Engine very rough at low rpms - why?

[Ostinato]

Originally Posted by Ostinato

I had a mechanic from Hinkley at Savannah GA on board. He said if was an injector there should be smoke but there isn't any. If it was valves he would expect the engine to start hard but it starts right up like it always has. We both heard a low rhythmic sound that I had not heard before. The sound appears almost instantly and disappears just as fast when increasing the engines speed.


Best guess now is that it is the dampening plate though I have never heard a bad one sound quite like this. These are a new type of plate for me with rubber bushings instead of metal springs so that may explain my lack of experience with it.

There are two things that bother me about this analysis. First is that the vibration is not felt at the starting rpm of 750. It begins at 850 rpm.



Second is the amount of vibration. When dampening plates have gone on me in the past there is noise but not this kind of vibration (or no noise or drive when it falls apart). Admittedly my experience is mainly with the spring loaded type.



Any thoughts?

[FPNC]

Unless these are very old Kubota's, 750 RPM is a low idle speed. Check the recommended idle for your particular engine and measure at the crank with a photo tach. Newer Kubota's idle at 850-950 depending on the model.

At 2500 hours you may need a valve adjustment. Scheduled check is every 800 hours but I have seen people go much longer. These engines still start pretty quick even with the valves out a bit.

Noise/vibration in gear could definitely be a drive plate. Not so much in neutral.

You may have a prop issue, dive and check. Even a small wrap of line can sound and feel hinky at different RPMS.

[a64pilot]

Originally Posted by a64pilot

How long has it been since you adjusted the valves?

I’d suspect as this wasn’t answered you either didn’t see it or don’t know, and it may be that they never have been adjusted.
A way to go about trouble shooting if the cause is unknown is to start eliminating possibilities, I like to start with easy and cheap first, and valve adjusting is free just costs a little time. And even if that’s not it, it’s good to check adjustment anyway.
If they are in spec it’s just a few minutes with a feeler gauge is all. But no money.

Also varnished up injectors that need cleaning very often show up as a rough lumpy idle and even a cold type knock that doesn’t go away, I used to know when my old Mercedes injectors were getting dirty cause it would knock like it was cold, when it was warmed up.
That was only a tiny bit dirty, cause a can of transmission fluid in the fuel would clear it up.

[Ostinato]

Originally Posted by a64pilot

A way to go about trouble shooting if the cause is unknown is to start eliminating possibilities, I like to start with easy and cheap first, and valve adjusting is free just costs a little time. And even if that’s not it, it’s good to check adjustment anyway.
If they are in spec it’s just a few minutes with a feeler gauge is all. But no money.

I appreciate your thoughts. I will deal with the valves in due course but at this point I would rather pursue a strategy which takes the symptoms and matches them with the most logical causes.

I recognize that my description of what this engine is doing can never adequately convey what someone standing next to it would experience. Unfortunately anyone trying to help me is hindered by this limitation. But I appreciate everyone's effort to help work through this problem and all suggestions useful even I don't pursue them immediately.

[wingssail]

Removing injectors:

Had some trouble getting an injector out after removing the bolts.

Cranked the motor and it came right out.

Recommend putting a towel over it (folded a few times) to prevent a dent in the overhead.

[s/v Jedi]

Check the engine mounts. Are they tight, not cracked and the rubber not old and compressed too much? Compare with other engine.

[Compass790]

Originally Posted by Ostinato

There are two things that bother me about this analysis. First is that the vibration is not felt at the starting rpm of 750. It begins at 850 rpm.



Second is the amount of vibration. When dampening plates have gone on me in the past there is noise but not this kind of vibration (or no noise or drive when it falls apart). Admittedly my experience is mainly with the spring loaded type.



Any thoughts?

When you said in OP it was running rough I assumed it was misfiring but now I see in your further explanation it's a vibration problem. Is that right?
Can you post a video or a link to one so we can try & listen for ourselves? It may be helpful to run it up & down & either side of the problem rpms.

[Compass790]

Forgot to add, take a video of good engine as well please, both engines in neutral & both in gear, if you can.

[Ostinato]

Originally Posted by Compass790

When you said in OP it was running rough I assumed it was misfiring but now I see in your further explanation it's a vibration problem. Is that right?
Can you post a video or a link to one so we can try & listen for ourselves? It may be helpful to run it up & down & either side of the problem rpms.

Great idea. Here are two links to video clips.


Starboard engine - https://1drv.ms/v/s!AivgDPPoXsHVgv4PSheUvKwT5y3WBQ

Port engine - https://1drv.ms/v/s!AivgDPPoXsHVgv4QixMZLDHwH4riDg

The issue is with the starboard engine. The final 15 secs of this clip will be most valuable I think because the engine rpm drops from about 1100 to 900 or so where the vibration is worse to to idle. Essentially it goes from very little to a lot to less. Sorry for this reverse presentation it is just the best segment that illustrates the issue.

The port engine video provides a comparison and does not exhibit the same degree of vibration.

Both engines are in neutral during these clips.

Both engines also exhibit a knocking sound that louder than other noises that I believe are the dampening plates. I put a stethoscope type device on the starboard and most the noise you hear when it is vibrating at its peak is coming from the transmission area.

[Shawn67]

Originally Posted by boatbod

Before you go crazy pulling all injectors I suggest checking to see which cylinder is misfiring. While running at idle, loosen the nut on the high pressure lines to injector #1 and see if rpm drops. Tighten and then repeat with injector #2 and so forth. When you find the cylinder with no change in rpm you have found the one with the problem.

Seconding this suggestion of loosening the fuel line to each injector one at a time to see what change happens with the engine. It should obviously shake much more when you open a firing injector but if you open one of them and there is no change the injector probably is sticking shut.

I had this problem sporadically on my Volvo MD7A. Occasionally when I went to leave the dock the engine just had no power and wouldn't throttle up. The volvo had such a heavy block and flywheel at idle it was hard to tell only one cylinder was firing. Sometimes when throttle up the injector would unstick and then run right. You might have something like this happened.

It also stuck partially open one time. That was easy to see as it was smoking like crazy and then unstuck and was OK.

After I finally figured out what was going on (thought it was an air leak to an injector) I ended up pulling the injectors, opening them up and then cleaning the seats with bamboo skewers along with very lightly polishing the needle valves bodies on a 4000 grit hone. After that they were fine. I read somewhere this could be caused by salts in the diesel fuel but don't know if that is accurate. Also had problems with my HPFP which could have been related if the main piston was freezing.

Shawn

[DeepFrz]

It seems obvious to me, although that might not mean anything, that the engine is not firing on one of it's cylinders and then all of a sudden it is. Shawn's post above, I think, has the answer.

[Wotname]

Swap injectors between the engines for fault analysis.

[Compass790]

Originally Posted by DeepFrz

It seems obvious to me, although that might not mean anything, that the engine is not firing on one of it's cylinders and then all of a sudden it is. Shawn's post above, I think, has the answer.

Agree with this
& solution is as in boatbods post #12. first isolate problem injector at the revs it's knocking. It may be somehting else but this should be your next step

[galynd]

Not highjacking but having same problem with yanmar 3gm30f. Perhaps my info can help. As I accelerate in forward, motor often shakes profusely at about 1400 rpms, then settles. Shaft often knocks on shaft tube. It also starts roughly unless I start at higher rpm.

What I've done thus far with no success:
Aligned shaft, replaced one motor mount, replaced cutlass bearing, had prop checked out, adjusted valves, increased idle to 1000.

I guess injector cleaner then injector removal next.

Maybe we can help each other. Any suggestions appreciated also.

[Ostinato]

It’ll be a week before I can get to the injectors on mine. I’ll let everyone what happens. Cautiously hopeful at this point