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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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Old 06-26-2016, 02:19 PM   #11
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Quote:
Originally Posted by mmwjr View Post
I have no idea what these black plugs are, can you post a picture?
Aren't they called prop vents? No idea what they do, though.

These guys are supposed to be pretty good. give them a call, describe the symptoms in detail, get their recommendation. Propgods.com > Mercury
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Old 06-27-2016, 05:56 PM   #12
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Ken, who is super nice and got back with me quickly, said this:

Should have 3 of them. one near the leading edge of each blade.

Its called the PVS system. (performance vent system)
Bascially, if the motor is bogging or hesitating when you initially hit the throttle, opening those vents up will let the motor rev up quickly.

If you open them too much, the prop will just ventilate to easily and holeshot will suffer. I would only open them if the motor was struggling to turn up RPM when you hit the throttle at first.

The way to take them out and not damage them so you can put them back in if needed. Take the prop off, and use a large philips head screw driver.
Push down on the center of that plug, and push it INTO the prop.

Give the motor a run with a couple plugs out if you want to see how it runs.
Then use a flat head screw driver to work the plugs back into the holes.

Now we know. I don't seem to have a problem with the engine bogging down when I accelerate so I'll just leave them in for now.

Sorry, I still can't figure out how to place a picture in a post.
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Old 06-27-2016, 07:50 PM   #13
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To post a pic from your PC, click on "Post Reply". Look above the text box and you'll see some editing icons, and one of those has a paper clip. Click on that and another box will open, click "Browse" and find the pic on your PC you want to load, then click Upload and then close the box.

None of which solves your prop issue. Did Ken give you any rec's?
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Old 06-27-2016, 08:45 PM   #14
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Thanks SS. Here it is.
Attached Thumbnails
prop vent.jpg  
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Old 06-27-2016, 10:02 PM   #15
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Ok so they are on the forward part of the prop hub. Adjust to let exhaust gases flow over the prop to provide some slip to allow it to spin up faster.

Never saw this before.
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