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The following six pictures show what's inside the Ampico Air Motor Governor. They will be used to help explain how the speed of rewind can be decreased.
Below is a picture of the rewind valve cover plate with the screws removed. Under the plate, there is a piece of motor cloth glued to the governor. It's there so that you can access the valve in case a repair is needed. Also, it seals the vacuum inside the governor.
Once the cloth is removed, you can see the circle of fiber board which serves as a 'stop' for the rewind valve, which is directly underneath. The fiber board serves no other purpose. The large screws on either side of the fiber piece need not be removed for this operation.
The fiber piece is glued in place and care should be used when removing it. But, since it only serves as a stop, it doesn't have to be perfectly flat anyhow. Under the piece is the rewind valve. Note that there is no valve facing on top of the valve. That's because the only purpose of the valve is to allow unregulated vacuum to reach the air motor during rewind. It does that when the rewind pouch, below the valve, pushes the valve upwards.
This picture just shows more clearly that the rewind valve has only one facing. It also shows the valve seat under the valve. You can also see part of the rewind pouch down inside the hole.
This last picture show the valve gap between the top of the rewind valve and the fiber board. The thickness of this gap is what determines the rewind speed. Decreasing the size of this gap will reduce the speed of rewind. The easiest way to decrease the gap is to glue a paper punching to the top of the valve. I measured the gap to be 3/32" or 0.090". So, decreasing the travel by 0.040" should reduce the rewind speed by about half. (See NOTE below.)
Although it might not be immediately obvious by looking at the following photo, understand that the flow of air through the governor (marked in yellow) during the play mode is from the large tube on the left, through the graduated opening, through the regulating bellow (which uses a pinch-off type regulator), then through a channel that runs under the rewind pouch, and out through the two supply tubes on the bottom right. When the system is in the rewind mode, that flow of air does NOT change at all. In fact, if you increase or decrease the Tempo during rewind, you will find that the speed of rewind changes. If you'd like to experiment a little, disconnect the hose leading to the rewind pouch and close of the elbow leading to the pouch with a piece of tubing with a knot in the end. Then move the Play/Rewind lever to Rewind. You will find that the Tempo Control controls the speed of rewind. If you want to get really clever, you can restrict the air going into the rewind pouch so that the pouch doesn't lift the rewind valve all the way up. In essence, what you're creating is a 'volume control' for the rewind vacuum. You can try this by squeezing the rewind pouch hose during rewind. As the air getting to the pouch decreases, the pouch will start to deflate, and the valve will draw closer to its seat, choking off the vacuum to the air motor. So, now you have two ways to decrease the speed of rewind.