I had a similar story where I bought an off the shelf 600 vac sec to replace an old one and and things went backwards bigtime. I transferred the jets over amongst other things but with my limited knowledge i couldnt get it anywhere near as good as the old carby. I eventually gave up and took it to a dyno tuner who got the timing 100% right and diagnosed an incorrect power valve. I cant remember the exact size that went in but it wasnt a small adjustment. I couldnt believe how far out the carby's stock power valve settings were at the time for a mild 302C I just assumed it would perform like the old one? The engine purrs like a kitten now it was the best thing I ever did.
