Some thoughts - I've run the same setup on my 250, didnt notice the diff in cyls.
How is your carb oriented? (Primaries forward, aft, inboard, or outboard - mine are outboard, carb mounted sideways)
Which Holley List# and what jets are in it?
Could it be that the whole thing is rich but there is an air leak sneaking in on 3-4 so they come out about right?
Just an idea, those air leaks are pesky and can even appear/disappear as it warms up as the intake is alum but the exhaust is iron - and they're bolted together (or do U have headers?)
