The speed adjustment on an EFI Yamaha is on the transaxle. It is a shaft that comes up out of the transaxle that is connected to a "V" shaped spring. The "V" shaped spring is held closer or farther apart by an adjustment bolt that squeezes it together. The "V" is set quite wide by the factory by not running the nut very far down on the bolt. To increase speed, the nut is tightened so that the "V" is narrower. (Some people use a zip tie to close the "V" spring down so that they can cut it when they go in for service.)
The "V" spring controls the speed of the cart by attempting to keep speed as determined by the transaxle at a certain point. It does this by making the throttle cable more or less responsive at the high end. The throttle cable from the gas peddle goes to one side of the "V" spring and the other end of the throttle cable on the other side of the spring goes to the engine.
Basically, when the cart is adjusted to a slow speed, more throttle from the gas peddle to the engine is only allowed if the cart is going slower than the speed setting. If the cart is going as fast as the speed setting allows, then the throttle cable is sort of 'disconnected'. This is accomplished by the spring being tighter or looser by being rotated by the speed shaft that it is connected to.
Tightening the nut and squeezing the spring essentially changes at what speed the throttle cable is 'disconnected'/ max'ed out.
Going downhill, your speed can exceed these settings because you are coasting. The engine is actually at idle and really has no control over the speed. So the speed adjustment is for engine driven speed levels.
Along these same lines, if you have an older gas cart with 8" wheels and go to 10" wheels, you will gain about 2 mph because the speed governor thinks it is still driving smaller diameter wheels.
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