Heat cross over passages. Designed to warm up the intake manifold. Olden day style manifolds had a corresponding passage to flow thru underneath. Some apps with old style choke stoves used them to warm up the choke for proper operation. Some apps also used it to pick up exhaust gases for the E.G.R. I would prefer them clean but it does also connect to the exhaust port and will push the excess out the exhaust manifold. Be cautious though, if one of the exhaust valves are open, the carbon can fall into the engine and can also hang up the valve off of it's seat and create a miss and make you scratch your head wondering why.