1960 Lark 40 Rubber Mounts? Rough Vibe :-(

[KirbyE]

This thing vibrates my boat as if there is no rubber isolating the powerhead from the transom. Feels like the wooden runabout will come unscrewed, and I will lose fillings from my teeth. Runs fine with lots of power. Idle is smooth. It's new to me, no known history on this powerhead but the mounts LOOK ok.
My other one (1959 J35 hp on same boat) is smooth and quiet at all speeds with 48 year old rubber. Both engines seem to wiggle on their mounts the same on land.
Do these motormounts get hard? Are they easy to change? Where can I get them? What else should I be looking at that might cause this kind of vibration? All input welcome. Thanks.

 

[F_R]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe :-(

Hm-m-m-m.. I wonder... The 1960 had a problem with breaking crankshafts. That's just a wild thought. See if there's anything unusual like the flywheel flopping around.

Maybe there's a chunk of something dropped down between the housings?

 

[KirbyE]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe :-(

1960 is a guess. It may be 61 or 62. It has the "larger" crankshaft and flywheel, and I'm sure it's on tight. Balanced I don't know about. It's not like a dead cylinder, or bad balance between them. I like your rock sitting on the rubber idea better. I will go there next. Thanks.

 

[KirbyE]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe

Nothing in between the housings. Had it all apart today and the rubber all looks and feels good.
I turned the engine by the prop (plugs removed) and I can feel the lower gear teeth. Maybe this is normal, maybe there is a problem. Never tried before. They behave smooth, just not silk. Hard to make a call in text on a forum.
I will sync the links again, and give it another shot when I don't have the kids in the boat!
The clue I was not giving before is that this is a replacement powerhead ($1 eBay) and the motor purred before I threw a rod last summer. The powerhead is most suspect, but had to remove / replace all the other stuff too. Lower gears were fine then.
OK then, what else am I missing?
Thanks again for the intrest, and I really enjoy lurking on this site !

 

[larky]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe :-(

have you done a compression test? another idea, is the flywheel out of balance for any reason?

 

[rodbolt]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe :-(

that engine is mounted to the midesection which is mounted to the frame with either 5 or 6 rubber mounts.
the mounts liked to break and then the motor would shake violently in the frame.
once one broke the rest were right behind it.
they are right much a pain to replace in a saltwater motor
dunno if they are even avalible from BRP now.
the upper front was usually the first to go and is the hardest to replace.

 

[KirbyE]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe :-(

I will do the compression. I have no reason to doubt the flywheel, but who knows. How would it get out of balance? How can I tell? How can it be re-balanced? It shows no dents or signs of damage. (Electrric start).
I'm confident it's not the rubber mounts at this point. It's a freshwater motor. All the fasteners came out easily. It doesn't "shake violently" It's more like the hull becomes a guitar resonator. Smooth at idle, smooths out at high throttle.
OBTW: Evinrude model 35520 SN 23821. The powerhead is white (Johnson) with 40773E and E01958 stamped on the welch plug.
I'll run it this weekend a play with mixtures, syncronization, whatever and see what I can do. Thanks 10e6 !

 

[F_R]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe :-(

Mounts are good, nothing lurking in there, big crank...Don't know what else to suggest. Let us know what you find. Maybe it will help the next guy.

But I still suspect metal to metal contact somewhere when the mounts are compressed under load.

 

[KirbyE]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe :-(

I tend to agree with F_R. I will try things like shoving a tennis ball in the gaps, and looking for distortion anywhere. There are a few painted bosses on the castings that are clearly there to retain the engine if the rubber gives way. Their paint is intact, so I don't think they are hitting. I found one of the coolant lines for the choke was contacting the inside of the cowl. Doubt this could do it, seems too light, but maybe. I will post my results, and again, thanks for the help.

 

[KirbyE]

Re: 1960 Lark 40 Rubber Mounts? Rough Vibe :-(

I ran it a lot this weekend:
1. It's not the rubber mounts, or debris etc.
2. The compression is excellent. 110 both cylinders.
3. The points are exactly 180 degrees apart (openning, using meter).

Everything seems to make a difference. The vibration is only at mid throttle. Idle and are WOT smooth. I changed props among 3 and each has its own character (11, 12, and 14 [badly worn] pitches). The 12 is the best for the weight of the boat. I played with high and low speed mixtures. I played with syncronization of throttle and ignition. I got it working much better, but these are time consuming activites ! I found that I like it better when the throttle opens sooner than the marker on the brass plate. I wonder if the shape of the brass ramp on the roller is worn, or could be modified? I did not play with the WOT throttle linkage, but I think that could be better too. Maybe I'm too fussy. My 35 hp is silky smooth, and I have been the only mechanic on it ever.
Another minor issue I have is the water takes maybe half a minute to start spitting, and you can hear the difference. The impeller is a few years old. Once it "primes" it's fine and quiet for the rest of the day. Thanks again !