Most carbs don't have an adjustable main jet. Is your carb a Zenith? It helps to let people know what you are working on. At least you did include a year. Some people don't even offer that much info.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.