be positioned further into the needle jet and allows less fuel to pass. For those whom we have just confused there are two items a needle jet and a jet needle. The needle jet is the object that jet needle slides into. Most of our tuning in this system is done with the jet needle and not the needle jet. The most difficult aspect of getting this system correct is to get the driver to drive you a session where you can run between ¼ and ¾ throttle so that you can read the plug and determine that you have the correct jet needle setting. If the plug color is white, lower the clip on the needle jet and richen the mixture. If the plug color is dark brown raise the clip to lean the mixture.

Addressing the Low End

Pilot Jet
The pilot jet affects just off idle through 1/4 throttle. The pilot jet is adjusted with the motor at idle. If the pilot jet is to lean the motor will hesitate when accelerating (motor in neutral) off idle. If the pilot jet is to rich the motor will smoke excessively and be unresponsive off idle. If you are adjusting the pilot jet by turning the screw between 1 and 2 1/2 turns and are having no affect then the pilot jet has to be replaced with a different size. The proper technique for adjustment is to turn the pilot screw ¼ turn for each adjustment and then test the motor performance. After tuning the pilot jet you may have to adjust the idle stop as you have likely altered the idle speed of the motor.

Throttle Slide Valve
The slide valve affects 1/8 throttle through 1/2 throttle. The affect is far more dramatic between 1/8 and ¼ and has a lessening impact up to ½ throttle. Slides come in a variety of sizes. The size ids determined by the area cut away from the backside of the slide valve. The larger the cut away the leaner the mixture as more air is allowed to pass the slide valve. For a richer mixture it is necessary to install a slide valve with a smaller cut away. Each throttle slide valve has a number either cast into or stamped into the throttle slide valve. If there is a number 1 this indicates a 1.0-MM cutaway if there were a number 3 it would indicate a 3.0-MM cutaway. The larger number would indicate to you that the mixture at 1/8 to ½ throttle would be leaner. For most karting applications only experienced tuners change throttle slide valves during race events.

Jetting a shifter requires you first to determine what system you are planning to address. You cannot address all of the systems from any single system. If you plan to address the low end the systems that you will be working with are the pilot Jet and the throttle slide valve. If you are struggling between ¼ and ¾ throttle then the jet needle and needle jet need to be addressed and corrected. When the engine is running poorly from ¾ to full throttle the work with the main jet.

Once you have gone through the complete process to establish your baseline then you can begin to rely upon other tools to assist with your jetting. You can begin to rely on some of the jetting slide rules from the carburetor manufacturers or you can rely upon on an air density gauge to assist in your tuning.

Always remember that there are a lot of factors that affect your tuning. Air density increases as the air gets colder. When the temperature drops the motor will run leaner and you will have to add more fuel. When the air temperature rises the motor will run richer and you will have to take fuel away. It is mandatory that you make jetting changes if air temperature changes 10 degrees. We do jet calculations for practice rounds to be sure that we are keeping the performance level up. Humidity is the moisture content of the air. Humidity affects our jet selection. As humidity increases the motor will run richer.

You may have run fine in early morning practice and may find that your race is not until late in the afternoon. You have to determine what carburetor systems need to be adjusted to keep tour kart at peak performance. This is where your record keeping becomes key. What happen the last time that you were in this situation and did you address the correct carburetor system? We see a far to many teams who only adjust the main jet and they loose the race because they can not get off of the low speed corners. Get familiar with all of the carburetor systems for your shifter and make them all contribute to your overall performance.

Bob Chiras
Crew Chief
AllKart International
603-432-4766 home
781-442-3045 office
bob.chiras@att.net