I'm with 85hokie. How does the tach read at an idle? With the throttle backed off till the motor is just barely running smooth, the idle should read about between 700 and 900 rpms.
When slowly increasing the rpms, do they slowly climb on the tach? Or start jumping higher then would seem normal? For example a very slight increase in engine speed and the tach rises 300 or 400 rpms.
Is the tach electical or mechanical (cable driven)?
If electrical, it could be a simple pulse count adjustment on the back of the tach. It's either pins with a jumper, a dip switch, or push and turn dial. With brief instructions written near the adjustment. This would be if at low rpm and reading other then 700 to 800 rpms. Say 1 rpm or showing an rpm of 1500 and the motor is barely idling for examples.
If mechanical it may be the ratio on the tach versus controller that bolts to the block could be off. Say showing 700 to 800 rpms, and slight increase in throttle and rpms on tach make a drastic change
Or it could be that if rpms seem to increase properly, the max rpm limit may not be adjusted. Which is the set screw where the throttle linkage connects to the injection pump. One set screw is low idle, the other is max throttle