Universal M3-20 vibration/shudder at certain RPM

[ValiantV]

Hi all,

Our "new" '92 Catalina 30 MKII has a Universal M3-20 diesel that seems to work well. Starts easy, runs smoothly, no smoke, etc.

During the sea trial, the surveyor and I both noticed that the engine has a pretty good "shake" to it at about 900 rpm - whether in neutral or in gear. It's pretty smooth at idle (looks like about 800 on the tach - but I'd take that with some salt) and dead smooth at 1,000 rpm and above.

Engine mounts look good, and since this shudder/vibration happens in or out if gear, we don't think it's a shaft alignment issue.

I read in Don Casey's big book that sone smaller (1-2-3 cylinder) diesels can have some pretty pronounced vibration, soon seeing if this is "normal" for this engine.

This only appears to happen just above idle speed, so some kind of "harmonic" cause seems likely.

Just wondering if anyone with this engine or engine/boat combo has seen this as well.

Thanks!

[Diesel Bill]

All though I don't have experience on this particular engine, My guess based on what you posted,(starts easily, runs good, no smoke, etc.) that it's the nature of the beast.
Just try to avoid that RPM when possible.

[Jd1]

I have Catalina 36 with the M35 engine and there is definitely a band of rpm where it shakes pretty darn good. My idle speed is a bit higher if my RPM meter is to be trusted and anything below about 1400 rpm shakes the boat. Once above that it is smooth as silk.

[Nineteen]

Hi
Being a 3 cylinder I think 850-900rpm will be the first harmonic range for that engine. There is nothing you can do other than operate the engine outside of that range.

Our 3JH3 has to idle just above 900rpm for that very range. If I have a cold start with a large load on the alternator I can see that it will cause the engine rpm to droop below 900 and she will shake unil I nudge up the throttle slightly.

Nature of the beast.

Cheers Tom

[Gary Matthesen]

When I first got my 1987 Catalina 36 with a universal 21 I cautiously left the unfamiliar harbor at slow speed. My heart sank as I suddenly had a great deal of vibration, tea kettle shaking on the stove, none of the crew said anything but Im sure they thought, poor guy bought a lemon.
Now after 4 years of owning it, as was said, slow down or speed up and everything is fine. It loves 2800 RPM and pushes me nicely at 6k+ with a clean bottom.

[Cadence]

It sounds like a harmonic but I would revisit the engine mounts.

It doesn't seem it should be transmitted to be noticeable.

[ValiantV]

Originally Posted by Cadence

It sounds like a harmonic but I would revisit the engine mounts.

It doesn't seem it should be transmitted to be noticeable.

One of the mounts was loose (upper nut backed itself off) so I thought that might be it. Shudder still there once it was tightened.

I guess I'll check the other mounts and may as well check the alignment while I'm at it.

I would think that if mounts did anything, they would harden with age. Is that a valid guess? The engine doesn't appear to move, but the vibration/shudder is very evident in the cockpit and through the whole boat. (Yes, teakettle rattling)

I guess I'll check all possibilities, talk to other C20 owners that have the M3-20 engine and see what their experiences are.

Thanks all!

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[ValiantV]

That, and once I verify my tach is in the ballpark, make sure the idle speed is set correctly.

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[Perna]

Softer engine mounts should do the trick.

A 3 cylinder has a first order rotating imbalance and a nodding moment in the plane of the cylinders, so they like to vibrate more than say a 4 cylinder or 6 cylinder. Every engine has a natural frequency of vibration of its mass on the mounts and the stiffness of the mounts should be chosen so that this natural frequency lies well below the idle speed, (theory says at 70% of this speed).

What it boils down to is that more flexible mounts will lower the frequency below the idle speed so the engine will only vibrate during stopping and starting as it should.

It's tricky because the mounts have to be flexible but strong enough to support the engine.

That's the theory anyway

Good luck

[Cheechako]

I have experience with that engine. I managed some C30 charter boats for a while. Nice smooth little engines. I wonder if one cylinder has an injector that needs reworked? I don't remember any bad vibrations from that engine. There's always a little at some low rpm, but not like some 1 or 2 cyl engines!If it has the original mounts it should be fine.
However, if there are no other problems and it only does it at that rpm, it very well could be just a harmonic thing as mentioned.

[Cadence]

Originally Posted by ValiantV

One of the mounts was loose (upper nut backed itself off) so I thought that might be it. Shudder still there once it was tightened.

I guess I'll check the other mounts and may as well check the alignment while I'm at it.

I would think that if mounts did anything, they would harden with age. Is that a valid guess? The engine doesn't appear to move, but the vibration/shudder is very evident in the cockpit and through the whole boat. (Yes, teakettle rattling)

I guess I'll check all possibilities, talk to other C20 owners that have the M3-20 engine and see what their experiences are.

Thanks all!

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The durometer of rubber increases with age. You are looking at 25 yr. old mounts +/-. You may need to replace them or live with it and avoid that RPM. I believe I would live with it.

[ValiantV]

Originally Posted by Cadence

The durometer of rubber increases with age. You are looking at 25 yr. old mounts +/-. You may need to replace them or live with it and avoid that RPM. I believe I would live with it.

Yes, I believe that (after checking for any other reasons) I will live with it for now. Prolly get around to changing out the engine mounts eventually if that's what it takes. Don't wanna have the old girl shaking herself to death and putting g undue stress on anything.

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[daveNZ]

I have just changed engine mounts on a Lombardini 1404M 40hp. It used to shake at 800 to 950, but perfect at 750, or at 1000 and higher. I could not pin it down to any one cylinder, or injector, or cylinder. Looked at fuel pressure etc. Asked every mechanic that would listen. Frustrated for 4 years. Changed (from Mackay ?? MD400 hard as hell) to Trelleborg Cushyfloat mounts and it is like a different engine. On first trial there is no more shuddering, cupboard rattling etc. Cant believe how much difference they have made. Hope that helps.

[Quadrille in JB]

The M30 on our 34’ O’Day definitely vibrates more at lower rpm. I replaced the fuel injectors and re-aligned the drive train after installing a new transmission and vibration has been reduced significantly. 3 new injectors from Ebay were $109 usd total for a Kubota 245 tractor.