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post #1 of 26 Old 03-13-2020 Thread Starter
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Black smoke in exhaust while in gear > 2000RPM

Calling all mechanics for ideas

I have a 1990 Catalina 30 with a Universal M3-20 (18hp) engine. I discovered about 3 seasons ago if you run the engine in gear over 2000 rpm there is black smoke/soot/particulate matter in the exhaust. Since then I've only run the engine up to 2000 rpm and can make about 4kts in smooth conditions. I'd like to go faster!

Description of symptoms:
  • Running the engine in gear (both forward and reverse) and exceeding ~2000rpm results in black smoke & soot exiting with the exhaust (seems to be unburnt diesel)
  • To be clear revving the engine in neutral to any RPM (even in the 3000-3500 range) shows no signs of soot.
  • Soot can be observed both while underway and while the boat is securely tied to a dock and put into gear.
  • Unknown if this is a new problem or has always been a problem- noticed it 3 seasons ago.
  • Unknown what max RPMs are- went to 2500 but didn't test any further.

Problem overview:
Black smoke is caused by partially burned fuel. When the fuel/air mixture increases there is insufficient oxygen present in the cylinders to complete the combustion process. Large quantities of carbon are then produced which appear from the exhaust as minute black soot particles

Causes of problem:
Insufficient air:
  • Air intake filter clogged (we have no filter)
  • Air intake restricted (intake looks okay)
  • Exhaust restricted (Exhaust riser was replaced, muffler appears to be okay, can't inspect the exhaust hose exiting the transom but suspect this is not the problem)
  • Leaking inlet or exhaust valves (compression tests are excellent so don't think this is the problem)
  • Poor engine room ventilation (not the problem)

Excessive fuel:
  • Defective injectors (ours are new 4 seasons ago)
  • Incorrect injector nozzle (purchased the Westerbeke/Universal OEM)
  • Injector pump incorrectly set (this hasn't changed)
  • Low fuel grade (a possibility but we have two filters)

Engine overload: As load increases the governor senses the slight decrease in RPM and adjusts the injector pump to deliver more fuel. If the engine becomes overloaded then the increase in fuel does not increase RPM and no extra air is being sucked into the cylinders. Net result - soot:
  • Boat bottom dirty (not the problem as we're power washed & bottom painted)
  • Propeller dirty/fouled (not the problem as we've thoroughly cleaned the prop)
  • Engine alignment incorrect (a possibility- engine was rebuilt and installed. How to measure this?)
  • Propeller incorrect (all research points to 13" props with varying pitch for 18hp Universal M3-20... we have a 15" prop- 3 blade)
  • Transmission incorrect ratio (nothing changed here)
  • Stuffing box (?)

Options:
To me the next most obvious thing to test is a smaller prop. I am debating between this and bringing the boat to a mechanic. If the prop really is the problem though I'd love to "try" and fix before having to pay for a haul out (she's currently on the hard). Anyone have any opinions?

Anecdotal evidence pointing to prop size issues:
  • Catalina Direct recommends 13x7 prop for this engine
  • Prop tip to hull clearance should be 10-15% of prop width. For a 15" prop that means 1.5"-2.25" of clearance. I have 1/2"!
  • Surveyor on purchase mentioned the prop size seemed extremely large

Last edited by jaredrada; 03-13-2020 at 04:21 PM.
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post #2 of 26 Old 03-13-2020
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Re: Black smoke in exhaust while in gear > 2000RPM

Your clearance suggests to large a prop, which will over load the engine at elevated rpm ! An inch plus over size may not seem like a lot, but it increases surface area exponentially, if the pitch is not changed, to allow the engine to rev up under load !
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post #3 of 26 Old 03-13-2020
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Re: Black smoke in exhaust while in gear > 2000RPM

If your prop is too big you can't run the engine at the proper RPMs and are building up carbon (unburnt fuel) in the exhaust system, which will get blown out when the engine is revved up.
Any prop manufacturer's site will have a formula for figuring out what the proper prop should be. Time to spend some boat bucks now or plenty more later when your engine breaks.
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Re: Black smoke in exhaust while in gear > 2000RPM

Testing a 13” prop is a good idea

Another thing to check is that the oil has not be overfilled, but since it occurs under a load ( with the prop spinning ) I doubt that’s it

You said you looked at the exhaust riser....is that the same as the elbow on your boat. That’s a frequent cause with even a small resriction. Carbon buildup there causes a back pressure
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Re: Black smoke in exhaust while in gear > 2000RPM

Quote:
Originally Posted by capta View Post
If your prop is too big you can't run the engine at the proper RPMs and are building up carbon (unburnt fuel) in the exhaust system, which will get blown out when the engine is revved up.
Any prop manufacturer's site will have a formula for figuring out what the proper prop should be. Time to spend some boat bucks now or plenty more later when your engine breaks.
Cant seem to find any information from the manufacturer- its a Universal engine which is now Westerbeke. But most of my Google research reveals people are using 13" props with pitches from the 7 to 11inch range.

Quote:
Originally Posted by chef2sail View Post
Testing a 13” prop is a good idea

Another thing to check is that the oil has not be overfilled, but since it occurs under a load ( with the prop spinning ) I doubt that’s it

You said you looked at the exhaust riser....is that the same as the elbow on your boat. That’s a frequent cause with even a small resriction. Carbon buildup there causes a back pressure
Definitely not the oil I change it myself and regularly check the oil levels.

Yeah the riser and the mixing elbow are coupled via some pipes. I've removed the entire thing and had it rebuilt- it had very minor carbon build up: https://imgur.com/gallery/hcJepvB
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Re: Black smoke in exhaust while in gear > 2000RPM

Engine manufacturers don't know squat about props, but as I said, if you go on the prop manufacturer's websites they can get you there. But you are going to have to know about your boat, engine and the reduction gear of the trany, if any. Also you need to know the taper of the shaft, the size of the key way and shaft diameter to order a prop.

7 to 11 inches of pitch is a huge range! A prop shop can maybe tweek a prop an inch or two of pitch, so you'd best be a lot closer than 7 to 11 inches.

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Re: Black smoke in exhaust while in gear > 2000RPM

The OP certainly seems to have their bases covered. While many things on the original list are simply being dismissed, if you take it all for granted, the prop would seem to be most likely culprit. Despite the soot, have you tried to run it wide open throttle, while underway, to check max rpm? Do you know what your engine is rated for? That is a critical measurement, even to determine whether you have the next correct prop. Use a $20 laser tachometer to measure, not the cockpit tach.

The other things I don't recall being mentioned are the stuffing box and transmission. What kind of stuffing box? If traditional packed, is it dripping correctly or is it possibly crushed down too hard? Transmissions are more likely to get too loose than bind. Any chance it could have been replaced along the way with an incorrect gear reduction ratio? Just a stab in the dark.


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Re: Black smoke in exhaust while in gear > 2000RPM

One additional engine setting that you should check is the injection pump timing. If your engine is injecting fuel into the cylinder too late it may not have sufficient time to burn before the exhaust valve opens. There is a timing mark and a sight hole on the starboard side of the engine's bell housing.


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Re: Black smoke in exhaust while in gear > 2000RPM

Another random guess. Some injectors require crush washers and others do not. I heard of poor fuel burn if these are accidentally switched, either one that requires it being installed without or more often one that does not require it, being installed with a washer. Either causing improper burn chamber coverage.


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Re: Black smoke in exhaust while in gear > 2000RPM

Quote:
Originally Posted by capta View Post
Engine manufacturers don't know squat about props, but as I said, if you go on the prop manufacturer's websites they can get you there. But you are going to have to know about your boat, engine and the reduction gear of the trany, if any. Also you need to know the taper of the shaft, the size of the key way and shaft diameter to order a prop.

7 to 11 inches of pitch is a huge range! A prop shop can maybe tweek a prop an inch or two of pitch, so you'd best be a lot closer than 7 to 11 inches.
My apologies I completely misread your post. Makes sense the prop manufacturer could help with selecting the right prop specs.


Quote:
Originally Posted by Minnewaska View Post
The OP certainly seems to have their bases covered. While many things on the original list are simply being dismissed, if you take it all for granted, the prop would seem to be most likely culprit. Despite the soot, have you tried to run it wide open throttle, while underway, to check max rpm? Do you know what your engine is rated for? That is a critical measurement, even to determine whether you have the next correct prop. Use a $20 laser tachometer to measure, not the cockpit tach.

The other things I don't recall being mentioned are the stuffing box and transmission. What kind of stuffing box? If traditional packed, is it dripping correctly or is it possibly crushed down too hard? Transmissions are more likely to get too loose than bind. Any chance it could have been replaced along the way with an incorrect gear reduction ratio? Just a stab in the dark.
Thats the thing- I didn't run it WOT and therefore I don't know the max RPM. I should have done this last season before hauling. So now I'm faced with the decision of making a guess and swapping out the prop or launching the boat as is, finding the max RPM underway, and then having to pay for a haul out. My mistake! I'm leaning towards launching the boat as is and collecting all the facts before swapping the prop, even though I'll most likely have to haul it again to swap props.

Thanks for the insight on the stuffing box- this isn't something I had considered. I believe its traditional packed but I'll have to launch to see if its dripping correctly. I'm a complete newbie in this area.
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