Tach problem

[kikdurazz]

Need a hand. Bought a used tach off another member installed it. The tach works but is reading wrong. Compared to the rpm on my SAFC at idle it read 250 rpm lower as you increase the throttle it goes out additionally. 2000 rpm it reads 1450, 3000 rpm on safc it reads 2000?

Is there a way to calibrate this or is the tach all messed up?

Thanks

 

[kikdurazz]

Just to add I had to remove the needle to replace the whiteface gauges. Could this have caused the discrepancy?