Yes Chuck, I understand what you have experienced. I have experienced the same issues many many times. I am the guy that is responsible for CMB building their 67, 80 and 90 engines with duel and triple plugged heads in the early 90's. Previous to that, (1988-1990) I built many duel plug heads for OPS and CMB 67's, 80's and 90's. I still have a 4 plug head for a CMB 100 that I tested. It had enough firepower to burn pure 90% nitro/10% oil in that engine. That was before CMB even had a 100 engine. The first CMB 100's came by way of Ranjit Phelan big bore P/L's.
We had set up's that were much faster than the single plug heads, otherwise we (Me and Art McMeans) would not have asked CMB to build them. The problem was that if the engine was not set up correctly or the wrong modification was made, the duel plugs did not work. There's a lot more to it than just the carb bore size. It's many things about the complete system including the fuel tank, needle valve, pipe and of course the engine and carb itself.
As I mentioned previously, we had two Jag 67's with CMB 67 (black & gold) duel plug engines in them and they both ran high 90's in racing trim.
And when I say same racing trim, the boats were just as we raced them (full turnfins included) with the only difference being an additional 10% nitro and a bigger prop. One boat used an ABC 65x110 (cut down 1667) and the other boat ran an ABC H-32. It takes a lot of RPM to run high 90's with an H-32. Those duel plug engines would do it.