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Old 11-05-2011, 11:50 PM   #1
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Onan blowing black smoke

Hi, I have a 6.5kva onan, mid80's vintage. It has just started to blow black smoke and what appears to be soot under moderate to heavy loading. I have recently taken a roughwater passage whilst running it and am wondering if I have disturbed the tanks enough to clog the fuel filters.

any advice, please.

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Old 11-06-2011, 03:55 AM   #2
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RE: Onan blowing black smoke

Cameron:* Welcome.* Your exhaust elbow is most likely*partially blocked.* Take it off and clean it or even it better see, if you can find an after market SS elbow.* In any case it might be time to replace it anyway.* They are*a maintenance item.* Good luck.

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Old 11-06-2011, 04:46 AM   #3
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RE: Onan blowing black smoke

one other quick fix...try running without the air filter(be careful what is around)...or just check it carefully...I'm pretty sure black smoke is possibly an indicator of restricted air flow. Other possibilities...clogged muffler, catalytic converter, or a diesel particulate filter; as well as a turbocharger malfunction (some not even applicable to your Onan). Leaky or stuck injectors can cause this also.
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Old 11-06-2011, 01:15 PM   #4
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RE: Onan blowing black smoke

Thank you both, I'm most appreciative.

I'll start with the air filter as that's probably easiest. As for the exhaust elbow I'll probably do that at the same time. Are there any things to watch for when removing it and as far as cleaning it out is that just a physical process or does it involve a chemical soak or suchlike.

Once again thanks, great to have this resource.
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Old 11-07-2011, 06:11 AM   #5
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RE: Onan blowing black smoke

Based upon your post, it seems a good servicing is in order. Marine age kills lots of gensets, especially one 25+ years old. Go to the Onan book and look up the most rigorous servicing schedule and do it. Items would include:
<ul>[*]Clean/replace air filter (annual)[*]Change oil and filters (annual)[*]Change fuel filters (annual)[*]Replace water pump impeller (every 1- 2 years)[*]Replace* mixing/exhaust elbow (if original don't think about getting another year out of it)[*]Adjust valves[*]Replace belts and hoses[/list]As suggested by other posters, your smoking problem may be unrelated to the above.*For additional details, look up *Onan generators on website for hints and issues.*
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Old 11-07-2011, 11:14 AM   #6
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RE: Onan blowing black smoke

As others have said,...

you're either not getting enough air flow

You're getting too much fuel, or the fuel is not atomized properly.

you have an internal problem affecting the ability of one or more cylinders to reach proper compression, resulting in poor combustion


you have a internal problem resulting in lube oil entering the combustion chamber (you mever mentioned if you're using oil)

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