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Mercruiser Tachometer

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allamerican170

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"I have a 1990 Larson All Amer

"I have a 1990 Larson All American 170 with a 3.0 L mercruiser alpha one. My tach seems to be about 1000 rpms off. The back of my tach (Faria) has a dial woth 6 settings (labeled 1,2,3,4,5,and 6. Any idea how many poles my alternator has, as this apparently determines which setting to use on the tach. or just what setting I should use on the tach?"
 
"Not sure, but I can go check.

"Not sure, but I can go check. Someone on iboats told me that's an outboard tach and may not read accurately for my I/O. I was running almost 6,000 rpms wot (according to the tach) with a 17 pitch prop. Ran great and popped out of the hole really nice. Problem is, max recommended rpms is 4800. I went to a 19 pitch prop and dropped down to 5400, with a noticeable lag in plane-out. To get near my target rpms, I'd have to go to at least a 21 pitch, which can't be right for a 17 foot All American. Plane-out would be horrible. That's why I'm thinking my tach is off. The tach was set on setting 1 (of 1-6)."
 
"The alt. has nothing to do wi

"The alt. has nothing to do with a tach reading on the GM 3.0 .Mando dosent have a tach lead for one.
Besides the 3.0 cant go 5400 anyway.
to verify a tach reading ,compare it against another known good tach.Then adj. accordingly if it`s adjustable."
 
"Thanks BT,

Then I assume t

"Thanks BT,

Then I assume the 6000 rpm reading I was getting was way off (due to the tach). Is there a way I can hook up a second tach to compare them? Directions would be greatly appreciated...I know just enough to be dangerous.
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"
 
"If you dont already have a sh

"If you dont already have a shop timing light/tach, you should.
Sears, Auto Zone, most good stores should carry one.Theres a bulletin setting the timing at 1 deg.
to keep the head gasket from blowing out.Most shop tachs have a power cord that hooks to the battery and /or a plug wire to read the RPM`s."
 
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