I had a similar issue with my 434, the first run was usually slower. I also had an issue when the car gets cold it would stumble if I come up on the carb too fast. I went from 9's to 7's and it made a big difference. Part of my problem is I believe my heads don't work well at low RPM's. I am also going to try to go back to 110 rather than 112 or 114 fuel that I have been running as I am also running about 13 to 1.