Check spark plugs and wires including terminals. check rotor and cap.
Check compression on #1 and al others to determine overall engine condition.
Get a
carburetor that matches your engines need. Case inpoint; HiPo 289 engines h
ad a less than 450cfm carburetor to produce 271 h.p. Granted GT350 had 715cfm carburetors. However those engines had a higher h.p. rating and to be abl
e to feed fuel needs at wfo. Additionally 390 FE engines often had less than 450cfm carburetors. You have way too much carburetor unless you are doing race only at full throttle. I suggest your problem solution should focus on that health of that single cylinder that is not functioning as it should.
consider the following formula: engine size × max rpm × efficiency ÷ 3456 = carburetor size
312c.i. x 5000 x 78% at best / 3456 = 578+
This would be a maximum need, Actual need more like 415c.f.m comparing the y-block to later more efficiently designed engines.
This would even be a optimistic efficiency of 245(h.p.) / 312c.i. the actual efficiency is less for the 312.
Have fun, ohn