On the schematic that zcar70 supplied it shows a speed adjustment. It that the recessed black slotted item just to the left of high center on the back of the tach? I have two tachs. One is over 200 rpms off and the other, just supposedly completely rebuilt, is 600 rpms off. Will that speed adjustment adjust the rpm reading on the tach?

I think it should take a small adjustment, try it.

Thanks for the input. I will try it. Ron