Whether the sled is stock or an all out, highly modified trail animal there is usually room for improvement over the stock or after market jetting and clutching specifications. Not to say that the manufactures or the aftermarket specifications are wrong. In most cases they are very close considering that most published specifications need to cover a wide range of riding conditions and rider styles. For an individual who wants to take the time and spend the money to fine tune his or her sled to the conditions that he or she will be riding there is usually some extra performance to be gained that can be used to smoke your buddy. That's what performance snowmobiling is all about, right?

Before any valid testing can be done, you must be sure that the sled is mechanically sound. The rear suspension should be checked for proper track alignment and track tension. Check the front suspension for proper ski alignment and toe out. Ignition timing: Using a dial indicator to verify the proper position of the timing marks, and then checking the ignition timing with a light is the only way to know that it is correct. I will go in to clutch and fuel system maintenance later.

Some handy tools to have would be a primary clutch puller, a grunt tool to remove the spyder from the fixed sheave, and a clutch alignment tool. Most of the carburetor work can be done with basic hand tools but it is handy to have a special tool box made up just for carb tuning. Contact your favorite dealer or performance company and ask for the following information, clutch spring charts, primary ramp or weight charts and jetting charts for your application. For making jet changes, monitoring atmospheric conditions is very important, so having at least a thermometer and keeping records of what the temperature is during testing is highly recommended. Some other handy carb tuning tools would be a barometer, humidity gauge and a R.A.D. (Relative Air Density) gauge, I will go in to more detail later on.

There are several ways to verify your progress or lack of progress while field testing, but the cheapest and in my opinion one of the best is to find a second sled that is comparable in performance with the one that you are field testing. Drag racing from a rolling start and getting some baseline runs before changing anything will usually give you reliable results.

The fuel system must be inspected for cleanliness, and carburetor for proper float setting. One very important item is carburetor synchronization. If the carburetors are not properly synchronized the throttle response will be very poor and can cause plug fouling. I prefer to use the Uni-syn carb synchronization tool made by Edelbrock. Using the Uni-syn, adjust the idle screws to synchronize the carburetors at idle. Loosen carb cable adjustment to ensure slides are bottoming on idle screws. Then check that the throttle slides are moving from the idle screws at the same time when the throttle is opened. If throttle slides are not opening together, lengthen or shorten the cable adjustment at the top of the carburetors.

The changes in atmospheric conditions from day to day has a lot to do with engine performance. So to maintain peak performance it is necessary to know what is going on in the weather world and to know how different types of weather effect performance. Barometric pressure is the pressure of the atmosphere or density of the air so a higher barometric pressure will have a higher air density or put another way the molecules that make up our atmosphere will be compressed more at a higher barometric pressure. So an engine running at a higher pressure / air density will receive more oxygen molecules and need more fuel than an engine running at a lower pressure / air density. The elevation also changes the air density, higher elevation will be lower pressure / air density and lower elevation will be higher pressure /air density. The air temperature also effects the air density. Colder air will increase air density and fuel requirements, warmer air will decrease air density and fuel requirements.

As you can see their are a lot of variables in air density and fuel requirements. So for a person who would like to have "spot on" jetting it is a good idea to have a thermometer, barometer combination or a R.A.D. gauge. The R.A.D. or air density gauge measures a combination of barometric pressure and temperature and translates that information into a percentage number. So after you have completed jet testing and are happy with the results, you would record the air density for that day example 80% and the main jet 360, this will be your base air density and base jet. When the weather and or elevation (air density) changes, example 88%, the following formula will give you the proper jet change:

When I field test a sled that I know will need both clutch and jet work I always start with the jetting because clutch tuning an engine that is not properly jetted is a waste of time. The low speed tuning (idle to 1/4 throttle setting) is calibrated by pilot jet changes and adjusting the air screw. Operate the throttle between idle and 1/4 and see if the engine revolutions increase smoothly. If the pilot jet circuit is too lean, increase in the engine speed will be slow and irregular. If the pilot jet circuit is too rich this would create heavy exhaust smoke as well as a dull exhaust noise. If you cannot maintain speed in this 1/4 throttle area while the throttle is held constant, the pilot jet circuit is too lean. To find a good starting point on the air screw, warn up the engine to operating temperature and adjust the air screw in (richer) or out (leaner) until the idle RPM is at its highest point (do not open more than three turns). One point to watch on air screw adjustment, if the engine idle to 1/4 throttle performs best with the air screw adjusted 1/4 turn or less it may be an indication that the pilot jet is too lean and the opposite holds true if the air screw ends up near three turns the pilot jet may be too rich. Its easier to adjust the air screw than to change pilot jets to find out if you are too rich or lean on the pilot circuit.

The mid-range tuning (1/4 to 3/4 throttle) is calibrated by the needle jet and jet needle. The jet needle tapers off at one end and the clearance between the needle and the needle jet increases as the throttle valve opening gets wider allowing more fuel in to the carburetor venturi. The air-fuel mixture ratio is controlled by the height of needle positioning clip that is inserted into one of the five slots provided in the head of the needle. The top slot #1 is full lean, the #5 slot is full rich.

The needle jet is changeable in most Mikuni carburetors. The fuel delivery can be adjusted by changing to a larger or smaller inside diameter, the size designation will have a letter followed by a number example, P-2. The number shows the inside diameter size in increments of .010 mm. Example, the difference between P-2 and P-4 is that the inside diameter of P-4 is .010 mm larger than P-2. The letter shows the inside diameter size in increments of .050 mm, the difference between P-2 and Q-2 is that the inside diameter of Q-2 is .050 mm larger than P-2. So a larger diameter needle jet will allow more fuel to pass or a richer mixture.

Dialing in the pilot circuit can normally be accomplished by listening, seeing and feeling what the sled is doing, but above 1/4 throttle the horse power and engine loading is increasing rapidly and the chances of engine damage because of improper jetting increase as well. So it becomes necessary to start reading plug and piston color. The best way to get a valid piston/plug reading is to hold the throttle steady at what ever throttle setting you want to check and run at that setting for at least 1/4 mile if possible, kill the engine before releasing the throttle.

Because of the differences in engine designs and hundreds of performance products available for these engines, I suggest contacting a dealer or performance company that is familiar with your engine combination and talk to them about reading your plugs and pistons. A couple of tips on mid-range jetting, start out conservative, it is easier and less expensive to change jets than a seized piston. Use the jet needle for changing fuel requirements first, its faster to change the "E" clip position than to change the needle jet. If you find yourself at full lean or rich and still needing more or less fuel that is the time to change needle jets. A rule of thumb here is one "E" clip position on the needle is equal to one needle jet size. So if your needle jet is a P-4 and the needle "E" clip position is #1 you could change the needle jet to a P-2 and move the needle clip to #2 and the fuel flow would be real close to the same as the P-4 and #1 clip position.

The tuning procedures for the main jet are the same as the mid-range except the throttle range will be 3/4 to full and of course you will be changing the main jet not the needle jet or needle.

Clutching It would be a real waste of time to tune clutches that are full of worn out parts so both the primary and secondary clutches must be checked for cleanliness and all worn parts replaced. The drive belt must be high quality and in excellent shape. Check clutch alignment and belt deflection, adjust belt deflection as tight as possible without the sled creeping forward at an idle. Primary spring charts will normally have two measurements, pre-load and rate. Pre-load is a measurement in pounds when the spring is compressed to a specified height. Rate is the increase in force when compressed a specified distance beyond the height of the pre-load measurement. So spring pre-load controls clutch engagement RPM (higher pre-load increases engagement, lower pre-load decreases engagement). Spring rate controls peak RPM (higher spring rate increases peak RPM, lower rate decreases peak RPM).

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