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Old 06-23-2016, 02:55 PM   #1
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Default Where is it?

Hi all and thanks for everyone's help in the past.

I'm having a problem with my Tach. I'm trying to find what my WOT is however, when I reach around 4500 rpm, the tach will sometimes jump up to 6000 rpm so I'm a little skeptical on its accuracy. It appears to be an original gauge. I'm trying to check all the connections however, I'm a little embarrassed to admit this, I can't find the coil. I'm guessing it looks like any other coil, round and cylindrical with two wires coming out of the end of it. My engine is tucked in a little bit to where I can't really see behind it but I'm guessing it's the same for most people. Anyway, it's a 1994 5.7 liter with a Thunderbolt IV ignition. I've stuck my cell phone behind the engine and took pictures to see if that would help but it didn't. If anyone has a picture of where things are located, that would be great.
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Old 06-23-2016, 03:02 PM   #2
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Cancel that. I suddenly remembered that I could follow the center distributor wire to the coil. I found it. Any info. on the tach problem would be great though.
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Old 06-23-2016, 04:24 PM   #3
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The grey wire on the coil is the tach sense, so check that connection.

So it only jumps near WOT?
When this occurs is there any change from the engine in sound or performance?
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Old 06-24-2016, 04:21 PM   #4
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No change. But then I start to wonder if the tach was inaccurate prior to the 6k rpm reading or if the 6k reading is now accurate.

The local boat shop suggested I turn back and forth a number of times the cylinder number indicator dial on the back. He said he's had a lot of this same problem come up this year and what he told me should fix the problem. A simple possible solution so I did it. We're going out on the boat today so I'll check it out.
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Old 06-24-2016, 08:16 PM   #5
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I doubt a 6000 RPM reading was accurate. That engine probably won't spin to 6K without floating the valves, which at the very least would have been audible.
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Old 06-24-2016, 11:15 PM   #6
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I agree with Dan unless way under proped then as he said the valves will start making noise.
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Old 06-25-2016, 04:20 AM   #7
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We just got back from the lake and it seemed that the tach was working fine. I had six people on the boat and at WOT the boat was running at 4400 RPM. Does that sound like a good RPM?

Is it possible for a boat prop to cavitate while underway? It seemed that as I turned pulling a lightly encumbered tube, and after turning and then straightening, the engine was running at the same RPM but we were just poking along. I stopped the boat and then started again and everything was fine. Wierd.
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Old 06-25-2016, 01:58 PM   #8
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Though you didn't mention what boat and drive you have, 4400 at WOT with six aboard is probably in the ball park.

It is entirely possible, and has a lot to do with how you have the drive trimmed. Once I get ours on plane I always trim the drive up and as long as steering input is minor it's fine. If I have to make a moderate correction it will cavitate until I lower it again and/or straighten it out. If I'm pulling a tube I leave the drive all the way down.

If yours is cavitating with the drive all the way down you may want to consider a different prop.

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Old 06-26-2016, 06:53 AM   #9
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Thanks for all the replies everyone.

The boat is a '94 2300SR with a carbureted 5.7 and alpha one and I think it has a 1.50 gear ratio and I'm running a Mercury Enertia 18p prop. I'm not sure what the diameter of the prop is but I'm fairly sure it's the correct size.

I keep the outdrive down when pulling anything. I'm wondering about the round black plug looking things at the base of the prop. I can't imagine they would have anything to do with it but the local dealership was wondering why I had the prop set up that way. The plugs are completely pushed in so they're level with the outer layer of the prop.
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Old 06-26-2016, 11:23 AM   #10
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Quote:
Originally Posted by Drenski View Post
Thanks for all the replies everyone.

The boat is a '94 2300SR with a carbureted 5.7 and alpha one and I think it has a 1.50 gear ratio and I'm running a Mercury Enertia 18p prop. I'm not sure what the diameter of the prop is but I'm fairly sure it's the correct size.

I keep the outdrive down when pulling anything. I'm wondering about the round black plug looking things at the base of the prop. I can't imagine they would have anything to do with it but the local dealership was wondering why I had the prop set up that way. The plugs are completely pushed in so they're level with the outer layer of the prop.

I have no idea what these black plugs are, can you post a picture?
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