YES!!! the linkage length is critical. There are two items that come into play, centrifugal force increases with the rpm, AND the effective travel of the governor changes with the rpm. Imagine a pendulum. The length of the pendulum has a lot to do with the travel, but we can't adjust this in a Lister. What happens is at low speeds the governor pendulum is at rest near the centerline of the pendulum. As speed increases it does not take much to effect a large movement of the linkage as the weights fly outward. As speed increases the pendulum weight flies outward in proportion to the speed, however, the movement is much less as the speed increases. (the weights rise a lesser amount in height than they do at slower speeds) All of this is compounded by the fact that increased speed results in more centrifugal force, along with the resulting change in height of the pendulum weights. These 2 factors, working together, is what determines the sweet spot, which will be different even on similar engines. Shortening the linkage rod too much will result in hunting, (the weights are biased toward the lower portion of the height-arc) while longer linkage will result in a lazy governor regulation. Turn your clevis in or out 1/2 turn at a time to find your "sweet spot". My GM-90 holds within 1/10 of one cycle within its load range with this method. Heavy starting loads do change the rpm slightly, about 2 hz., but it quickly recovers without overshoot or hunting.
Mike Montieth
Listeroids/USA