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Old 30-03-2009, 15:05   #1
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Fouled Injectors > Low RPM & Smoke?

Hi folks (PMd Never Monday as well, as he ran the prop #s for me last year)

Wrote last year about being overpropped. The prop calc recommended about a 14x8.5 or 15x 7.5 prop, (my current 3 blade fixed is 15" x 10")

With my 2gm, I can hit 3600 in neutral, but only about 2000-2200 rpm in gear (clean bottom and prop), before lots of smoke and no increase in speed.
Not lacking speed as I run around 6kts at 2000rpm or so, and hull speed is not much more than that.

Took prop to an experienced guy today to have it depitched a couple of inches, or -1" diameeter and -1" pitch, or so.
He said that he could do that, but didn't think the a couple inches would make that much difference- he was really concerned that I only rev so low now.

Question: Could bad injectors (plugged, bad spray, etc) cause me that much smoke, and inability to rev higher (combined with likely being somewhat overpropped as well)?

Don't mind paying to have the prop depitched some, but wonder if there are other issues as well. Hate to depitch too much, if it could be the injectors bad, possibly giving similar symptoms.

I don't mean to beat a dead horse, but do not want to splash the boat, only to find I could have done something different. I will get the injectors checked anyway, but have to decide on how much to modify the prop ASAP.
I am looking to purchase a used feathering / adjustable pitch prop, which would be great - having the ability to adjust in water. (not sure if deal will go through yet)


My trannie is a 1982 km2a (not p). I believe it is 2.62:1. He told me to verify that, as if it were, say 2:1, that may explain things a bit. Not at the boat till the weekend, so will check then.


Thanks, in advance, for your time, and patience!!
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Old 30-03-2009, 15:28   #2
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What size boat do you have? 6knts at 2000rpm on a little 18hp GM2 is alot. If you're hitting 3600rpm in neutral and can't get past 2000-2200 in gear without black smoke my guess, without being there, is the prop.
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Old 30-03-2009, 17:10   #3
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I would say overpropped. It would be a shame to loose the diameter, How much is a new prop in the right pitch?
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Old 30-03-2009, 18:39   #4
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Tellie - the boat is 30'LOA, 25'7"LWL, disp 8600lbs(book) 10,000lbs (crane).
I too felt that I was hitting the speeds of 5-6kts quite quickly, at too low of rpms, and therefore must be overpitched. The smoke seemed to confirm this, when trying to push past 6 kts.
I likely will still get 2" of pitch taken off the prop, and see what happens.
An experienced member mentioned that low compression could be causing the smoke.
If the boat performs much better with less pitch, and I can hit near max rpms without smoke, I'll be happy, If I can't reach max rpms without smoking, it will not be the pitch at that point, so then it may be time for a compression test. It starts very easily, so I had never realy questioned / checked the compression.

Cheech - I can have my prop taken down two inches in pitch, and none in diameter, if I choose. The experienced prop guy said that removing diameter would not do too much anyway, as it is has fairly narrow blades, more pointed, than fattly rounded, at the tips. There is not alot of surface area in the last 1/2 inch!

I don't mean to waste members time, as I have received alot of good input in the fall, when I wrote about being overpitched.
I just was taken back a bit today, when the experienced prop guy figured there must be something more than pitch / overpropped, to account for smoke after only 2200rpm, or so. He figures reomving two inches of pitch will only buy me a couple - a few hundred rpm, and that's still about 900 short of max rpms 3600 at WOT!

Regardless, I will use the boat this summer, as the engine worked fine last year - likely didn't put more than 60 hours on her last year, and then decide if I need to do more next year.

Thanks!!
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Old 31-03-2009, 08:40   #5
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Did the engine blow black smoke since you first got it, or did this develope over time? If engine ran good once, problem should be other than prop. Yes, fouled injectors can result in lose of power and unburnt fuel (Black smoke). Injectors are relatively cheap ($80 new, $40 reconditioned), and getting them seviced is never a bad choice anyway. Also wondering if there would be some carbon buildup inside pre-combustion chamber. Another thought would be to remove the head and get it cleaned (again relatively cheap, and they will do injectors at the same time).
Anyway, I am not a mechanic, only my thoughts on a possible solution, good luck.
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Old 31-03-2009, 08:52   #6
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Exo - I have only owned it for 2 years (the engine is 1982), so it may have had this propeller, and possible symptoms for years, who knows. It does not smoke at all up to 2000 -2200 and seems to have lots of power - in fact it drives the boat to near hull speed at these low rpms.

I think that the previous owners may have been quite happy motoring in / out of harbour, to go sailing, and engine / prop worked fine for this.

I would like to have the engine work as well as possible, and that's why I am trying to make sure it is not overpropped, etc.

I will likely have thye injectors checked / cleaned / refurbished, or buy new, if not much more, just to make sure thye are woring well.

I will not remove the head, or go that deep now, until I have tried changing the prop, and cleaning the injectors.

I will do a compression test soon, or if prop changes and injector cleaning does not allow me to rev near max rpms.

As mentioned, it has likely worked this way for quite some time, and allows me to motor OK, but I am just trying to make the engine as happy as possible, especially in case I get into big headwinds and chop.
We have been out a couple times motoring directly into 25kt headwinds, and 8ft chop, and have been down to 1 1/2 kt boat speed or so. Hoping to work the engine less, with less prop and higher rpms.

Appreciate the advice!!
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Old 31-03-2009, 09:25   #7
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To me it sounds like your definetly overpitched. I would keep the same diameter and just repitch. I have an adjustable pitch propellor and have played with it a fair amount when I first installed it. It was overpitched on first installation and the engine had all the same symptoms as yours and when I dialed into the right pitch all was fine.
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Old 31-03-2009, 10:20   #8
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Ditto on what "Jade" just said.
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Old 31-03-2009, 10:51   #9
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Jade - thanks for advice. Roughly how much rpm change did you get, when changing your pitch, an ince or two, for example??
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Old 31-03-2009, 12:21   #10
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Well to take things in a completely different direction. I would check your exhaust elbow. If it is carbon clogged(coked up) you will get these symptoms as well. This is quite common on Yanmars. I strongly doubt that your prop became suddenly over pitched.

You seem to be trying to get your boat to go much faster and as I'm sure you know it will only go so fast no matter how much you add power. A 25.5 ft displacement hull is only going to do 5-5.5knts+/- no matter what you do.
You cannot fool with physics.

I just rebuilt a 31yo Yanmar ysm8 and found some truly amazing things about that little engine. Firstly it was only carrying about 180lbs compression. Too low to start easily but with a bit of ether away it went and although a bit sooty it would run all day long. It had a burned exhaust valve, bad rings and a worn cylinder. A piston wrist pin that was .008 out of true and a few other small items. Yanmars will run when most other engines will refuse to go making us think they do not have a problem when they actually might. Denial

The 31yo injector was worn but making a good spray according to the diesel shop here by the house. I replaced it anyway but the old one goes in the spares box just in case. The pre chamber was damaged by me when I took the head in for the new valves and guides(plus an exhaust valve seat) so it got replaced too. Even a light coating of pencil eraser sized barnicles on the prop would make the engine bog down and create a soot trail in the water and on the hull.

I would check out the exhaust first and replace if required. Even on my worn out engine(before rebuild) this helped somewhat. Check your compression with a diesel engine compression tester, not one for a gas engine or you'll burst it. 300 to 600lbs is not unusual for a diesel although I do not know exactly what a Yanmar is supossed to carry.

1982 was a long time ago for any diesel but for one on the water its really old. They take a lot more punishment than say your lawn tractor of the same age. Most get less maintenance than they should and a marine environment is a tough place to live.

I hate to say it but your engine may be in need of some more in depth repair and I think you are starting at the wrong end.............m
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Old 31-03-2009, 13:14   #11
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Cantx- thanks for the input - my answers remarks are in CAPS!!

Well to take things in a completely different direction. I would check your exhaust elbow. If it is carbon clogged(coked up) you will get these symptoms as well. This is quite common on Yanmars.

-- HAD THE ELBOW OFF LAST YEAR - CLEAN AS A WHISTLE. SOAKED IT IN LIGHT ACID JUST IN CASE!!!

I strongly doubt that your prop became suddenly over pitched.

--AGREE WITH YOU - i DON'T THINK IT HAS SUDDENLY BECOME OVERPITCHED, i THINK IT'S BEEN THAT WAY FOR A LONG TIME, LIKE FROM WHEN THAT PROPELLER WAS PUT ON (WHO KNOWS HOW LONG AGO!!. BEEN LIKE THAT SINCE I BOUGHT IT 2 YEARS AGO.)

You seem to be trying to get your boat to go much faster and as I'm sure you know it will only go so fast no matter how much you add power. A 25.5 ft displacement hull is only going to do 5-5.5knts+/- no matter what you do.
You cannot fool with physics.

--AGREE WITH YOU!! NOT REALLY TRYING TO DO MORE THAN 5.5 - 6KTS. JUST TRYING TO FOLLOW ADVICE OF OTHERS, AND GET THERE AT HIGHER RPMS, LIKE BEING ABLE TO COME CLOSE TO GETTING THE 3600 MAX RPMS, THAT I CAN GET IN NEUTRAL. BELIEVE THIS IS BETTER FOR ENGINE.


I just rebuilt a 31yo Yanmar ysm8 and found some truly amazing things about that little engine. Firstly it was only carrying about 180lbs compression. Too low to start easily but with a bit of ether away it went and although a bit sooty it would run all day long. It had a burned exhaust valve, bad rings and a worn cylinder. A piston wrist pin that was .008 out of true and a few other small items. Yanmars will run when most other engines will refuse to go making us think they do not have a problem when they actually might. Denial

The 31yo injector was worn but making a good spray according to the diesel shop here by the house. I replaced it anyway but the old one goes in the spares box just in case. The pre chamber was damaged by me when I took the head in for the new valves and guides(plus an exhaust valve seat) so it got replaced too. Even a light coating of pencil eraser sized barnicles on the prop would make the engine bog down and create a soot trail in the water and on the hull.

--HULL AND PROP WERE COMPLETELY CLEAN WHEN TESTED. NEW ANTIFOUL ON SANDED BOTTOM.

I would check out the exhaust first and replace if required. Even on my worn out engine(before rebuild) this helped somewhat. Check your compression with a diesel engine compression tester, not one for a gas engine or you'll burst it. 300 to 600lbs is not unusual for a diesel although I do not know exactly what a Yanmar is supossed to carry.

-- WILL TRY TO DO A COMPRESSION TEST, IF I CAN GET A TESTER.

1982 was a long time ago for any diesel but for one on the water its really old. They take a lot more punishment than say your lawn tractor of the same age. Most get less maintenance than they should and a marine environment is a tough place to live.

I hate to say it but your engine may be in need of some more in depth repair and I think you are starting at the wrong end.............m

--ENGINE REALLY DOES WORK WELL, STARTS EASILY, RUNS WELL - NO SMOKE, OTHER THAN WHEN TRYING TO REV MORE THAN 2200 RPM OR SO.
HAVE HAD SEVERAL EXPERIENCED FRIENDS WHO OWNED / SERVICED OWN HEAVY DIESEL EQUIPMENT AND DIESEL TRACTOR TRAILERS FOR 20+ YEARS ON THE BOAT - THEY SAID THE ENGINE WORKED AND SOUNDED GREAT.
- AS MENTIONED ABOVE, MY ONLY VISIBLE ISSUE IS NOT REACHING MAX RPMS.
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I DO APPRECIATE ALL OF THE ADVICE!! tHANKS!
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Old 31-03-2009, 13:31   #12
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The experienced prop guy said that removing diameter would not do too much anyway, as it is has fairly narrow blades, more pointed, than fattly rounded, at the tips. There is not a lot of surface area in the last 1/2 inch!
That means you don't have a power boat prop on the boat. They have mickey mouse ear looking blades. I had that where the diameter and pitch were right but the prop type was wrong. It made for a very over propped boat. had been that way since it was launched. I noticed it at low RPMS. I could not idle as low as I should have been able to not rev as high. Switching out the prop actually got me just a tad more speed but the engine ran much better and got higher rpms. If you have the numbers on the engine and the transmission the prop guy can compute the proper prop dimensions. It is still pretty scientific by the numbers and not just a wild guess. If it's wrong making it right with a re pitch is not that much. I think 3 inches is the max they can do. In my case I had to buy a new 3 blade - not cheap.

Once thing you could have is build up in the exhaust manifold. Carbon can build up there and knock the top off your RPMs. I had that too. It's not an uncommon problem with sail boat engines. To pull the manifold will require a new seal. They are not dirt cheap. It was suggested to me as being a cheaper route to finding out. just guessing isn't really going to get you answers.
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Old 31-03-2009, 14:14   #13
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Hi Paul - Below is a picture of the prop on the boat. The prop guy said it looked like a sailboat prop, but not quite like the ones he was used to. They do sailboats, but mainly fishing boats, and larger props.

Does it look like a typical sailboat prop???

Hadn't thought of the exhasut manifold. Will price a gasket, and have a look at my bolts, to see how bad they look. Have torches, so I can heat the bolts, to come out easier, if necessary.
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Old 31-03-2009, 14:44   #14
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Does it look like a typical sailboat prop???
Yes, it clearly is. Power boat props have very round blades at the tip. As iI say "Mickey Mouse Ears".

Youi might want to get some help. Folks that work engines a lot can tell a lot more than you can describe to people here. Troubleshooting by email is really hard. We all can only make a best guess of possible things and a lot of things could be possible. At least ask around if you are not sure who can get you help at the boat.

The guy that fixed my exhaust manifold also missed the problem for a while too but when it got bad he knew right away. That's the way it can go where it needs to be really a big problem to be easily noticed. BTW, they had to send the manifold out to be machined. Tearing into it blind can get expensive.

All the best of luck.
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Old 31-03-2009, 15:51   #15
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I couldn't tell how much I was actually changing it as I would dive on the prop and turn a bolt to adjust the pitch. But I can tell you that I had another sailboat that I had put a brand new 3 blade Michigan Prop. After one season it seemed as though it was overpitched and I wanted it changed. When I brought it to the prop shop they discovered that all three blades were pitched differently ranging about 1.5 inches. He recommended resetting it to what it was supposed to be. That did the trick.
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