1969 Merc 1250-Severe slippage

JasonJ

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Aug 20, 2001
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Day before yesterday, wife and I go out on boat for fun/sun. She is driving, we motor out of no wake zone, and she gives it full throttle to get on plane. Motor is screaming, we are slow to get on plane, float tube flies out back, she freaks out, we stop. Retrieve float tube, I drive, we head to favorite anchoring spot. Boat seems slow for rpms, I just attribute it to extra weight. Then the rpms start to fluctuate. We are cruising along at around 4500 rpms, and the rpms would climb to near 6000, then fall back. It did it repeatedly. I stopped, checked the prop for weeds, nothing. Again took off, it came up on plane just fine, and then would do the rpm climbing/falling thing again. It clearly wasn't a powerhead thing, as it sounded fine. It was acting like the load was changing. It was acting like the prop was cavitating or the hub was starting to spin. We got to our spot, did our thing, and returned, the problem still persisting.

This morning I decided to take it out to see if it would do it again. It was much worse. It got up on plane and was accelerating normally and then the rpms started to climb to the moon and the boat slowed and fell off plane. I reduced throttle to about 2500 rpms and the motor hummed along fine but the boat slowed to a near stop, then there was a dull thump and the rpms dropped. The boat came back up on plane and then the rpms started to climb and it fell back off plane. Okay I thought, prop hub is dying, I'll switch to the spare prop. It got even worse with the other prop on. It will cruise fine at displacement speed, there is no grinding sound or anything wierd soundwise coming from the motor. I have had lower units go on me before and I could tell those were a tragic gear event. This just feels like a huge slippage.

I am going to drop the lower unit and hand rotate the driveshaft to see if I can feel ruined gears, and check the splines on the shaft. My question is this: Is there any other soft joint in the drivetrain that could slip and fail like the hub of a propeller? Would this be located where the driveshaft couples to the powerhead or in the lower unit if such a coupler exists on this motor? I feel like there is none, but I want to make sure. The only other thing I feel it could be is stripped driveshaft splines, although the pee stream was powerful. If the splines were stripped it would have to be at the lower unit end or the water pump would not be pumping when the boat is near stopped and the motor is screaming. The other, more horrid possibility is the lower unit gears are roasted. I guess I won't know 'till I drop the lower unit. Any help is much appreciated.
 

JasonJ

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Aug 20, 2001
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Re: 1969 Merc 1250-Severe slippage

Dropped the lower unit, splines looked fine. Pulled the lower unit apart, everything looked fine. Had difficulty looking up leg to see splines on crank so I pulled powerhead off. Splines look fine. I am at a loss. I will further investigaet the splines on the crank, perhaps the whole thing cracked and spreads under load. Can't think of any other reason for this to happen on a setup that has not only worked fine this year but for years past. Can't be the prop, the odds are against both my props failing at the same exact time...
 

jheron

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Jul 21, 2004
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284
Re: 1969 Merc 1250-Severe slippage

For what its worth I think if the splines where slipping you would see obvious evidence in the form of shredded metal...
Jon
 

jheron

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Jul 21, 2004
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Re: 1969 Merc 1250-Severe slippage

Also you can put a punch mark on the hub and the prop to see if its slipping.
 

Clams Canino

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Jan 10, 2004
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2,179
Re: 1969 Merc 1250-Severe slippage

Yes.... probably a spum hub in the prop - that's the obvious 1st guess. Shouldda waited for an answer here to save a LOT of needless work.

-W
 

JasonJ

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Re: 1969 Merc 1250-Severe slippage

Yeah, spun hub came to mind initially. It doesn't explain why my spare did the same thing. There is one other possibility that was posed over on the Old Merc site. Cavitation caused by the warmer water. It seems like it shouldn't be a factor, its the same lake, same motor, same boat (except for the bass conversion, but that took weight off of the front, the back lost little weight) but it sure ACTED like cavitation. Needless the say, it is back together. I think I set a personal record for how quickly I can rip an outboard apart and put it back together.

I will run it again tomorrow. I will do two things. First I will mark the prop to check for spin. Second, I will take the hydrofoil off so the stern can settle in the water more. If it still does it I'll have to live with it and not travel as far. I can't have it out of commission. On the plus side I now know the internals of my lower unit are fine....

Besides, I was bored, needed something to do...d:)
 

Bob_VT

Moderator & Unofficial iBoats Historian
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May 19, 2001
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26,019
Re: 1969 Merc 1250-Severe slippage

Jason think outside the box...

A motor will rev to full rpm's even if not firing on all cylinders. Could you have a coil going bad? It just sound like it it is the top half of the motor.

If you search you will see alot of posts where full rpms can be reached and not on a full set of cylinders.

Bob
 

JasonJ

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Re: 1969 Merc 1250-Severe slippage-update

Re: 1969 Merc 1250-Severe slippage-update

Thanks Bob. I only have one coil.

I removed the foil and ran it this morning. The issue is greatly reduced. It drives fine, and stays hooked up for the most part. I could not get it to blow out, not even in turns, and the boat gets on plane just fine without the foil. I am thinking the warm water is the issue. It has been at or near 100 degrees for a week now, and the water temp is near 80 degrees in the part of the lake I had the issue. Removing the foil allowed the prop to better stay hooked up, but the new side affect now is the rpms are exceeding 6500, whereas they were around 5900-ish rpms before. Again, don't know if this is because of the water temp or the reduced drag of the foil being gone. Either way it works now, but now I am wondering about getting a prop that will hook up better in these circumstances. maybe a stainless in the same 13x17 pitch will bring the rpms back in line and bite in the warm water.

Can you even get a stainless prop for this motor?
 

jheron

Petty Officer 1st Class
Joined
Jul 21, 2004
Messages
284
Re: 1969 Merc 1250-Severe slippage

If your boat is docked in the water all the time maybe haul it out and check the bottom of the hull for somthing that could cause the cavitation...
Jon
 
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