Explorer Speed Control Problem

[Explorer]

Hi all: I have an Explorer that is having trouble reaching top speed. Sometimes, a programmed RPM of 4200 only yields 3800 on the display.
The RPM's can usually be increased using the RPM +- JOG switch, but, sometimes the only result is a rush of air out of the manifold.

The RPM JOG switch operates properly when used to LOWER the RPM's, BTW.

I just replaced the manifold (leaking) recently. I did not replace the solenoids, but I have new ones if necessary.

I suspect that my drive belt may need to be replaced. Any thoughts?

Also, do I need to replace the timing belt? I am not exactly sure of it's function-

Thanks,

Explorer

[machintek]

The timing belt drives the back gear.
The spindle speed is not accurate. It is an approximation. The control uses a linear taper potentiometer as a voltage divider in place of the typical dial to tell the control about where the shaft is that pulls on the chain that pulls on the plate to move the variable drive disc to change speed. There is a adjustment on the top of the head to vary the pivot point of the plate to achieve full travel of the mechanism. Oil for the air motor and solenoids is important as well as the proper air pressure. There is a switch on the hi/low lever to tell the control what speed range you are in. The potentiometer position is adjustable to tweak the speed display. It is in the can at the top front of the head.

George