Tachometer calibration and fuel gauges

dlogvine

Master Chief Petty Officer
Joined
May 4, 2015
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839
I changed the tachometer gauge on a 1984 boat with OMC 3.8L v.6 engine. Had to exchange it once since the first tachometer that I've got through eBay was dead. The other one, medallion B-3150 is working but looks like it shows up RPMs way too high. I can even hear it by the ear, when it is about 3000-3500 rpm it shows 5000-5300. The setting is on 2 which corresponds to v.6 The question is, since the tachometer has calibration nob, how can I calibrate it to show approximately right RPMs?
The other question is about the fuel gauge. I had to change the float in the tank, the old one was all soaked and was sinking into the fuel. The cover of the float was all warped, then the previous owner tried to seal it with some sealant which was all eaten by gas. So when I hooked up a fuel gauge to the sender it started showing up a full tank and keeps showing it after an hour of boat running. Any ideas about what could be wrong?
Thx
 

gm280

Supreme Mariner
Joined
Jun 26, 2011
Messages
14,605
dlogvine, first it sounds like you tach need changed from the "2" setting. What other options or setting are there? Obviously that 2 setting is not the correct one for your engine.

As for the fuel gauge, it sounds like it is stuck in the up position. So you will need to remove the sender assembly and verify it is moving freely. And then reseal it up again with the proper rubber seal. JMHO!
 

Silvertip

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Sep 22, 2003
Messages
28,771
The tach normally has settings 4-6-8 cylinder auto'styloe engines which can correspond to other settings for "alternator-style" (pulse) signals. The "2" setting is certainly not correct for a 6 cylinder 4-stroke. That engine produces 3 pulses per engine rev. So check your instructions again. The next question involves if this tach is for an outboard, Inboard/I/O or is it a universal. If it is outboard only you have the wrong tach if there is no setting corresponding to 3 pulses/rev. As for the fuel gauge, if the tank was full when you left the dock, it is possible the reading could remain at full. These things are not as linear as you might think. It all depends on the attitude of the boat, shape of the tank, and the sender itself. The arm is adjustable and if not it can be bent a bit to compensate for ONE-END of the scale. You cannot adjust for one end of the scale without affecting the other. Adjust it for an accurate EMPTY reading. After all -- you know when the tank is full because you filled it.
 

dlogvine

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May 4, 2015
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839
I looked for the setting of this tachometer, it has 4 possible values, 1,2,3,4. 1 corresponds for 4cylinders, 2 for v.6, 3 for v.8 and 4 for 10post. The tachometer is used on inboard engines. Should i try other settings? I will try to check in the gauges repair store which is the correct setting, but all of the online resources i found, state the setting 2 corresponding to v6 engine.
 

dlogvine

Master Chief Petty Officer
Joined
May 4, 2015
Messages
839
No Title

This is how this tachometer looks like (very similar to it).
 

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Silvertip

Supreme Mariner
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Sep 22, 2003
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28,771
When you installed the tach using the "U-shaped" bracket, did you insert the insulating bushings in the two holes that the top two posts on the tach pass through?? If not, those two posts need to be electrically isolated from each other. Other than that, you either do not have a V6, the tach is bad, the settings are not correct, or the connection at the engine is not correct. Since I can't see your boat -- I'm without a solution for you.
 

airshot

Vice Admiral
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Jul 22, 2008
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5,864
As far as your fuel gage goes...make sure the float mechanism is working correctly and is free to float. What id did was to empty my tank then start adding 2 gallons at a time. I used a marker to make a small mark each time I added 2 gallons until the tank was full. Now I had markings to show how much fuel I had left in the tank.
Now my tank is only 25 gallons so if you have a larger tank you may wish to add more gas each time to get the feel for how your gage reads. I was lucky as my gage drops 5 gallons at each quarter mark right on the money. When the gage hits the E mark I have just 5 gallons left, I do continue to check it often by watching how much fuel I put in according to the gage reading....just to make sure something doesn't change.
 
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