Originally Posted by lg1969
I don't think so. 427 vette engine came with 3 Holley carbs with 2 vacuum secondaries. It sound to me more the center carb is leaning out. He may have to reduce the exhaust crossover from heating the center carb. It can also may have an internal vacuum leak inside the carb. Could be a warp throttle plate, too.
Thank you everyone for responding to this puzzling problem. The carbs are definitly Holley, the engine has been run with the fuel cap loose and fuel lines checked for fuel flow restrictions.
The next thing, I am assuming he hasn't checked this, is to block off the heat riser port, and reinspect the carbs again. I'll let you know what he finds out.