Volvo Penta MD3b vibration

[Hortense]

My MD3b starts and runs fine but between 900 and 1400 rpm the vibration from the engine would rattle your teeth out! The engine vibrates the original stuffing box really badly and I’m worried about the shaft seal and beyond.
I was thinking of changing engine mounts - but are there any other points I should look at.
I thought injector timing might be an issue - but it starts so easily and can run at very low revs quite smoothly. The vibration calms down at 1000 revs and upwards to a top rev reading of about 2000rpm.
The vibration is worse when in gear but is still very evident when just revving the throttle.
Any advice would be much appreciated.
Steve

[NickScheuer]

Our V-P MD17G behaves similar. I just run it either faster of slower than the "vibration" RPM. We usually cruise at about 8 kts and dock at several hundred revs so I can shift gears without bothering with the throttle.

[Sunsetrider]

I have the same "un-sweet spot". I just try to avoid it.

[WillieC]

Take a real good look at your engine mounts. If you have the same as mine, they are fairly cheap OEM replacements. They are designed from the factory to absorb oil from the ever leaky VP engines and thus fail every thirty or so years. And then check shaft alignment. Once all that is done, and while you are at it take your injectors in for evaluation and calibration if needed, the engine will still rattle your teeth out. Our sweet spot is 1700 rpm when all the cabinets, dishes, coat hangers, etc. reach harmonic convergence. Motor on!

[Hortense]

SORTED!
upon close examination I found that both starboard engine mounts had snapped - the main bolt had sheared under the rubber and on levering that side of the engine the whole thing lifted. No wonder she was vibrating a bit!!!!
Now fitted 2 new mounts.

[WillieC]

Most excellent! Now keep oil and fuel off of them!

[WillieC]

Most excellent! Now keep oil and fuel off of them!

Take the time to figure out how to check your shaft alignment. Who knows when last it was adjusted and maybe it was done with worn, or busted mounts, or not at all. Mine was horrible. I was trepidatious at first since there is precious little info out there how to adjust these old beast since they use shims. But it isn't that big a deal after all. I made various shims out of scrap metal I had laying around, galvanized tinplate, old electrical boxes, fluorescent light fixtures... turns out when all was said and done, the original bronze shims of varying thicknesses were good enough. Just a bit tedious. I also installed a flexible RD Engineering Shaft Saver as I needed the extra length, which also gives a bit more tolerance.

My first suspicion of alignment issues before I even knew it had a name, was watching the shaft where it enters the tube under the floorboards while running. Mine is fitted with a PSS shaft seal, not relevant to alignment. The shaft would rise and fall almost a quarter inch (6mm) during each revolution. I thought, how amazing that the Swedes could get away with this! We are going to get along just fine! Then I did my homework.

Carry on!

[hetek]

"Rise and fall almost a quarter inch during each revolution"? Yikes!

I had hoped for the Swedish "acceptable tolerance" feature myself, but when my prop shaft builder said he was going to machine face my coupler and lap it and my prop to the shaft and dynamically balance the whole thing... It all now seems to be a bit more involved.

At least I'll be pretty sure it won't be the source of vibration if I have any.

[DesertAlbin736]

Probably not room for one in the engine bay with a VP MD3B, but of all the things I curse our PO about & think we paid too much, the AquaDrive they put in when they re-powered with the Yanmar is one thing I'm glad we have & sure smooths things out. We run at 2200 to 2350 RPM with 16X12 prop & it's very smooth and not much louder than driving a car. Things do shake a bit at low idle speeds in neutral under 1000 RPM, but smooth out at a 1000 RPM fast idle. Right to left in this photo, shaft coupling to thrust bearing, CV joint unit, coupling to tranny. As you can see, the smaller 24 HP 3GM30F is rather a small, short engine. I imagine this being a 1971 model boat the OEM engine was likely a AD21. That's why, interestingly, in these early boats the part of the engine box that's above the floor boards is shorter than those in later models, and the floor boards extend to the front edge of the starboard bench seat. Aft of the exhaust elbow the box is shorter in height & under the floorboard. In this shot you can also see the 20 gallon holding tank I put in a couple years ago where batteries used to be. In Desolation Sound, which is mostly no discharge, there are no pump out facilities north of Lund, and the one in Lund costs $10 CAD to use.

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