Maybe you are struggling with a fuel issue. My 261 runs a pair of weber 38mm carbs on a Clifford manifold. This set up was new to me last year. I previously ran 32/36 weber pinto carbs. Did not have a problem when I ran the motor up. 5/16 lines. When I ran the 57 at last year's trifive nationals, ran the carbs out of fuel every run. The video is my first run with the new set up.
You can hear the miss and popping as I pulled into second gear. Is this what yours sounds like? Even though this motor is not making all that much power, probably 200 or less, I still ran it out of fuel with the mechanical pump and 5/16 lines. So maybe you start there and upgrade the lines, pickup and pump, install an electric pump like a carter 4070 and eliminate the fuel supply as an issue.
www.youtube.com/watch?v=1xJbck9QQH8
