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Can worn valves mimic rich idle mix?

PlymCrazy

PlymCrazy

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Both of my SBC tail pipes lay black patches on the shop floor for the first several minutes after cold start up at idle. One has an Eddy 1405 and the other is a Rochester 2bbl. I haven't touched the idle mixture screws since I've owned either of them but plan to in the next few days to try to improve on this. As I'm overthinking here, I wondered if valve seats or valve seals could ever get so worn that they produce symptoms that mimic a "rich carb condition" out the exhaust? I would tend to think if that were the case you'd have poor compression in the cylinder with the worn valve(s) at the very least? Please correct my thinking as you see necessary.
 
Likely part of what your seeing is the condensate when cold in the pipes/muffler picking up the carbon in the exhaust system and then tossing it out the rear.
Somewhat unavoidable in an old car and engine.
Do they use oil? Does it come through the valve guides?
Best thing you can do is check the choke function make sure it opens quickly enough.
Start it and drive it out of there letting it idle has no benefit.
 
Don Frelier said:
Likely part of what your seeing is the condensate when cold in the pipes/muffler picking up the carbon in the exhaust system and then tossing it out the rear.
Somewhat unavoidable in an old car and engine.
Do they use oil? Does it come through the valve guides?
Best thing you can do is check the choke function make sure it opens quickly enough.
Start it and drive it out of there letting it idle has no benefit.
Click to expand...
One is confirmed it does not use oil. The other I have not owned long enough to verify yet. I keep these (2) cars closest to the overhead doors so I can roll the tail end out to start and idle before taking them out. When it's raining out I'll leave the door open and put a pad under the tailpipe to blow on while inside.

But hypothetically speaking, is it even possible for worn valves to mimic a rich condition?
 
sam dupont said:
You can check for worn valves with a vacuum gauge.
Click to expand...
Ok, that makes sense. For a minute I was looking at things upside down in my head. Is there a certain amount or percentage of loss that is acceptable when putting a vacuum on it?
 
Last edited:
I think it's just condensation, my fuel injected 318 does that with 3" single exhaust and a 36" dynomax truck muffler. I have to remember to cardboard my garage door or black and rust particles on white. Engine idles over 18" vacuum.
 
After either car warms up, the black stuff at idle goes away. Upon hard acceleration one car will blow black smoke, but not while cruising. Haven't put the other car through the paces just yet. And likely won't. Not into hot rodding that one.
 
Oh yea all 3 of my old motors which all have been freshened with low miles and tuned appropriately all shoot black **** and soot out when fired cold in my garage from the condensation in the pipes. It gets even worse if you start them and shut them off alot to move them around without letting them warmup properly. Part of this fantastic hobby haha!
 
Sounds like I'm in good company lol. I do try to let them run long enough to burn the moisture out of them but sometimes it ain't in the cards for the circumstances.
 
Well, I'm much more confident that what I'm seeing is normal for the vehicles' age and setup, but...

My original question still stands. Could a valve or valves be worn so bad they could mimic a rich carb condition as in the "sooty exhaust" example here? Hypothetically speaking, could there be any way unburned fuel could get passed an excessively worn valve and out the tailpipe giving the appearance it's running rich? Seems unlikely, but impossible?
 
What is that old saying.....................
Anything is possible.
 
Remember too that a lean condition will stink as well. I had my stroker motor set lean and swore it was running rich but it was actually a bit lean and when I fattened it up the "stink" changed to a different smell, so both are possible.
 
91r/t said:
Remember too that a lean condition will stink as well. I had my stroker motor set lean and swore it was running rich but it was actually a bit lean and when I fattened it up the "stink" changed to a different smell, so both are possible.
Click to expand...
Bullseye..
To me, too lean smells "sour", and too rich is "sweet":p
 
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