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Black Soot from Tailpipes on Startup

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Good evening all.  

My '78 Bronco has the factory 400M engine installed and blows out a small amount of black soot on first startup of the day; can be seen on the concrete of my drive way.  Prior to my purchase, the Bronco had set up for quite a few years so there could be quite a bit of particle buildup in the intake, valves, pistons, etc.  I don't think it is the exhaust as I had it completely replaced during the partial refurbish.  My Dad and I reset the carburetor and idle speed per factory instructions after I replaced the spark plugs, plug wires, coil and ignition module earlier this year; is it possible we have it set too rich? A third and final theory is the 10% by volume enthanol gas mix I have been running in it doesn't burn very well and is leaving some fuel residue in the pipes when I turn the engine off. Would running a higher octane ethanol free fuel fix it?

The engine starts well on cold mornings, doesn't smoke or use oil (107,000 original miles when purchased) and idles well once it gets warm. Any recommendations or information would be greatly appreciated.


Jim Meek

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Yo Jim,

No smoke is Very good!

I assume plugs looked good.  Any with carbon deposits? 

See reading_spark_plugs.jpg

PCV Valve; 

Check it.  Pull it and shake it, should hear a rattle.  Check vacuum line.

Location & Depiction in Parts Diagram in a 78 400M 
Source: by 78brncoxlt

Have a great New Year Jim!


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Good morning Al and thanks for the information. The plugs were clean a few weeks back when I pulled one to check for fouling. The carburetor looks like it has been replaced, and after sitting up for so long, it may be time for a rebuild or thorough cleaning. I changed the oil last night in my rig, and will pull the PVC valve and check it this morning; PCV valve was replaced earlier this year so it should still be good. I'm not very good with carburetors, so I will probably take it to a shop to have it cleaned and reset to factory specs. I'll let the forum know how it turns out and what the root cause of the soot was.



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