72cc polini build

[1man8scoots]

Contra controls the variation. Too much tension and like Benji said it won't open fully. But at the same time if you're too light it will variate prior to fully being in the PowerBand. I've seen an engine lose 15 mph from a contra.

[benji]

Hitting the power band too low will cause lower top speed?

[benji]

What if you use a really stiff clutch spring set, say 6,500-8000 rpm clutch springs. Will that put the power band high enough to overcome too-quick of variation?

[Wheelman-111]

Greetings:

Clutch triple springs only determine the engine RPM when the clutch engages. If they're very stiff, and the engine spins up fast enough to move the belt along the pulleys prior to engagement, then by the time the clutch bites you're already fighting "2nd gear" as you try to take off.

You assess the weight/Contra spring balance once underway. The "average" engine RPM is determined only by this balance. Stronger spring OR lighter weights = engine RPM stays higher. Heavier weights OR weaker spring = engine RPM runs lower.

The tranny has to match the engine's best-power RPM for best performance. In extreme cases of heavy Contra or super-light weights, the engine spins up so much RPM that it falls off the high side of the torque curve and loses power. You can even have a situation where the variator won't fully shift, losing top speed as well.

On the flip side, if the weights are too heavy for the spring, the engine is held below peak power while the variator shifts all the way to "high gear" too soon. A low-RPM struggle to accelerate with engine power well below maximum ensues. Think about driving a 5-speed stick at 20MPH and flooring it. Not much happens quickly.

Experiment with this, perhaps based on the experience of others, by first choosing a spring, then by switching rollers to achieve the optimum total-gram load for your particular engine. If you switch pipes, carbs or just about anything else on the power-production side, a corresponding adjustment to the transmission will most likely be necessary.

[1man8scoots]

Hit the head on the nail wheels.