Okay so my chevelle is a 71 with a 383 Holley 770 street avenger carb. Had the car the past couple days and no problem till today I put 30$ in 93 octane fuel in it and drove it 2 miles home fine. Left 30 min later and after about 3 miles then it started running rough. Got back toy college and the car will stay running but sounds horrible when driving. If I put it in neutral it misfires till about 1500rpm n then goes away but in drive it's nonstop