The tach on an 83i is cable operated and best I can tell connects to the fuel pump on the lower left side of the engine. I'd disconnect the cable at the gauge and see if the cable is still intact or connected properly.
To remove the cable housing from the fuel pump, remove the inner bolt that holds the pump onto the cylinder head. The bolt clamps the tach housing to the pump. Be sure not to lose the rubber seal on the end of the cable, or you'll have an impressive oil leak when you put the cable back on.