Or from late or retarded timing. Its not burning in the cylinder and is being pushed out the exhaust. Its almost always a timing issue. I run these engines with thermo couples in every exhaust port to keep an eye on things. I also will run the vacuum advance hooked right up to manifold vacuum for break-in to keep the exhaust temps down, and to make sure we don't dilute the oil from incomplete combustion during the break-in period. 30-45* of timing at no load 2000-2500rpm will not hurt anything.
