[NeighborsComplaint]

That's reversion from a cam too far advanced and an overly rich mixture. The exhaust gases get sucked back up into the intake tract. In extreme cases it can go all the way up to the plenum and even form a cloud above the carburetor. Oversized headers on a motor operating at low to mid-range rpm can also contribute to reversion as the exhaust gas velocity is too slow for efficient scavenging.