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SVArgo · 09-05-2015, 14:09

Our 3GM30 is shooting out black smoke at 2500rpms.

What are some possible causes?

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J Clark H356 · 09-05-2015, 14:14

Do you have a MaxProp?

Could be dirty prop or if MaxProp, worn and needs to be refurbished. See what you Max rpm is now and compare with what your previous Max RPM was. If it has dropped, could be prop related.

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**Tellie** · 09-05-2015, 15:31

Fouled prop or fouled bottom or clogged exhaust elbow are the first things to check.

shakey doug · 09-05-2015, 16:40

There is only one cause = fuel to air ratio.

SVArgo · 09-05-2015, 16:46

We just splashed the boat after a new prop and bottom paint. Also, I dove and everything is clean and clear so I'm concerned it's more internal.

The engine didn't have an air filter before and we just put in one, so maybe it's getting less air than it used to and is blowing out the carbon?

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SVArgo · 09-05-2015, 16:48

Quote:

Originally Posted by shakey doug

There is only one cause = fuel to air ratio.

So which is more likely, not enough air or too much fuel?

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**Wotname** · 09-05-2015, 16:49

Quote:

Originally Posted by SVArgo

We just splashed the boat after a new prop and bottom paint....

New prop or new prop paint???
If new prop, then a new size or not?

SVArgo · 09-05-2015, 16:54

New prop. It's the same size and pitch, but had to find a larger shaft size, with a reducing coupling.

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SVArgo · 09-05-2015, 16:57

The boat is fairly new to us, and the last dew days have been then first real out and about, so this engine is all new to us.

We replaced all the hoses and belts, oil change, brand new water pump">raw water pump, new alignment, air filter, etc.

So unfortunately we have no idea how the PO ran it at what RPMs. There was a lot of differed maintenance.

I'd think that there may be a lot of carbon built up, so we treated the new fuel with seafoam.

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**Wotname** · 09-05-2015, 17:15

OK, so possiblity overproped from way back...
You report black smoke ar 2500 rpm
What rpm can you get with WOT.
It should be around 3400 to 3600 rpm.
Anything much less with a clean bottom and clean prop indicates a prop sizing problem if you have a clear air inlet and black smoke. Technically you could have a gear box ratio issue but this is highly highly unlikely.

With enough air, the black smoke indictes too much fuel at the rpm set. Your engine is expecting to reach 3600 so at WOT, it is delivering enough fuel for that rpm. If external factors (prop size) prevent that rpm, black smoke is the result.

The air supply remains a constant, fuel supply is varied, not enough rpm gives a rich mixture, thus the black smoke.

DeepFrz · 09-05-2015, 17:25

Contrary to the above post the air supply is not necessarily a constant. Try running without the new air filter and see what happens. May not be your problem but is worth a try as it is something that you have changed.