1974 Mercruiser thunderbolt tach conversion

[Farnswj]

I retrofitted my 427 mercs with GM hei ignitions....port motor still shows approx. double rpm on tach...gray tach wire on hei isnt even connected...where is this seemingly spurious signal coming from ?

 

[alldodge]

Took a bit of looking but found the 427 was listed as Merc 325
The Brown wire goes to the Tach.

Which HEI distributor did you install?

 

[Farnswj]

Took a bit of looking but found the 427 was listed as Merc 325
The Brown wire goes to the Tach.

Which HEI distributor did you install?
View attachment 327872

GM marine grade from Marine Parts Source in Whitehouse, TN.

Took a bit of looking but found the 427 was listed as Merc 325
The Brown wire goes to the Tach.

Which HEI distributor did you install?
View attachment 327872

GM marine grade from Marine Parts Source located in Whitehouse, TN. Thanks a bunch for the schematic my friend