Although clutch calibration is a complex process and may be beyond the scope of the average rider, the cost of having a good shop perform the work is relatively inexpensive compared to many engine component modifications. And it can produce significant performance improvements for both recreational and aggressive riders. 

Look for clutch wear

Before consideration is given to modifying the clutch "tuning," clutch components must be inspected and determined to be in good condition. (These include outer cap bushing, spider roller, spider roller bushing, clutch weight, clutch weight bushing, sliding sheave bushing, spider buttons and the belt surface). 

An example of excessive wear of a spider bushing (left).
For comparison, a new bushing appears on the right.

The only way to inspect many of these components is to completely disassemble the pulleys and inspect the parts. If significant wear is present (see photos), all the clutch tuning in the world will not resolve performance problems. 

The moveable sheave half shaft is scored
and fretted due to bearing failure on the moveable sheave half.
The pitted and rough surface on the disc-shaped sheave half
is due to belt abrasion at high-gear operation.

(Note: Never disassemble the primary clutch of any snowmobile without a clutch compression fixture and necessary safety precautions.) 

Additionally, the belt fit must be adjusted, the engine properly tuned and the correct gear ratio selected to accommodate riding style. 

It also is important to recognize the dramatic impact of drive alignment. Misalignment will not significantly reduce the ability of the drive to get the power to the track, but it will cause a severe reduction in belt life and, to a lesser degree, clutch component life. 

On hyper sleds, which are ridden aggressively, clutch maintenance is recommended at least twice a year. For normal sledding, clutch maintenance is recommended once a year by most manufacturers. 

The ideal clutch system

An ideal clutch system will follow the curve shown in Fig. 1. This curve represents a wide open throttle acceleration run to maximum speed. The three major components of the curve are the clutch engagement (region A), the "low gear" acceleration (region B) and the up-shifting acceleration (region C). With proper tuning of the clutch and drive components, today's clutch systems are capable of replicating the curve rather closely. 

Fig. 1

Such performance depends on the system's ability to match clutch engagement speed to the engine/sled characteristics in order to eliminate any "bog" as the belt is engaged. Other requirements are maximum engine speed control and sufficient belt tension to minimize belt slippage. 

Clutch engagement speed

The clutch engagement rpm requirement is determined by several factors including engine performance characteristics, altitude, sled weight and secondary gearing. The belt should engage at as low an rpm as possible yet avoid bogging the engine. 

In general, the higher the engagement rpm, the less likely the occurrence of bog. Too high of an engagement speed, however, is very hard on the belt and drive components. 

Maximum engine rpm

It is important to control maximum engine rpm to a narrow range during wide-open throttle acceleration. The maximum rpm must be controlled by the system to a range that coincides with the peak engine horsepower rpm (see Fig. 2). 

Fig. 2 Snowmobile Two Stroke
Power Curve (for hyper sled).

On many high performance engines, the curve is steep on both sides of the maximum horsepower rpm and any significant deviation from that rpm can mean a drastic reduction in horsepower available to power the sled. Modifications to the sled can change the shape of the curve and move the peak horsepower rpm to a different range. 

Assuming the peak horsepower rpm range for a sled is known, most clutch experts agree that the primary pulley should be "adjusted" to hold the engine in that range during full acceleration. This is best achieved by changing the clutch weights. Normally, more weight will lower the maximum engine rpm and conversely, lighter weights will increase the rpm. 

Some clutch tuners also will experiment with the shape of the weights. Changing the shape of the weights is best left to experts with good track records for getting the most out of the system. 

When testing for the ability of the system to hold the engine speed to the correct rpm, a condition of over-speed generally is caused by clutch weights that are too light. However, a very aggressive belt or an aggressive cam in the secondary pulley can cause the secondary pulley to "lock" in the closed condition and not let the drive upshift causing a momentary over-speed condition. 

Both the condition of a belt and its construction can have a significant effect on maximum engine rpm. This is a function of the dynamic coefficient of friction which describes the resistance to movement between two rubbing surfaces such as the belt's sidewalls and the pulley. 

Recreational belts (for sleds with up to 500 to 600cc engines) have a higher coefficient of friction due to the fabric materials used as reinforcement. This can result in lower engine rpm. High-performance belts are constructed with aramid fibers in the undercord material that are "slippery." They offer tremendous strength and wear resistance, but have a lower dynamic coefficient of friction that can result in engine over-speed. Aramid-reinforced belts require higher squeezing force which means higher operating tensions for a given load. 

Snowmobile owners are advised against changing belt constructions or suppliers to obtain proper maximum engine rpm. There are several snowmobile belts on the market and many are good - so select the belt based on quality and performance. You can then adjust the clutch to get the most out of the belt construction that's chosen. 

Minimizing belt slip

Adjusting how hard the pulley halves squeeze on the belt is simple in concept, but difficult to achieve since it will affect the complete balance of the system. 

If the system needs more tension, in concept it is possible to change the primary flyweights and the secondary torque sensitive cams to increase or decrease belt tension. Two major issues must be considered, however. 

• First, it is difficult to determine if the system needs more tension to reduce slip.  
• Second, making any significant changes in both the primary and secondary components can upset the balance of the system and affect functional operation of the system at partial throttle or during back-shifting operation.

To correct the maximum engine rpm, adjust the primary pulley. Unless drastic changes have been made to the engine, this should only require small changes in the weight of the clutch weights. 

If more than a couple of added grams of weight still does not reduce the engine rpm to an acceptable level, then the system is probably not providing sufficient tension and the belt is slipping. If this is the case, a more aggressive secondary pulley cam should be installed. This will increase the squeeze on the belt and should minimize or eliminate belt slip. 

Changing the secondary pulley cams almost always requires recalibration of the primary pulley to "re-balance" the system. Unless there have been major engine modifications, cam and clutch weight changes should only require small changes to optimize performance, assuming that someone with less than expert knowledge has not altered the system. 

Whenever clutch calibration is considered or modified, careful consideration should be given to the impact of secondary gearing. Improper secondary gearing can cause some of the symptoms of improper clutch calibration and/or magnify clutching problems