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Old 08-20-2013, 06:54 PM   #1
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Default Re-visiting Coupler problem

In my previous post about my Drive feeling like it was slipping.. I rectified the problem, got a new matched drive.
Boat ran fine.. had some other issues, working out the bugs, but now I have a returning High RPM issue.
I can cruise this boat decently but it is cruising in the 4000s.. opened up goes up over 5000.. and just to see what it
was going to do I put it WOT.. If this tach is correct, it hit 6000 rpms, and I take it there is some sort of rev limiter? because the boat was
shutting itself down into 5500 range by itself.. now, I had a 19p prop on it and was getting up there.. put a 23 pitch prop on and same..

I have read that the engine coupler does not slip, it is either bad or not bad.. is this true? Would a bad engine coupler allow my boat to
get up to 32/34 mph at all and consistently although at very high rpms? there is no smell of burning rubber and this boat with the 19 pitch prop jumps right up on plane.. again, just higher rpms than I believe it to be running at. I put the 23p on it to see if I could bring down the rpms..

Not being able to think of anything else.. Rings? .. if there is compression problems, will I see this same RPM problem no matter what prop size/pitch I use? I take it that I am looking for 150-160 psi on the 7.4 big block?
Engine Temp is consistently 180 not higher..
I am going to investigate this tomorrow..

All the fuel problems seem to be corrected, that I had in previous posts.. but now, back to this RPMs issue..

I went out on my friends 95 bayliner 2855.. has 7.4 same engine I believe.. His boat runs 38- 40mph at 4200 rpms easily, it does not sound like the engine is reving really high like mine does at 25 to 30..

Thanks!!
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Old 08-21-2013, 02:37 PM   #2
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JR,

1st - don't run that engine faster than 5K for anymore than a few seconds. You're lucky that it survived when it hit 6K. I don't remember the year of your boat/engine, or even what boat you have, or what drive you have, but if it is fuel injected then yes, it does have a rev limiter, though I don't know what it is set at from the factory.

Regardless of what the tach says, you can easily hear if the engine is running faster. If you reach a certain indicated speed, say 32-34 as you indicate above, and then try to push it faster, if the boat gains little or no speed but you can hear the engine running faster then you have either a bad prop or a bad coupler.

On the other side of the coin, if the engine speed (RPM's) is NOT increasing once you hit a certain MPH speed then you need to look at other areas.

So, given the symptoms you've described above I don't think you have an engine problem.

Dan
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Old 08-21-2013, 03:59 PM   #3
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a slipping coupler is like driving a car with a slipping clutch.

it works OK, when cold, and slips when hot, or under a load with four fat chicks on the boat.

so, yes, you can get to 34mph, with the current and no load on the boat.

simple way to check:

to go on plan, crank the boat up full throttle, out of the whole from a dead stop, and see if the boat pulls out of the water or the RPM just spike.

A normal boat will tractor right out of the whole at 4,300 rpm.

simple.

couple engine pull and re stall is about $2,500.00.

don't fell bad, some guys with twins, have to pull the motor to get the starter changed.
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Old 08-21-2013, 07:22 PM   #4
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have you check the hub of your propeller? if it slips this will change your rpm's, this is what maxum recomends for your 2700scr for a prop with that drive ratio 1.81:1 and engine 18-1/4 X 19 RH PROP.
good luck
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Old 08-23-2013, 12:24 PM   #5
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Jr, both the coupler and the prop have a break away hub. Typically when these fail the rpms will reach 2000 then shoot up quickly and the boat will slow down because of slippage. As Dan said don't go over 5K, if you can get to this rpm and still have throttle left you are under proped.

Yes compression should be ~150 but good or bad compression will have nothing to do with over reving.

Regarding the temp get a handheld IR thermoter ~$50 and measure the t-stat, manifolds, elbows to see what temp they are at. I suspect either the temp sendor, or gauage is off. Also merc installed the sensor in the t-stat housing and use a gasket that have brass eyelet to complete the ground, I have see these wear which increase he resistance to ground and results in a higher temp reading.
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Old 08-23-2013, 01:52 PM   #6
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Mike, do B3 drives have rubber hubbed (break away) props? I've not had the props off of my B3 but I've looked at the outer prop pretty closely, and poked at it with a screw driver, and it sure looks like it's solid/one piece to my un-trained eye.

JR, the short of it is this; there are only two things that can cause the engine to rev up significantly with no cooresponding increase in speed. The prop(s) are slipping or the coupler is slipping.

Dan
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Old 08-23-2013, 02:20 PM   #7
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Dan,
I really don't know for sure. I have a B2 and it does. Guess I was just assuming they all do. How long have you had your boat? Thought you would have pulled the props by now if nothing else just to regrease the splines to prevent corresion.
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Old 08-23-2013, 09:08 PM   #8
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Thanks Mike.

Have had ours for 5 years now. Props have been off once, season before last, to have them inspected but I wasn't the one that pulled/re-installed them.

Dan
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Old 08-23-2013, 10:54 PM   #9
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Dan,
after ready a few articles that I googled (not 100% backed as I have gotten bad info before) it seems the B3 has solid hub and not rubber, but I'd look myself to be sure.
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Old 08-24-2013, 01:45 PM   #10
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Thanks Mike, my poking with sharp objects supports what you found; solid hubs.
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