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Old 31-03-2009, 19:37   #16
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Again, 6knts at 2000rpm, and very close to hull speed. It is unlikely you have a compression, injector or carbon build up problem. If you did it would show up sooner under that load. In the pic you posted it looks like a pretty healthy prop for a 2GM. I don't know what hp your making at 2000rpm but it's not the rated 18hp. So you're making 6knts at about maybe 12-14hp, not counting the alternators energy requirement. That's pretty impressive for a two cylinder GM2. I have two Yanmar 2GMs on a 38' Cat and I can't get 6knts with them both at 2000rpms. Again as others said, no one can make a real diagnosis on line. But my money's still on the prop.
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Old 31-03-2009, 20:11   #17
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I have reviewed the posts/opinions...here is mine.

If you achieve near hull speed(6.78 is theortetical hull speed) at 2000 RPM
and
you cannot get above 2500 without black smoke
the vessel is over propped.

As Tellie said, it is unlikely that you have a compression problem.
There may be some carbon buildup on the injectors and in the exhaust
but I would not do anything to the eine untill you get a prop with less pitch mounted.

As far as a compression test goes....unless you do this for a living...you probably do not want to invest in a diesel compression tester kit.
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Old 01-04-2009, 05:09   #18
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Tellie / Chief - Thanks for the input! I tend to agree with you guys, and think the engine is running pretty strong.

Dropped the prop off this morning to have 1" diameter, and 1" pitch taken off. I had thought about having more taken off, but this will make it 14x9", which is close to the recommended size/ pitch. The prop guy didn't

It may not improve it as much as I like, but shoudl be better than last year. Also keeping an eye out for a used folding / feathering prop.


Will report the findings in a couple months, once she is splashed.

Thanks again to all who took the time to help!
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Old 01-04-2009, 07:15   #19
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Simple / Highest Probability Things First

Main symptom is black smoke and inability to reach rated RPMs under load. No black smoke or other indications of an engine problem at lower speeds and at idle. Neutral revs are fine.

So the engine is heavily loaded. You only get that way from what it is being asked to drive. All signs point to the prop - especially since you've already had a look at the exhaust elbow. If it was bad injectors you'd likely see that under all conditions.

Re-pitch is what I'd do as you have decided. The rest of the engine check-this, check-that suggestions can easily be done waterborne.

This is like deja-vu all over again... as you know. I went through the exact same sequence on my last boat, checked the exhaust elbow & turbocharger, yadda yadda. Prop pitch was way too much. Fixed that and all was well.

Good luck and be sure to tell us how it comes out.

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Old 01-04-2009, 08:18   #20
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I had the same symptoms as you on a 3QM30. Repitched the prop from 18X13 to 18X11, it helped but didn't solve the problem. Two years later the head gasket blew and has bad since I bought the boat 13 years prior. Now the problem is solved. Is your oil on the dip stick black?? mine was. Not any more. Good Luck, and may you find the problem.
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Old 01-04-2009, 08:27   #21
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oil is definitely black, but I thought that was pretty normal!!!

Firgot to mention, it is raw water cooled, so it only operates at around 130 degrees, so the salt doesn't wreck the engine. This low operating temp is likley not good for efficiency, but that's the way it was designed 25 years ago!
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Old 01-04-2009, 08:28   #22
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AEG - I would have thought your oil would be milky, from water, if the head gasket was bad.
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Old 01-04-2009, 08:42   #23
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The exhaust was going into the crankcase and going out the exhaust and putting soot all over the stern. Mine is raw water cooled also. I thought the black oil was normal too. But it isn't black any longer. Mine used to hammer alot, thought that was normal also, it runs a lot quieter now. More RPM's than before the head gasket was changed.
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Old 04-04-2009, 07:10   #24
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Be careful with the numbers, the current prop 15x10 3 blade is close if you are using a 2.62:1 version of the KM2A, it's within an inch of my calculations. This would indicate to me a problem with the engine or maybe the shaft as you would not be overpropped enough to give you the problems you describe.

Does the propeller shaft spin freely out of gear?

The KM2A was also available in 2.21:1 and 3.22:1 ratios so make sure people are using the right numbers to calculate, further are you sure it is a 15x10 propeller that's fitted, it may have been repitched and not remarked to show the new size.

Before getting into engine mechanicals, I would make sure the exhast has no restriction, the fuel flow to the engine is good and not restricted, I don't just mean clean filters, but the fuel pick-up in the tank is clear, there are no restrictions in the fuel hoses, the fuel pump is to specification and the fuel is fresh and clean.

This can take time and be frustrating, but be methodical. You can certainly check a lot of things before calling in a preofessional. A quick check on your fuel system would be to get a gallon of fresh diesel and plumb it directly to your fuel pump with a new length of pipe, this will eliminate most of your fuel delivery system, including the tank and the fuel in it. You will need to allow the old fuel already in the pump and injector lines burn off before the fresh fuel is used before futher testing.

As the engine starts easily, there cannot be a lot wrong with it mechanically, but if the above does not work for you, then a compression and leakdown test is in order. This would also be a good time to try new injectors as they have to be removed to do these tests.
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Old 05-04-2009, 17:17   #25
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R.Furburough - thanks for the detailed reponse. I will answer the various ideas in CAPS.

Be careful with the numbers, the current prop 15x10 3 blade is close if you are using a 2.62:1 version of the KM2A, it's within an inch of my calculations. This would indicate to me a problem with the engine or maybe the shaft as you would not be overpropped enough to give you the problems you describe.

i WILL TRY AND REMEMBER TO CHECK THE GEARBOX TO SEE WHAT THE REDUCTION IS FOR SURE.

Does the propeller shaft spin freely out of gear?

YES, SPINS FREELY OUT OF GEAR!!

The KM2A was also available in 2.21:1 and 3.22:1 ratios so make sure people are using the right numbers to calculate, further are you sure it is a 15x10 propeller that's fitted, it may have been repitched and not remarked to show the new size.

PROP HAD NOT BEEN REPITCHED - WAS 15X10. i DID HAVE IT MACHINED / REPITCHED TO A 14X9 A FEW DAYS AGO.

Before getting into engine mechanicals, I would make sure the exhast has no restriction, the fuel flow to the engine is good and not restricted, I don't just mean clean filters, but the fuel pick-up in the tank is clear, there are no restrictions in the fuel hoses, the fuel pump is to specification and the fuel is fresh and clean.

WILL TRY AND CHECK THE EXHAUST FOR RESTRICTIONS. MOST OF THE EXHAUST HOSE WAS NEW 2 YEARS AGO. ALSO TOOK APART / CLEANED OUT RUBBER SILENCER AT THAT TIME. I DO NOT HAVE A WATER LIFT MUFFLER, JUST A RUBBER SILENCER, THEN A HIGH LOOP, BEFORE EXITING THE THROUGH-HULL.

RE: FUEL - STARTED LAST YEAR WITH BRAND NEW MOELLER DIESEL TANKS, (NEW PICKUP), NEW FUEL FILTERS. tHERE WAS NO SEDIMENT OR WATER IN BOWL LAST YEAR AT ALL.


This can take time and be frustrating, but be methodical. You can certainly check a lot of things before calling in a preofessional. A quick check on your fuel system would be to get a gallon of fresh diesel and plumb it directly to your fuel pump with a new length of pipe, this will eliminate most of your fuel delivery system, including the tank and the fuel in it. You will need to allow the old fuel already in the pump and injector lines burn off before the fresh fuel is used before futher testing.

As the engine starts easily, there cannot be a lot wrong with it mechanically, but if the above does not work for you, then a compression and leakdown test is in order. This would also be a good time to try new injectors as they have to be removed to do these tests.

THANKS AGAIN FOR THE ADVICE!!!
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Old 06-04-2009, 19:46   #26
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Have you checked the cooling elbow for obstruction?
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Old 08-04-2009, 04:47   #27
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Chief,

I took the elbow off (it was very clean / unobstructed) 2 years ago. Soaked it in Muriatic acid solution at that time, as I was running it through the block anyway.
Back then, I had been tryig a few things, as it had started to overheat once.

Ended up turning over the scored cover on the raw water pump, and it seems to pump much better now.
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Old 11-05-2009, 13:48   #28
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UPDATE: A combination of having 1" of diameter, and 1" of pitch taken off the prop, as well as being more forcefull in pushing the engine through some smoke (likely hasn't been revved up high in a while), has lead to success.

My tach, which may be off by a bit, now shows 3200 rpms in gear, at 6.5kts, compared to about 3400 in neutral. Last year it smoked hard at about 2200rpms, and I was afraid to push it much further, feeling that it was overpropped. I believe it partly just need a good cleaning out. It is still smoking very slightly (blue), at the 3200 rpms range (rings may be letting some oil by, as it is 25 years old) but it is running strong, and doing hull speed. The boat just went in the water a week ago, so I have only motored for a couple of hours.

It is definitley smoking way, way less at 3200rpm, than it did last year at 2200rpm.

(of course, had I pushed it more last year, it may have cleaned itself out, and smoked less somewhat, without the propeller change)

Thanks to everyone for all of the advice!! Definitely appreciated!!
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Old 11-05-2009, 13:51   #29
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If your fuel is old, it maybe smoking due to that. The fuel was tested in my old trawler when I bought it and it was only >60% combustible! Had it pumped, the smoke cleared out after some running with fresh fuel....
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Old 11-05-2009, 15:05   #30
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It is last year's fuel. However, last spring I put in a new diesel tank, so it should not be as bad as fuel that has sat in a 20 year, sludge filled tank.

I am hoping that it will smoke less with more use, new fuel, etc. Even if it continues to smoke, it is minimal compared to the amount last year.
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