Do I need a Lower Unit Rebuild?

I'm dealing with a '77 Mercury 1150 / 115HP - Ser. Num 4569747

I having an issue where when I go down on the throttle and hit the 2000 RPM range my boat seems to gently ease out of gear and the engine revs. It always happens when the boat is about 1/2 way to plane and when it happens the prop still turns but the RPM's go way up. I can come out of gear and can hear the clutch dog come out of forward gear, then when I put it back into gear there's a nice crisp engage into gear - no grinding noises or anything odd sounding. I checked the L/U oil and there is no metal whatsoever and the oil has no water.

I was hoping it was a spun hub issue, but I marked the hub and took it out to replicate the problem, when I pulled the prob off all my marks were still lined up, so I know it's not the prop. The prop shaft looks great, and like I said there are no odd sounds or vibrations coming from the motor. When the RPM's rev up the boat sort of maintains the current speed, till I back off the throttle and I can leave it in gear at any speed below where it seems to loose the gear and it will maintain that speed... ie, I can "troll" around at say 1500 RPM's with no issues, but when I go to 2000+ RPM is when it seems to slip out and the RPM's start to climb quickly.

Does it sound like my Lower Unit is shot? Do you guys recommend a rebuild or find a used unit? Could it be something with the shift shaft? If I decide to probe inside the L/U what tool is needed to get that initial "nut" off that threads onto the outside of the gearcase?

Thanks for the help.

Re: Do I need a Lower Unit Rebuild?

From what you say the clutch dog may not be fully engaging the drive gears in the LU.

I'd first understand the full range of the shift linkage on the engine. Remove the remote cable and move the engine shift linkage through it's full range ensuring that any time you go to R you do it slowly and slowly rotate your prop so that these gears can mesh.....R unlike F is a square box gear shifter arrangement where the two mating surfaces have to be directly in line for engagement. If you try to force it you will tear up something. Mark N and the max limits of F and R.

Then set your controller in N and shift your engine to N rocking the engine linkage to ensure that you are in the middle of the N position. Roll the deadman bronze barrel up or down the threaded portion of the remote cables at the engine until you can put the end on the shift linkage and the barrel will fall into the slot provided for it.

Go to the control box and shift and when in a gear, go back to the engine to check and see if you got the engine linkage to move to the places previously marked. If not work on your control box. If you do, go out and try it........remember, any time shifting to R with the engine off, you must rotate the prop while feeling your way into engagement.

Mark

Re: Do I need a Lower Unit Rebuild?

Thanks for the help Mark, I've got some ball games to attend today but I'll give your advice a try tomorrow. I just feel like the L/U is not damaged just due to no metal and no water in the oil. It sounds like it is in tip-top shape until it revs up and looses the forward momentum.

One thing I was looking at last night was putting it into gear and trying to turn the prop shaft. Seems like a while back I read that in one gear you should be able to move the shaft either way and there should be a ratchet noise, but in the other gear you shouldn't be able to hand turn the shaft. Right now when I put it into Reverse I cannot turn the prop shaft at all, there's just a tiny bit of "slop" which lets me turn the shaft back and forth perhaps 1/4 turn before the shaft makes contact with something inside the L/U which stops the turning motion. When I put the boat into Forward it lets me turn the shaft clockwise and I get the ratchet sound about every 1/2 turn or so. When I try to turn the shaft counter clockwise in Forward the shaft will not move at all. Does this scenario sound right?

Re: Do I need a Lower Unit Rebuild?

Absolutely normal. Rotating the prop with finger pressure only:

Forward: Prop will turn (ratchet) CW, will not turn CCW
Neutral: Prop will turn freely in either direction
Reverse: Prop will not turn in either direction (except for the 1/4 turn gear mesh you are feeling).

Gerry

Re: Do I need a Lower Unit Rebuild?

Sounds like the prop hub is slipping, if the engine was jumping gear it would be accompanied by a loud bang as it re engaged

Re: Do I need a Lower Unit Rebuild?

""When the RPM's rev up the boat sort of maintains the current speed,""

What Laddies said. And it likely wouldn't engage until you put it back into neutral.

And the boat would not maintain any speed if it had slipped out of gear.

Perhaps your scribe lines on the prop and housing were not in the right place.

Try a different prop, you should have a spare anyways.

Re: Do I need a Lower Unit Rebuild?

I agree that it sure sounds like a spun hub, but my scribe lines definately hit the hub, the rubber, and then the outer aluminum housing that the fins attach. Unfortunately I just got this boat from my late Grandfather and it did not have a spare. Let me back up a summer ago with some more background info: Last year I was running this boat at WOT at full speed with no issues. One day while at WOT the motor reved and I instantly came off the throttle (scared the crap out of me!). What happened that day WAS a spun hub as the rubber gasket was shredded on the old prop. I pulled the spare (the prop that is now in question) and put it on but haven't ran the boat until this year. I assumed the "spare" was a working prop for this boat, but unfortunately my Grandpa isn't here for me to verify this. Is it possible that instead of a spun hub, that the spare is not the right prop for this motor and instead of spinning the hub, the fins are losing their "grip" with the water and that is why I am reving the RPM's at the same point????? That could also explain why there is no odd sounds and how I can ease off the throttle and still keep forward momentum until I try to give it more gas.

So...........is this the right prop for my '77 Mercury 115? It's a 3-blade aluminum prop with the numbers 48-32746A3 21P stamped into the side of the prop. On the center hub it has 48-32746-2 stamped.

I'm not sure how to "read" these numbers other than I assume the 21P is a 21 Pitch? Could my hypothesis be correct and the prop is losing it's grip with the water and at that point the prop just free spins till I back off the throttle to a point that it can get bite in the water and keep the boat moving at 5-10mph????

Re: Do I need a Lower Unit Rebuild?

Do you still have the old prop for reference.

Re: Do I need a Lower Unit Rebuild?

No, I do not have the old prop.....

Re: Do I need a Lower Unit Rebuild?

Those numbers sound like a standard merc prop with a 21" pitch. A bit too much pitch for a heavy boat. What boat do you have it on?
iS the motor tilted all the way down? If not, as the boat approaches planing speed the prop can ventilate and rev up.

Re: Do I need a Lower Unit Rebuild?

Chris, the boat is a Fish and Ski style boat with a max capacity of 1350lbs of gear and persons. I'll post a pic of the boat. The boat has never had this issue before so I know the motor height is ok and yeah, the motor was trimmed at the proper angle.

Re: Do I need a Lower Unit Rebuild?

I am not sure what to recommend. I had a '77 Merc 1500, spinning the same prop on a much lighter boat. It perfomed well. With that prop on a heavier boat and less HP, it may be a bit overpropped. When a Merc is overpropped, it simply won't rev up, and may stay at just fast idle.

However, I do not know how the motor could rev up and the boat not move faster, without the prop hub spinning. As was said, you would hear grinding or banging if the clutch dog was slipping.

Re: Do I need a Lower Unit Rebuild?

bsutravis said:

Chris, the boat is a Fish and Ski style boat with a max capacity of 1350lbs of gear and persons. I'll post a pic of the boat. The boat has never had this issue before so I know the motor height is ok and yeah, the motor was trimmed at the proper angle.

View attachment 147833View attachment 147834

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That rig ought to do 40 with a 21P prop and a couple of folks. I don't see trim cylinders but at the tilt position you have it, it shouldn't have a blow out problem from being pitched too far out. If you think it may, suck it in a couple of tilt holes (prop closer to transom) and try it.

I guess the confusing thing here is that you said that you marked your prop and tested it and it passed. That sure looks like the smoking gun unless whatever you grandpa may have hit to spin the hub on the old prop didn't tear up something in the LU, but if it did you would hear metal to metal noises when idling in F.

I get on here and get yourself a 21P SS prop or at least a 19 if you don't waterski as the SS will be more efficient and is cupped equalling a couple more inches of pitch at WOT. Course, ideally, go to a local dealer that will let you test before you buy and get just what fits your operating profile. Will pay more initially, but the extra is worth it in terms of enjoyment for the many years that you will surely use the boat.

Mark

Re: Do I need a Lower Unit Rebuild?

I got a larger diameter prop with a 19 pitch... gonna test it tonight so I've got my fingers crossed. I'll post the results.

Re: Do I need a Lower Unit Rebuild?

Amazing! The 19 pitch on the bigger prop solved my problem!!! I was running on the river which only allows me a short distance to run WOT so I didn't get any top speed data, but with the new prop I ran right past 2000 RPM and the boat ran great! I knew in wasn't the Lower Unit, but just couldn't figure it out. Amazing what dropping to the 19 pitch did!!!! Thanks for all the help guys....now I can try to fine-tune and get my old Merc running like new again.