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1978BroncoCustom

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.

v/r

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!

Al

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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.

v/r

Jim 

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Good morning Al and my apologies for the delay in providing an update. Think I may have narrowed down the root cause of soot buildup in the exhaust. As soon as the temperatures got above freezing the soot level decreased. I noticed when the outside ambient temperatures were in the single digits with a negative wind chill the choke plate would stick causing the engine to labor some at a lower rpm with a strong smell of gas. When the ambient temperatures warmed up the choke worked normally indicated by a gradual increase in rpm as the motor warmed and the plate opened up in the carburetor. Plan to take it to a mechanic tomorrow to have the carb and choke checked/adjusted and the curb idle reset. Hopefully that will correct the cold weather soot issue. 

Here are some photos of my rig for your viewing pleasure. Will update the thread as soon as the mechanic checks it over and makes corrections.

Jim

53878903323__7AD01B61-EB89-4553-BC9C-51E838957389.jpeg

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53878953525__D0AA823D-A7B3-4A28-AACC-88EBABB3A6FD.jpeg

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Yo,

Sounds good on the choke.  Installed a choke cable on shipmate's Chevvie while in Milwaukee (1965) due to temps below 0.

Your Bronco looks Great, inside and out!!!

Put pics in the Album section.

Al

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Yo Jim,

Thank you, fills out the gallery very well!

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Hadn 't made it to the mechanic yet to check carb/choke. This past week I filled up the tank on my way home from work. Next evening when I drove in for my shift the truck would not accelerate normally and would seem to "bog down" when I pressed the accelerator to speed up to highway speeds. Suspected bad fuel at first so I put a bottle of gas treatment into the still full tank and changed out the fuel filter; no joy on the post maintenance road test. 

This afternoon before work I was going through the recommended trouble shooting techniques in the Haynes Manual for lack of normal acceleration/power. I have eliminated everything but timing (slipped timing chain), distributor and coil.  The coil was replaced last year about this time, but I picked another up at the parts store on my way in to work. Tomorrow afternoon I will replace the coil, recheck plug wire connections and pull a couple of plugs to check for fouling. If the coil doesn't fix the problem it will be time for a mechanic to trouble shoot/repair the issue. 

Of the three remaining areas to check (coil, timing, distributor) all could account for the soot due to an inefficient fuel burn in the cylinders. Stumped just a little on the cause of the "bogging down" as it starts right up on cold mornings, idles well and doesn't smoke (fouling) with the exception of a strong fuel smell during idle warm up when the engine is very cold (parked outside in below freezing weather overnight).  Please advise if I am missing anything in regard to trouble shooting; recommendations or similar experiences would be appreciated.

Jim

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Yo JIM,

I didn't see this last post until tonight.  This site has been showing error messages.  The site owner is trying to fix this issue.

Is the engine at normal operating temperature  when it bogs?

See http://recarbco.com/tech-www /basic-troubleshooting/hesitation-2/

Then look @ Hesitation, deadspot or stalling that only seems to occur after the first mile of warmup.

#1 perpetrator is choke due to strong "fuel smell" and tge Recarbco list.

More tech articles at http://recarbco.com/tech-help/basic-troubleshooting/

Good Luck Jim!

AL

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Good morning Al and thanks for the links; both are great and I plan to keep them on my profile page for future reference.

The gas treatment must have worked its magic as the Bronco ran fine and accelerated normally on the drive over to the mechanic this morning. The bad fuel theory may have held true as some of the smaller deposits or water that could have been causing a line constriction/fuel starvation during power demand may have dissolved over the three days it sat in the garage. As cold as it has been, and the number of nights it sat outside in freezing temperatures while I was at work, there is a possibility that I may have had some ice  deposits in the fuel system.

When I dropped the Bronco off I passed word via the receptionist that I had changed the fuel filter and suspected bad gas, but wasn't completely sure. Once the carb, choke and curb idle are reset the mechanic plans to take it out for a road test.

When I get it back I plan to replace old vacuum lines and a few other sensors that are associated with the carburetor to see if it improves power demand response.

Jim

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Good morning Al,

Sorry for the delay in closing out this thread.  Mechanic checked the Bronco over, adjusted carb and the choke.  The choke plate had been out of adjustment for quite some time and the fuel air mixture was too rich so all the above was causing the soot on start up.  He recommended that either I add a bottle of fuel treat or fuel up with higher octane gas every third tank if I planned to run ethanol mixture in it. I still plan to replace the vacuum lines as a precautionary step as they to are almost forty years old and have hardened.  Thanks again for the links as they will be a great resource in the future.

Jim

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Yo Jim,

Glad it's resolved!

If you can, print out the 79 manual, etc because  it will be like gold someday.  So many websites have gone belly up in recent years including mine.  See my signature below.

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