Tachometer conversion

[Farnswj]

I recently retrofitted my 74 mercruiser 427 engines with marine GM high energy ignition systems. I completely gutted the Thunderbolt ignitions. The port engine tach reads approx. double the actual rpms. I havent even connected the gray tach wire on either engine so where is this signal coming from fellas ?

 

[Scott Danforth]

if you have twins, you also have an engine sync gauge

both tachs need to be connected for the "system" to read properly

 

[Farnswj]

if you have twins, you also have an engine sync gauge

both tachs need to be connected for the "system" to read properly

Thanks for the info...only the port engine tach is operational...the synch is located on the flybridge, and is disconnected...any additional info would by greatly appreciated