"Yes to your question, mixture
"Yes to your question, mixture screw controls each "bore". These are idle mixture screws. If you close one or both, the engine will stall.
You closed one screw with no effect which means there is a vacuum leak somewhere. Most common is in the base gasget. You said earlier that you do not have a pcv valve. If you have a vacuum hose connected the the back of the carb base plate and the other end is in usually the left (port) valve cover and there is no pcv valve attached to this hose and looking at the hose, all you see is a black hole, than you have a serious uncontroled vacuum leak caused by the missing pcv valve. Although the pcv valve sucks air, it has internal restrictions to control amount of suck. To compensate of this added air being sucked out of the crankcase, the carb is rejeted from previous years of no pcv's. You should put a pcv valve in this hose and then check the idle again. The usual starting position of both idle mixture screws is 2 turns from lightly seated. Final adjustment is backing out mixture screw until rpm does not increase then 1/4 turn c.w.
If there is still no response to rpms with this adjustment then look for vacuum leak in base gasget. The backfire can blow out this gasget. Worse case is vacuum leak in intake manifold gasget. Do not forget to check any vacuum hoses to anything else for leak. If you pull the carb, remove cast iron base plate from the carb, make sure there are no cracks on the base plate. If there is a vacuum fitting on base plate, a crack is common from the bore thru the treaded hole were the pcv hose is threaded to the base plate. If there is heat ports, clean them and check for cracks along the passages also. Install new gasget between base plate and carb and tighten. Install new base gasget between base plate and maniforld and tighten.
Don't try to r&r the carb yet, just check the carb assembly screws for tightness and replace the base gasget. Set idle to 500 and adjust mixture screws, if there is no vacuum leak, there should be a response in rpm's to mixture screw adjustments. If not, the only other place a vacuum leak would be is the intake manifold gasget/s"