I hope Carbking steps in if I mess up on the info.

Bill it helps to identify the Carter W1 model you have. Also the altitude (higher--> leaner) where you live and whether are not you plan to run E10 gasoline or non-ethanol. If you are running E10, a lean rod may make your problems worse. Usually run one size richer because E10 has less energy per volume of gasoline. Also your truck may be a 3100 and not a 2100.

Carter W1 684S (from my book 1941-51 Chevrolet)

75-485 standard .067"-.067" to .062"-.062" to .060"-.046"

75-517 1-lean .068"-.068" to .065"-.065" to .064"-.049"

75-562 1- rich (no other info)

Any more questions, I would contact Carbking. He is a busy guy so be polite.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."