I have a 1965 Mustang Coupe with an unmodified 289 and 3 speed trans. It has the original autolite 2 barrel carb with the original aluminum tag on it. I have rebuilt it several times and am not sure it has been reassembled correctly. The engine runs very well but on occasion when decelerating it will backfire through the muffler. I think it is running slightly rich. I know the idle mixture screws should be adjusted about 1.5 turns counterclock wise from the bottom. However, I am running them all the way in. I bought news idle mixture screws and tried to readjust. The engine idles very smoothly but the screws are again all the way in. The car runs very well except every now and then it will backfire. The timing has been set according to the shop manual specs with a timing light. The fact that I am running the idle mixture screws all the way in indicates that something is wrong. I have been told that running the screws all the way in should cause the engine to stall. Could it be getting gas throught another path?

Thanks for any help.

 

you want the idle to be as low as possible while remaining smooth and practical. keep turning the screws out until you hear the engine start to run rougher, then turn it back in a bit, do this for both screws.