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Old 30-06-2019, 09:54   #1
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Engine very rough at low rpms - why?

I have a cat with two 29hp Nanni (marinized Kubota) engines with about 2500 hours on each. Both have run very smoothly until recently when the starboard has started running rough at low rpms. Both engines idle at about 750rpm.

The starboard engine smooths out at around 1100 rpm. The starboard engine in all other ways performs like the port. Any ideas of what’s wrong and/or how to troubleshoot it?
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Old 30-06-2019, 19:01   #2
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Re: Engine very rough at low rpms - why?

Normally that's a fuel problem.
1) Clogged filter? water in fuel?
2) air leaks?
3) governor/throttle linkages lubed & working freely?
4) Injectors ok?

Start with easiest first. Check fuel filter. If you have a squeeze bulb in your system wrap joints in tissue paper & pump it up as hard as you can & check for leaks.
Check your governor & throttle linkages & lube them.
Then you can pull injectors & check spray pattern.
Hope that helps
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Old 30-06-2019, 20:13   #3
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Re: Engine very rough at low rpms - why?

Yes it does. Thanks.

Racor looks good - no water or sediment - but I’ll change it anyway. No leaks. Not sure why filter issue would not just get worse with higher rpm.

Have never pulled injectors. How do You check spray pattern when they are pulled?
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Old 30-06-2019, 20:20   #4
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Re: Engine very rough at low rpms - why?

Try some injector cleaner... I have used “sea foam” in past. Pour into primary filter at least half the bottle.

Good luck
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Old 30-06-2019, 20:45   #5
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Re: Engine very rough at low rpms - why?

If there are fuel injected trucks where you are, there's probably a shop that does injectors. You might just take them in and have them checked. It's not usually terribly expensive. And if they need servicing, they'll probably be able to do it for you.

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Old 30-06-2019, 21:05   #6
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Re: Engine very rough at low rpms - why?

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Originally Posted by Ostinato View Post
Yes it does. Thanks.

Racor looks good - no water or sediment - but I’ll change it anyway. No leaks. Not sure why filter issue would not just get worse with higher rpm.

Have never pulled injectors. How do You check spray pattern when they are pulled?
Engines dont always work in logical ways re fuel filter only affecting idle tho I wouldnt bother changing filter if it looks ok. I'd be more suspecting air. You need a workshop manual really to remove & check injector spray pattern as different manufacturers have different injector spray patterns. I SUSPECT but dont KNOW that yr Kubota has a very similar injector to a Yanmar. If so look at this vid posted by another CF user to see what it looks like. The spray pattern show in vid is ok. You dont want a solid stream or spraying out to one side.
You may find a Kubota injector removal & test vid on youtube
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Old 30-06-2019, 21:57   #7
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Re: Engine very rough at low rpms - why?

Did you check the on engine fuel filter, and the air cleaner as well.
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Old 09-07-2019, 10:39   #8
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Re: Engine very rough at low rpms - why?

I have change out the fuel filters and tried Sea Foam. No luck. I have now ordered a new set of injectors. They are fairly inexpensive. Hopefully the old ones will not be too hard to get out for my go at changing injectors.
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Old 09-07-2019, 11:57   #9
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Re: Engine very rough at low rpms - why?

Quote:
Originally Posted by Ostinato View Post
I have change out the fuel filters and tried Sea Foam. No luck. I have now ordered a new set of injectors. They are fairly inexpensive. Hopefully the old ones will not be too hard to get out for my go at changing injectors.
You should have the injectors rebuilt. Pensacola injectors did mine and it was very cost effective.
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Old 09-07-2019, 12:46   #10
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Re: Engine very rough at low rpms - why?

I will have the old one rebuilt for the second engine. I’m in transit and don’t want to sit around waiting for them plus logistics withOut a car is difficult.
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Old 09-07-2019, 13:23   #11
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Re: Engine very rough at low rpms - why?

How long has it been since you adjusted the valves?
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Old 09-07-2019, 14:14   #12
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Re: Engine very rough at low rpms - why?

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.
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Old 09-07-2019, 16:37   #13
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Re: Engine very rough at low rpms - why?

It's possible one of the plungers in the injector pump is sticking
I'm dealing with a big straight six John Deere with this issue right now. # 1 and #2 work at high revs but stick at lower revs. You can try this: Mix a jug 50/50 of injector cleaner and diesel, feed it to the pump and shut it off. Let it sit overnight. Then run on normal fuel and give it a good hard run, get the engine hot. Might clear it.
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Old 09-07-2019, 17:03   #14
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Re: Engine very rough at low rpms - why?

You can of course, just swap the injectors with the port side engine. If the symptoms move to the port engine, then you know one (or more) injector is the problem.

If not, then keep looking for other issues on the starboard engine.
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Old 18-07-2019, 04:50   #15
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Re: Engine very rough at low rpms - why?

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.
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