So i have been restoring my car for 4 years and finally got it done. I put a 454 Chevy big block bored .030 over with a Holley 750cc carb. with a jet size of 74. I have driven the car for 2 days and it ran perfectly. Coming home last night it started popping really loud against the floor boards because im running straight headers and was missing. Today i changed the spark plugs because i used these plugs on breakin period and they were gas soaked. I started it up to drive it and it ran great again until i got about half a mile down the road again and it started missing again. It wasent popping like last night but i think if i would have kept driving it it would have because the further i went the worse the missing got. I checked the timing and it is set at what it was before so i know the distributor hasent moved. Do i need to go down on my jet sizing or what. I dont know what to do from here. HELP PLEASE!