68 chevy losing power mid throttle

[luke1968c10]

I have a 68 c10 with 250 straight six 3 speed on the column.
It idles great and is smooth. When moving around just over idle it runs good. After about 1/4 throttle it loses power and refuses to accelerate until the throttle gets all the way to the floor. After it's on the floor, it runs great. I replaced the carburetor with a new one and it's still the same. I have upgraded the ignition to the HEI distributor. Plugs are not very old.
Any advice would be greatly appreciated. Thanks.