Engine running rough

[Phil]

Recently purchased 900ss 1981 seller said it used to run rich on back cylinder (been stored for 4 years)so got the baby home and got to work after check over tried firing up did fire up but only on right side so took back carb off gave it a good clean this time it fired up on both pots but was spitting and back firing on back pot after a couple of minutes i noticed left hand down pipe glowing red hot stopped bike let call down and adjusted mixture screw but still seems to get hot to worried to run it long just in case it gets red hot again any ideas?thanks phil