Have to agree with Bob Mathern. Main jet seems a re herring as it is only relevant after 3/4 throttle.
Also agree about getting electrics/timing right and looking for leaks.
But after electrics and leaks are confirmed ok I’d forget main jet size and start with slide sizes (6/3 is a good place to start) but I’d also be looking at needle jet size and needle position combinations. Needle jets are pretty cheap so a range of 105 / 106 / 107 is reasonably cheap to get. Also make sure the needle itself is in good nick.
Good luck
