1981 IT465 won’t start. It ran fine for the shop that built it, and for me 1 week ago on our maiden voyage. It has new bore, piston , rings and seals. Once I figured it out, it starts within a few kicks no problem. Idles fine, accelerates well ( no stumbles etc ), plug color a nice chocolate brown after tooling around a grassy field ( it wasn’t a chop throttle test ). Now, it will fire on 2 or 3 kicks, but will die when I crack the throttle or turn off the choke. It will backfire at times when kicking, but no kick-backs at all. I swapped the CDI unit thinking that it was the problem, but get the same result with the 2nd one ( not new ). Spark is a nice blue/white. Any ideas would be much appreciated. I was all set to go riding Friday, but ran into this condition and am out of things to try. HELP!