Mercruiser tach setting

[AllAmerican170]

I have a 1990 3.0 L Mercruiser with an alpha one drive. Does anyone know what the dial on the back of the tach should be set at (choices are 1, 2, 3, 4, 5, and 6. It apparently has to do with the number of poles on the alternator, whatever that means. The tach is a Faria brand 6,000 rpm tach. Thanks for any help.

Pete

 

[Don S]

Re: Mercruiser tach setting

That's an outboard tach. Won't work accurately with an IO engine.

You may try position 1.
If it has a 12 pole option, it may work. Just check it with a known accurate shop tach.

 

[AllAmerican170]

Re: Mercruiser tach setting

Thanks Don,

You're the first person (including 2 boat dealer mechanics) that has given me any useful information. Is it possible to hook up a second tach to test the in-dash one before I replace it? If so, do I do it at the tach or at the coil?, and how?...I'm no mechanic and know just enough to be dangerous.

Here's the situation: With a 17 pitch prop, the motor runs at about 6,000 rpms (on the tach). Switching to a 19 pitch drops me down to about 5400, with a noticeable drop in plane-out. Max recommended rpm for the motor is around 4800. To achieve this, I'd have to go to a 21 pitch prop, which doesn't make sense at all for a 17 foot Larson All American. I'm thinking the tach is off. That would make sense if it's the wrong tach. Any suggestions for the right one?

Thanks again,

Pete

 

[Don S]

Re: Mercruiser tach setting

Here's the situation: With a 17 pitch prop, the motor runs at about 6,000 rpms (on the tach). Switching to a 19 pitch drops me down to about 5400, with a noticeable drop in plane-out.

Your engine wouldn't be alive today at those rpms, and the drops with the pitch changes are impossible.
By a dwell tach meter at an auto parts store, or Sears, or ebay or something.
That tach is not what you need for that engine.
An IO tach only goes to 5000 rpm. But it still has to be set correctly and verified with a shop tach.

 

[AllAmerican170]

Re: Mercruiser tach setting

Thanks Don, I'll do that

Pete