That size motor should really like that carb. The carb has 2 mixture screws and a center air screw in the front. You may want to lean the mixtures in, then add some air by backing out the bigger one in the center. Beyond that, take a vacuum reading then look into the power valve circuit. I can't recall if that's mechanical on that one or vac. If it's mechanical, look at the needle setting, and look to see if the secondaries are stick open a lil bit. I did se one case where a guy backfired a time or two gettin dialed in and it bent the secondary shaft holding them open and running pig rich.
If none of that works, you may be too low on the lead, or initial timing. Try bumping that forward and see if it helps. You will need to adjust idle speed when it's all tweeked in. Worse scenario, the cam is in wrong. It has been known to happen. Sometimes cams be ground a bit off. That's why good builders use degree wheels to check.
