change the settings on the tach to pick up the pulse at a different time. since you can put it on any engine, whether it be 2 or 4 stroke, it had settings to allow for different fire timing. read the instructions, i think you need to put it on 180 degrees.
When i first installed on my CDI motobecane, i maxed out at like 19,0000 rpm and was like WHOAAAA, then i changed the settings and it was all good.