is it back firing thru the carb or exhaust? intake would be intake valve not working or thru exhaust exhaust not working. as was said in a previous thread pull valve cover off and the coil wire. check the push rod if its there. check valve lash at bottom dead center for looseness. take all spark plugs out. crank the engine and watch the 4 intake and exhaust valves of the 2 cylinders in question. if they don't move or move little compared to the others, you have a bad cam lob. others reasons, v-springs rocker arms, push rods will be obvious. 1 more thing is if the lifter has collapsed the rocker arm may not move. reason for removing all s-plugs is the engine will turn over much easier as there will be no compression in any cylinder. gook luck. take your time and be thurl.