symtoms of lean and rich mixture

this may help a little...... running rich. black sooty spark plugs, exhaust will smell like fuel, also may have black smoke from tail pipes. hard starting that may be helped by holding the pedal to the floor and cranking over motor (flooded )
lean.... exhaust stinks, may smell like rotten eggs . white spark plugs, the plugs may even have small spots on them, spots are not good ,may be piston particles.
carb back fires are normally a lean condition or a vaccum leak. a lean carb may surge a cruising speed with steady throttle, lean can also cause over heating.

to fix......verify before changing the carb. check your engine grounds, you need to have a ground from block to body, next check charging system and battery, next check the ignition , and timing.....if any of these are not right you will go nuts trying to fix the carb... take a few minutes and save a few hours.....
heres a couple quick tricks to get a general idea of a rich or lean condition...
if it is rich ..slip off a small vaccumn cap or line. this will add air and lean it out some... engine will respond by running smoother and may raise idle a bit ..
if it is lean.... you can richen it a little by closing the choke a little bit by hand, or use your hand to restrict the air flow. engine will respond by picking up idle and/or smooting out.
smelling the exhaust is the easiest. if you do not know how it should smell, just sniff the tailpipe of a good running car.
last.....watch the spark plugs... white after running is no good.. actually dangerous to pistons. again to lean..
black plugs.... means unburned fuel ....to rich
tan or light brown plugs ....this is what you want to see.....

hope this will help ..sorry it is so long.........Ron