Black Smoke Near Head

Hi,
I tried a search but things seemed confusing. I have a 1971 Plymouth Scamp with the 225 slant 6. It starts great. It can sit for 2 weeks and still start great. It does idle a bit rich though. When the car starts there is no smoke under hood. Once you rev it, you can see a puff of black smoke come out roughly where the choke pull off linkage goes to the manifold. Back to idle no problem. Rev it quickly again, you get the puff. Could it be a intake/exhaust gasket with a hole?