Sounds to me like the metering rods are bouncing. 16hg is still lower than what those carburetors come "sprung" for. I think they are more tuned to what a stock engine pulls.....like around 21hg. I'd go to a stronger spring.
The engine is 69 340 block, .030 over with KB 243 pistons and a Summit Camshaft, Hydraulic Flat Tappet, Advertised Duration 276/288, Lift .441/.441, X heads and MP electronic ignition, not all that radical of a build IMO . But yes I think its running rich no matter where I set the mixture screws.
