340 intake on a 318?

We tried, but, nothing seemed to help. Swapped manifolds and the problem disappeared. Like I mentioned, I've seen tons of similar combinations that had no problems, but, these two did for some reason.....and the only change made that cured the problem was a manifold change to where the ports matched. All the intakes involved were dual plane, but, they may not have the same exact internal design. I can't explain it either. I figured that the only flow problem you would have would be at high rpm's. It sure wasn't the case here though. It was right off idle that they fell on their face.