I have a new to me 1986 225 Four Winns with a 5.0 mercruiser 230 hp Alpha 1 . All my gauges work properly except the Tach. When turn key on the tach pointer goes well below zero. When running what I think is close to WOT it says approx. 1200 RPM. Does anyone know if this is a tach problem or a wiring problem? I cannot find a matching replacement. I would hate to replace just the tach and not have them match. Any help would be appreciated.