I put a honda tach on my 1600cc VW trike and it reads twice the rpm is there an adapter to fix the problem?

The tach is off a honda shadow about 1983-85 and is hooked up to the neg. side of the coil on my 1600cc VW engine. can the tach be modified or is there an adapter that will work.I put a honda tach on my 1600cc VW trike and it reads twice the rpm is there an adapter to fix the problem?Why not just get a tach made for a 4-cyl engine.I put a honda tach on my 1600cc VW trike and it reads twice the rpm is there an adapter to fix the problem?Load diodes on the input circuit to cut down the voltage.