your timing may be out - did you set it with a strobe light? Reading back over the thread, you have the same problem we all have - over all my suds, 33s and sprints the timing marks seem all over the place.
the advice seems to be - get it running, then slacken the 10mm bolt that locks the distributor. Twist it gradually while looking down the timing inspection hole and shining a strobe light down there. At idle the first line should be showing. As you rev it should advance to the second set of lines.
Some white paint (twink/crayon) makes it easier to see the marks, though a scratch with a screwdriver would also work.
Twist one way to advance, the other to retard. If it's a bit out, that could explain the backfiring.
There is a basic method to carb setup - but if they are working well enough to get you round the block, you're doing ok. They WILL keep filling up with crud for a while - did you put a filter inline? You already know how to clear blocked idle jets; you may well be doing this for a while, especially if the engine had been sitting. Varnish and crud will get disturbed and sucked through for a bit.
Next time you have the tops off look down into them and visually confirm that all 4 accelerator pumps are squirting the same amount. Get the car running and warm, and listen (an old piece of fuel hose is a good stethoscope) to the four carb throats - they should all be sucking the same. There are air screws you can turn to adjust this, but first check that all four butterflys open and shut equally - you can't tune them if they don't. Assuming they do, (there is a throttle stop screw you can adjust to make sure they do), use it to set the idle speed to a steady speed (about 1000rpm).
Now you find the air bleed screws, and wind them in or out. Adjust them on each carb to increase the idle speed, then bring it back down by backing off the throttle stop. Once you have the idle circuit set to the ideal you have done the basics - a surprising amount of your driving depends on these being right, so it's worth getting it as close as you can.
If the engine is reluctant to settle back down to a steady idle, or stalls when you pull up, the temp fix is to use the throttle stops to raise idle speed, then spend some time with the air screws. If it is really hard to get this right then your carb butterfly spindle may be worn, allowing air down past them and confusing the airstream.
Someone else has already mentioned float height - if you find the car seems to run out of fuel going up long hills this could be the culprit, and if fuel is overflowing as before, likewise, but I've found the most common problem is air leaks due to worn gaskets. A bit of careful spraying of carb cleaner/'start ya *******' at areas you suspect of having an airleak will confirm it as it'll suddenly rev up.
Hope all this helps. It can be frustrating (I went through a lot of this getting my sprint working properly with its new 1.7 engine) but it's great when you put your foot down and the revs go past 3000. My first Alfa was a S3 sud single carb and we converted to 95hp. It was a revelation...