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Discussion Starter #1
I have a 1990 21' foot commander open bow with a 454 built by harman marine (looks to be stock?) and a berkeley 12je pump. at full speed the speedo says 55 and the tach says 3,000 at idle it looks around 600. is this right or do i need to clean, adjust or replace something?
 

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Most tachs have a switch for 4,6, or 8 cylinders, make sure you are set on 8. If so maybe it's a bad tach.
 

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Also what kind of speedo? Your speedo could just be wrong as hell.. Does the engine push any farther than 3K
 

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Discussion Starter #4
It never shows above 3000 rpm but I have gps'ed my boat at 56mph so the speedo is correct it is just that the tach only shows 3000 wide open. I checked and the tach is switched to 8............?
 

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Discussion Starter #5
My buddy who knows a little about engines and i know hardly anything but maintanence said it should probably run around 4600 to 4800 wide open
 

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My buddy who knows a little about engines and i know hardly anything but maintanence said it should probably run around 4600 to 4800 wide open
Your friend is correct , most likely your tach is in error , It would be helpfull to know if there have been "ignition system " changes over time that may have affected your Tach? , has it always been this way ? something new? Call or post if you need more help Tom
 

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Borrow or buy a timing light with a built in tach. Snap on and mac sell them, you can buy cheaper versions at an auto parts store. Hook up the timing light, and check it against your tach.

Ryan
 

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Yes, compare to a known good tach, I use a Dwell-Tach meter myself, never saw a timing light tach (although @ work we have a strobe-tach).

Sure sounds liker the tach is wrong.

Since a tach reads every ignition pulse, then divides by teh switching setting (4, 6, 8 cylinders), Am I mistaken that the switch being wrong will make the tach read HIGH, not low (on a V8 anyway). Sometimes I get my mental math reversed.
Switch set to 4 cyl would read 2x RPM, and to 6 would read 1.5x RPM.
 

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Had the same problem once .. The setting on the back got bumped and it was in between settings. Sometimes the contacts get a little corroded too ... Switch it back and forth between settings a few times and then see what she reads at the 8 setting.
 

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Discussion Starter #10
to be honest i've had the boat for 3 years and never paid any attention to the tach..... took my buddy out and he is the one that noticed it from the passenger seat. i just drive the thing and have fun then fix whatever breaks after research it is fun learning. since i have owned the boat there have been no changes to anything.
 

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From 1990 prob. a VDO gauge... There's an adjustment screw behind the tach just go WOT, set it at 4600 or so and you'll be back in the right range.

Tach is prob. just fine.
 
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