Diagnosing Merc 140 Clatter Sound -- what to check next?

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myoldboat3

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Apr 6, 2023
RO Number
34841
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GM Engine 140 HP 4-cyl 181 cu.in. s/n 3888148
Mercruiser "Late Model I" Stern Drive s/n 3901959
1974 Harris Flote Bote, 28' pontoon

Hello all,

Fast cruise these days for the pontoon is 3400 rpm for the Merc 140 -- I take it easy on the old girl. A few days ago I was taking it even easier on the engine, medium cruise at 2800 rpm. Suddenly the engine & boat started shaking and the engine died with a cough. Sat a few minutes. It started right up. Oil pressure, temp, charging all good. Limped back to the boat lift at low rpm.

No weeds on the outdrive and prop. I didn't see any prop damage.

I've been trying to diagnose the problem:

Oil - no water
Compression test - good
Spark plugs - dry and clean
Spark - good

It starts immediately and idles perfectly. In neutral, running at 2000 rpm causes no problems. Momentarily going to 3000 rpm (I know, not great without a load) still causes no problems.

Took it for a short test cruise. Everything is fine at idle up to 1500 rpm, maybe even 1800 rpm. At 2000 rpm I start to hear the clatter, which gets very loud with increasing rpm. The shaking of the engine/boat increases as the clatter gets louder.

My thoughts were:

Blown head gasket - but compression is good and no water in oil
Bad plug or wire - but spark is good and the engine runs great in neutral
Rocker arm nut loosened or bent pushrod - but the engine runs great in neutral; could this act differently under load?
Flapper blocking exhaust - but the engine runs great in neutral
Failed engine mount - but they look fine, and the engine runs great in neutral
Timing - I found my timing light so I can check the timing at idle in gear, but the engine runs great in neutral
Distributor advance - but the engine runs great in neutral
Shift cable adjustment - would the noise change with rpm?
Gimbal bearing - but the engine runs great in neutral with no clatter

I have a mechanic's stethoscope, but I was doing the test cruise alone. I'll need a helper so I can listen to parts of the engine and exhaust while cruising, if you think that will tell me anything. Without using the stethoscope, it really sounds like the clatter is coming from the rear of the rocker cover or inside the exhaust.

Finally, the center post connection of the ignition coil was filled with liquid which I'm almost sure is oil from inside the coil. I have the new coil now, but I'm trying not to change a lot of things while diagnosing. Could the coil act differently when the engine is under load vs. in neutral? I wouldn't think so.

So:

Any other ideas?

Thank you in advance.

Steve
 
Could the noise generated by something in the outdrive? Bad coupler?
 
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Years back the coupler failed two years in a row (turned out to be bad splines on the sterndrive yoke). It wasn't fun pulling the engine twice. Anyway, each time the coupler failed, there was never any shaking or indication, other than the boat just sitting in the water while increasing throttle (a bad feeling).

Also, the coupler is driving the outdrive shaft all the time, so the engine wouldn't run great in neutral. I've been thinking that the behavior is load related because the clatter sounds like it's coming from the engine. But you got me thinking, maybe it's just happening in gear--I need someone to lay on the rear sundeck and watch the sterndrive while increasing throttle gets the clatter just starting. And maybe I'll try more throttle in reverse to see what happens.

Thanks. Anyone got more ideas?
 
Have you checked the prop shaft seal and outdrive oil(I know nothing about these......ha).
 
Sounds like a timing issue, is the rattle your hearing detonation (spark knock)? Given what your seeing in the coil would point me in that direction - it's been years, but I have had a bad coil be fine at idle and low speeds, but go crazy when the engine get's a load on it. You need to eliminate timing as a problem - detonation like that will destroy the engine and fast. You can do a quick check with the timing light, get someone to drive and put the light on it and see if the timing advances - I don't know the specs for that engine off hand, but it should advance at least 15deg BTDC. If it does not advance, I would go ahead and replace the coil AND start looking at the distributor - I bet it is a mechanical advance which would further explain how this could just start happening. The advance mechanism can get rusted, corroded over time and stop functioning.
 
Great input, thanks. I will replace the coil, then check the timing. Then when I have a helper, I will check the timing advance. I have a Pertronix Ignitor ignition module, which has been rock solid for decades. According to their documentation, the advance is still performed by the distributor's mechanical advance. So it could be the distributor. Here are the Mercruiser 140 advance curves from the service manual. The advance is added to the base timing. It's interesting that my engine's clatter is starting at about 2000 rpm, which is where the advance tops out.

Distributor Advance Curves sm.gif


Finally, I saved this years ago. I apologize that I can't recall which forum it was on:

So as a Mercruiser mechanic, we have learned that OEM timing is a little on the dangerous side. We have seen the 3.0 blow head gaskets over and over, different boats, different ages. It turns out that Mercruiser timing advance is too advanced. It leads to blowing head gaskets when set to factory specs. We do not set it too advanced anymore. One to two degrees retarded from what the engine calls for, will save the motors and eliminate head gasket issues.
Click to expand...

My base timing spec is 6 deg BTDC, so maybe 5 or 4 deg would be safer. Thoughts?

Thanks.

Steve
 
Bob:
Have you checked the prop shaft seal and outdrive oil
Click to expand...

Actually, I just thought about this last night. I have not checked those yet. It's difficult to get to the outdrive with the boat on the lift (deep water). Maybe I can get someone I trust to do it from a kayak.
 
myoldboat3 said:
Bob:


Actually, I just thought about this last night. I have not checked those yet. It's difficult to get to the outdrive with the boat on the lift (deep water). Maybe I can get someone I trust to do it from a kayak.
Click to expand...
Seriously, not a bad idea. Maybe a flat-bottom jon boat would be a better choice.
 
myoldboat3 said:
Bob:


Actually, I just thought about this last night. I have not checked those yet. It's difficult to get to the outdrive with the boat on the lift (deep water). Maybe I can get someone I trust to do it from a kayak.
Click to expand...
It occurred to me this morning that your marina may have a 'work barge' they may let you use. My friends that own my local bait shop/marina have a very simple one built on large styrofoam floats.
 
I have an Otter Island swim platform that we use when changing the impeller. We pin it under the aft end of the pontoons by lowering the boat lift. It works great.

But when it rains, it pours. We've changed out 3 of the 5 cables on the boat lift, and one of the remaining cables snapped. The old galvanized cables were getting corroded and jamming. We'll replace them soon, but until then the boat raises leaning far left and no way to get high enough to pin the swim platform.

On top of that, I started the boat to back it off the lift in idle, and now the sterndrive is making a loud, constant noise. Ugh. I've got some feelers out to mobile mechanics to come take a look.
 
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