i have 71 402 cbb.. lower end had rings put in.new cam 286,296 advertised duration 540-540 lift hyd flat tappets rotela t oil,,heads gone through,new valves,springs,guides etc etc.summit hei distributor 2 years old driven to car shows 4 per month,,rpm air gap intake,holley 670cfm carb..timming 12 initial,,38 total in by 3000..floats checked,new coil.modul 15"vacume at 800 rpm.vavles re adjusted..car runs good at 900 rpm after it heats up it starts running rough and backfiring through exhaust..get out and have to move distributor retarted like 10 inches then it starts i have to advance it to get it to run.then it runs decent for a while ..also double roller timming chain 1/4 in play any help,,, thank you much