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Tuning

Tuning

Jetting

JETTING: Always remember to change one carburetor component at a time and keep a record of your changes and effects.

IDLE to 1/4 THROTTLE: The AIR JET, PILOT JET and FUEL SCREW are most effective in this range When you want a leaner mixture use a LARGER AIR JET, SMALLER PILOT JET or turn the FUEL SCREW in. The opposite holds true for a richer mixture.

1/4 to 3/4 THROTTLE: The JET NEEDLE is the most effective component in this range. Raising the needle by lowering the “E” clip position at the top of the needle will richen the mixture. Lowering the needle will lean the mixture.

WIDE-OPEN THROTTLE: Changing the MAIN JET affects this range. Select the size, which offers the best wide-open throttle performance, then there is the option to install one size larger MAIN JET for ideal engine longevity.

FUEL SCREW:

(TM 33) This screw is located at the bottom of the float bowl, the screw meters fuel and opening the screw (counter-clockwise) out results in a richer mixture.

AIR JET, PILOT JET, FUEL SCREW, AND SLIDE CUTAWAY:

Keep in mind that the fuel screw gives a good indication of a properly sized Air Jet and Pilot Jet. The Air Jet and Pilot Jet calibrate the mixture from both the idle bypass and the idle orifice. If the idle screw is properly adjusted, but the engine does not have good response when the throttle is snapped wide-open, it is usually a sign of a lean mixture and the Air Jet/Pilot Jet will need to be replaced with one size (richer) and the fuel screw re-adjusted.
Consequently, if the throttle is only partially opened, such as in a trail riding situation, and the sled loads up (slow to build rpm) when the throttle is returned to wide-open, it is usually a sign of a rich Air Jet/Pilot jet. If the Air Jet or Pilot Jet does not clean up this part of the circuit, the slide can be substituted for one with a different cutaway (not recommended). The higher the number, the larger the cutaway will be, allowing more air to the nozzle leaning the mixture and, conversely, a smaller cutaway will richen the mixture with a greater effect up to 1/4 throttle.

JET NEEDLE:

Straight diameter section – By going to a thinner needle, there is a larger area between the jet needle and the needle jet supplying a richer mixture. Length of the straight section – This determines at which point the needle taper will start relative to the clip position. If you have to run a needle in the highest clip position (needle raised out of nozzle), a needle with a longer straight section should be used.
Needle “E” clip position – This works in conjunction with the length of the straight section.
If the engine is too rich above 1/4 throttle, raising the needle “E” clip (effectively lowering it into the nozzle) will lean the mixture. Needle Taper – A larger taper will result in a leaner mixture in the first half of the taper and a richer mixture in the last half of the needle. Needle Jet: The needle jet (nozzle) controls the fuel/air mixture up to 3/4 throttle. How it overlaps with the jet needle depends on the jet orifice inner diameter, air bleed holes and type of nozzle.
Most modern Japanese carburetors use a fixed needle jet (nozzle) assembly, which cannot be removed, but with the TM 33 replacement is possible.

Caution: this is where MOST burn-downs occur. To see where you are on the “MID” (needles) run the engine half throttle on a long straightaway hit the kill button or turn key off while holding throttle in “MID” position (USE EXTREEM CAUTION NOT TO CRASH INTO ANYTHING OR ANYONE ELSE). Then pull the spark plugs. The parts of the plug you should be looking at are the positive electrode and first 1/4 of the ground electrode.
Best power will usually result in a cardboard colored insulator tip and light colored ring around the tip of the positive electrode. For the ground portion should be a light cardboard color running no more than halfway towards threads. The first three threads should be oily if you’re running the correct heat range.

MAIN JET:

Fully load the engine (wide-open throttle) on a long straightaway or long hill, hit the kill button or turn key off while holding throttle wide-open (USE EXTREEM CAUTION NOT TO CRASH INTO ANYTHING OR ANYONE ELSE). Then pull the spark plugs. The parts of the plug you should be looking at are the positive electrode and first 1/4 of the ground electrode. Best power will usually result in a cardboard colored insulator tip and light colored ring around the tip of the positive electrode. For the ground portion should be a light cardboard color running no more than halfway towards threads. The first three threads should be oily if you’re running the correct heat range. Keep in mind that the different types (race gas or aviation) and different brands of gasoline can give different readings.

Exploded view TM flat slide (http://www.sudco.com/images/exptm.gif) Buy the Sudco Mikuni book if you want to tune.

Article from Snow Connection Magazine

The mysteries of carburetor tuning can confuse even the Merlin’s of engine tuners. How fuel flows through the fuel circuit and gets delivered to its final destination–the combustion chamber–can be mystifying, but through the help of aftermarket gurus, the puzzle can be pieced together and begin to form a solution.

Changing main jets in a carburetor isn’t necessarily a question of ‘if’, but rather a question of ‘when’. What we mean is that if you have ever fouled a spark plug in 40-degree weather, or melted a piston in 20-degree-below weather, you have fallen victim to carburetor jetting.

Rejetting a carb isn’t that difficult, just inconvenient. When it’s below zero outside, you don’t want to mess with removing an airbox, loosening clamps, turning carbs upside down and spilling fuel, just to find out after you put it all back together that you should go one size larger yet–and do it all over again.

Matching your gearing and clutching

You have to consider clutching first, and where the clutches most efficient 1:1. In our case we ride in the trees, creek beds, and hill climb. Our modified shop sleds run an average track speed at wide open throttle, while climbing in good powder of 60-70mph with a ground speed of about 40-45mph depending on how steep the terrain is, so to get the greatest efficiency from our clutching, and gearing we want to be as close as possible to 1:1 in our target condition.

We choose to run a ratio of 2.25:1 (20/40 with 8-lug drivers) on our mountain sleds, with the 2″ Camoplast Challengers, which puts us close to 1:1 in the condition we ride, for the greatest clutch efficiency. Essentially lower gearing equals more torque making it easier to quickly apply a heavy load on the motor and clutches. This allows the clutches to work through their FULL shift range at lower speeds, instead of over loading them (excessive belt slippage) and forcing them to work under high load and low ratio (high gear) at low speeds. The critical portion of clutching is getting a FULL shift at the correct RATE, so that your loading the motor as fast as it can possibly take it, without over loading it.

If it’s done correctly you should NOT lose RPM on a long climb or at the max speed that you’re geared for in fact you will gain RPM when you pass the maximum shift of the clutches (not recommended). Most people also don’t realize that the factory gears sleds for 10% over the max speed the sled will ever reach, this is their way of keeping people from running their new sled across a lake WFO, and damaging parts from excessive RPM. A problem we’ve see all to often is people change gearing without compensating in the clutching department, then ASSUME that the gear change had a detrimental effect.

If you gear down, you can clutch more aggressively because with lower gearing you can apply the load to the motor more rapidly. The motors power producing rpm range is fixed, and it’s how you get that to the track, then to the snow (clutching-gearing-track length, and lug height) that’s critical. Between 40-50% of the crankshaft horsepower is lost during transmission to the track. Even if the “other guy” has more power at the crankshaft, that doesn’t mean he’s getting it all to the track or applying it to the snow in the most efficient means, he may only be getting 40% to the track while your getting 50%, and you don’t have to be a mathematician to figure out the end result.

This is where you can gain a competitive edge. TEST, TEST, TEST, and know the correct set up for the conditions you ride. Do not trust “Joe bag-o-donuts” that rides in a totally different style, conditions, and has different modifications convince you that you should be running exactly what he’s running, that’s a fatal error far too many people fall victim to. We can help you find what works best for YOU in the conditions YOU ride in.

There is no “magic” clutch set up that works for everyone in every condition. If you understand how clutching and gearing work TOGETHER, and have a target to shoot for you can get that extra sled length or more ahead of the “other guy”.

Hartman Inc provides a detailed set-up sheet for our customers, so there are no problems during installation of our products. See example below

SETUP INSTRUCTIONS
setup_form_th setup_form_th
8.5×11 JPEG Acrobat PDF file

Click The images above to download the setup Form.

updated 7/20/2016