My 340 getting 6 mpg

Dan Georgino said
Wooo, 750cfm is way to much carb. 600cfm is plenty. Then play with your jets till you get 18 to 20 vacuum inches at idle. What is your vacuum at idle now?

750 is correct for a 340. You can use a spread bore for better fuel econ. but you need to ck the rest out first.