[Pete Munroe]

If I understand your problem...it's rough...surging? at mid power...like freeway speed?

I put a new Holley 1850 replacement carb on my 67 Corvette back in 1993...the original carb was shot, warped and rotten, but had original jetting circa' 1967.

The car was geat under all conditions except trying to cruise on the freeway at a steady 70 or so.

It would surge on an almost regular basis.

I pulled the float bowl, the new replacement Holley was several jet sizes leaner the stock one...I took the jets out of the old carb metering block...put them in the new carb, problem gone away.

Apparently is was "lean surge".

You say you changed intake manifolds...like from a dual plane to single plane?...anyway, perhaps the jetting is now a little too lean for the speed of the airflow in the new carb...maybe the velocity is lower due to bigger passages and runners and the fuel is dropping out of suspension at medium power...

Anyway something else to check if the only thing you changed was your intake...maybe it effects the mixture at midrange.

Pete