Tach question from 3cyl to 4cyl

[pita482]

Just swapped out my 1990 90hp force power head to a 120hp power head, have the idle set at 900rpm but seems kinda fast, was wondering if my stock US marine tach in my bayliner from my 3cyl is reading the 4cyl properly, thanks in advance for any input.

 

[GA_Boater]

The tach is counting pulses from the stator and not the number of cylinders firing like a 4 stroke.

Do both stators have the same numbers of poles?

 

[pita482]

I'll have to check

 

[jerryjerry05]

Your older 90 was probably Prestolite ignition.
From the pics I see the new is Mercury.
​They used 2 different ignitions on the 93/120
The older is a 20 pole Prestolite and the new is 12 pole Mercury..
So it probably won't work???

On the back of the tach is a black arrow.
​It is to set the amount of cylinders firing.
​Try setting it at the 4 cyl. and see what happens??

 

[pita482]

You are correct jerryjerry, I just looked my prestolite is 20 and my thunderbolt is only 12, I have a prestolite ignition for the 120 and it has 20 as well, I ran the thunderbolt ignition instead because I was told it was a better ignition, I'll check my stock tach for the arrow, thanks for your quick response guys, taken her out now for some water tunning.

 

[pita482]

Well that did it my stock tach had a 3cyl 20 pole and a 4 cyl 12 pole switch on it!
Guess I should have looked before posting but it was late last night and needed some input. Thanks again guys!