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Home > Technical Articles
So You Want To Go Field Testing?
By Allen Roberts

Whether your sled is virgin stock or an all-out, highly modified trail animal, there is usually room for improvement. From time to time, even aftermarket jetting specifications can use some tweaking. Not to say that manufactures' or aftermarket specifications are wrong ... in most cases, they are very close -- considering published requirements need to cover a wide range of riding conditions and rider styles. For and individual who wants to take the time and spend the money to fine- tune his sled to the conditions he will be riding in, there is usually some extra performance to be gained--you know, that extra edge will help to smoke your buddy. After all, that's what performance snowmobiling is all about, right?

Sled Preparation

Before any valid testing can be done, you must be sure 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 checking the ignition timing with a light is the only way to know if it is correct. (I'll cover the system maintenance later).

Most of the carburetor work can be done with hand tools, but it's handy to have a special tool box made up for carb tuning. For making jet changes, monitoring atmospheric conditions is 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. (This will be covered in more detail later on.)

There are several ways to verify your progress, or a lack thereof, 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 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 important item is carburetor synchronization. If the carburetors are not properly synchronized the throttle response will be poor and can cause plug fouling. The best tool I have found for synchronizing carburetors is the Uni-syn carb synchronization tool made by Edelbrock. To synchronize the carburetors, remove the air box, loosen the carb cable adjustment, open the throttle valves to the full throttle position. Reach inside the top of the carburetor venturi with your finger and feel for the bottom intake port side of the throttle valve. If you can feel the bottom of the valve protruding into the venturi, tighten the carb cable adjustment until the throttle valve is flush with the carburetor venturi. This will insure you are getting full throttle. Next, with the throttle valves set at idle, see if the throttle valves are bottoming on the idle screws. If they are not, adjust the idle screws until they make contact with the throttle valves. If there is not enough idle screw adjustment or engine idle is too high, it may be necessary to loosen the throttle cable adjustment slightly. With this complete, warm the engine to running temperature. Using the Uni-syn, synchronize the carburetors by adjusting the idle screws. After the idle screw adjustment is complete, make sure the throttle valves are not opening in synchronization with each other, lengthen or shorten the throttle cable adjustment as necessary.

Changes in atmospheric conditions from day to day has a lot to do with engine performance. 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 affects performance. Barometric pressure has a significant effect on performance. 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 barometric pressure and air density; higher elevation will be lower pressure/ greater air density. Air temperature also affects air density. Colder air will increase air density and fuel requirements, warmer air will decrease air density and fuel requirements.

The humidity has some affect on jetting requirements, but it is small compared to temperature and barometric pressure. The oxygen content of air is higher than water. So high humidity will require slightly less fuel than low humidity.

As you can see, there are a lot of variables in air density and fuel requirements. For someone who likes 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 relative air density guage, measures a combination of barometric pressure and temperature and translates this information into a percentage number. After you have completed jet testing, and are happy with the results, you would record air density and main jet for that day. For example, if the air density is 80% and the main jet is 360, this will be your base air density and base jet. When the weather and/or elevation (air density) changes, for example 88%, the following formula will give you the proper jet change.

Rounded off your new jet size will be 400.

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 to rich, this will create heavy exhaust smoke and a dull exhaust noise. If you 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, warm up the engine to operating temperature and adjust the air screw in (richer) out (leaner) until the idle RPM is at its highest point (do not open more than three turns). One point to watch the air screw adjusted 1/4 turn or less, it may be an indication the pilot jet is too lean. The opposite holds true if the air screw ends up near three turns-- the pilot may be too rich. It's easier to adjust the air screw than to change the pilot jets to find out if you are too rich or lean on the pilot circuit.

Throttle Valve

The size of the cutaway of the throttle valve affects the air-fuel mixture ratio when the throttle valve opening is between 1/8 to 1/2 throttle, especially in the range of 1/8 to 1/4 opening. As the cutaway increases size, with the throttle valve opening kept unchanged, air flow resistance is reduced and causes the amount of air intake to increase, resulting in a lean mixture. On the other hand, the smaller the cutaway, the richer air-fuel mixture will become. Because of the expense, I would try a lot of pilot/airscrew and needle jet combinations before changing or cutting the throttle valves. But if you have a lean boggy spot or poor fuel mileage just above engagement, and changing the pilot circuit doesn't help, it may be necessary to change the throttle valves.

Needle Jet and Needle

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 the carburetor venturi. The air/fuel mixture ratio is controlled by the height of the needle positioning clip which is inserted into one of the five slots provide in the needle head. The top slot (#1) is full lean, the (#5) slot is full rich.

The needle jet is changeable in most Mikuni carburetors. 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, P-2 for example. The number shows the inside diameter size in increments of .010mm. Example: the difference between P-2 and P-4 is the inside diameter of P-4 is .010mm larger than P-2. The letter shows the inside diameter size in increments of .050mm, the difference between P-2 and Q-2 is the inside diameter of Q-2 is .050mm 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 usually be accomplished by listening, seeing, and feeling what the sled is doing, but above 1/4 throttle the horsepower and engine loading is increasing rapidly, and the chances of engine damage because of improper jetting increases as well. Therefore, 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 whatever throttle setting you want to check, and run at least a 1/4-mile, and kill the engine before releasing the throttle. There is less risk to your engine if you begin your testing at the lower throttle position before attempting full throttle runs.

Because of the difference in engine design and hundreds of performance products available for these engines, I suggest contacting a dealer or performance company familiar with your engine combination. And talk to them about your reading your plugs and pistons. Before closing, here is a couple of tips on midrange jetting: Start out conservative, it is easier and less expensive to change jets than a seized piston. Use the jet needle for changing midrange fuel requirements first. It's faster to change the "E" clip position than to change the needle jet. If you find yourself at full lean or rich, and you still need more or less fuel, this 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. 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 the #1 clip position.

Main jet

The tuning procedure 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.

Like I said before, there's usually room for improvement. Following the above tuning recommendations should get you that extra burst of power and performance you've been seeking. The most satisfaction you receive, however, will be the picture of your bewildered buddy, scratching his head a few miles back on the trail.

"Setting the World's Performance Standards"