Regular puffs of black soot coming up from carb

[Animal Lawyer]

My mechanic swapped to the new engine & t5. Runs great, major what a difference 100hp and overdrive makes. But there is one issue, the new engine rebuilt 1970 351c mild cam (don’t know numbers), external oil mod, blue thunder manifold and a 680 cam SA carb keeps making small puffs of black soot from carb when idling (at least that’s when I saw it) coming out of ram aircleaner, inside of filter cover and carb are filthy with it, but car runs great and doesn’t seem to be losing oil.. Any thoughts?

 

[Don C]

Possibly a leak at the manifold gasket around the exhaust crossover or a porosity in the manifold allowing exhaust gases to enter the manifold. Or an intake valve not closing intermittently, or a valve timing problem. With a mild cam there shouldn't be enough valve overlap/early valve opening to cause that much reversion.

I would also pull one of the breather caps to see if you're getting puffs out of it. If you are the crankcase pressure may be entering the intake through the PCV valve. Make sure the PCV valve is functioning correctly, high pressure should cause it to close.

 

[Sheriff41]

Tight intake valve?  Does it idle smoothly?