V8's have a passageway through the cylinder heads and intake manifold that allows some exhaust gas to move from one cylinder bank to the other, under the carb, and exit out the other manifold when one side is partially plugged off by a closed heat riser valve. That's impossible on a stovebolt six with either an updraft or a downdraft intake. Just take the manifold assembly off and fix the heat riser- - - -it's an easy job. I usually use stainless steel for the shaft and flapper valve, install a new bimetal spring, and fix it once and for all.
Jerry
