Posted by seestheday on May 14, 2013; 2:23pm URL: http://vintage-and-classic-hondas.81.s1.nabble.com/cb750-popping-backfiring-tp4040698p4040741.html
Like Lucky said, you need to do a full tune-up.
Tires - are they older than 3 years? If so, plan to throw them out and get new ones, doesn't matter how much tread is left - you REALLY don't want a blow-out at speed.
Oil/filter change - Rotella T 15W-40 is decent and cheap
Valve lash - get them between .004" and 0.006" (very important!)
Compression - Ideally at 170 psi and all 4 within 10 psi of eachother, but 150 is considered to be quite good. Bike shouldn't run at all if they get below 100 (most engines won't).
Spark Plugs - NGK DE8A (stock) or DR8EIX (Iridium upgrade). Don't use anything else. Replace them if they're fouled.
Fuel filter - make sure you use one (paper is best) and put it between the petcock and where the fuel goes into your carbs
Fuses - check them to make sure they're all in good condition
Battery - get it load tested at an auto parts store (every one I've been too will do it for free), or just plan to use a brand new one.
After all of this is done, you should then start to look at your carbs and check your timing.
Regarding the carbs - you mention that you jump from 2700 to 3100. Are you sure that you put the jets back in the right spot? Here is a quick video I made showing the locations:
Also, has the bike fully warmed up when you are hearing the popping, or is it almost cold?