Motorcycle Repair: Cb750 #2 carb issue, vacuum leaks, throttle shaft

Question
QUESTION: I am restoring or freshening up a dohc 1980 CB750 and am having some issues with #2 carb. I have rebuilt all four with new orings, jets, gaskets, needles, air valves, and the accelerator pump. 1,3, and 4 are great but # 2 is popping or backfiring at a steady rpm, mostly around 3500ish. The #2 is different that the rest being that it has the main fuel feed and has the accelerator pump. I have gone through the carbs mainly #2 many times now checking my self. I can find no debris, all of the ports appear to flow freely, no missing parts, cannot find any vacuum leaks, floats are not adjustable, etc. when building I did soak housings and all parts in a carb vat overnight. Problem occurs no matter what position I place the air mixture screw. I have swapped a few parts between carbs to cancel out issues with no change. I have confirmed that is a fuel and not an ignition problem. I don't know what I'm missing but I'm not seeing something. Any words of advice?

ANSWER: Did you remove the throttle blades from each carburetor and remove the throttle
shafts to inspect the shaft seals? 1 on each end of the shaft. Sometimes the solvent or cleaner will damage the shaft seals and will suck air in causing that carburetor to lose vacuum.
I rebuild Carburetors for a living...www.viragocarb.com
Did you also use a new accelelerator rebuild kit?
Were all the needles ends or points in good condition?
Also did you check the plastic tops on the needles to insure that they were tight.
Sometimes they come loose and work their way up a bit on the needle, causing the needle
to lower a bit causing a lean fuel condition.
If you cant figure out the issue. Contact me through my website. I have email and phone contacts in there.

---------- FOLLOW-UP ----------

QUESTION: Tore down again this morning. Needles appear to be fine and I have swapped them around from carb to carb to verify that was not my problem.  I did use a new kit for accel pump. The throttle shaft doesn't really have a seal, just a bushing and more of a dust shield or dust seal. Not too much play in it that I can tell. I did find a few poor orings on the tubes that connect the carbs together. I'm going to replace those and try again.

Answer
The seals I mentioned are usually U-Cup shaft seals (rubber) then the plastic dust cover
over them. You may need to syncronize the 4 carbs. its important all 4 throttle blades be in the same position. There are tiny holes up inside the mouth of the carbs next to the
throttle blades. adjust the idel screw up until the blades are just behind the hole openings.
Then check and adjust each 1 to show the same amount of hole in each carburetor. This will make the 4 blades all open the same amount at the same time.
Then set the idel back down. the pilot jet (fuel Mixture Screws will have to be adjusted also.
Each carb will be different. I reccomend checking out a (Gunson Colortune Tool) on Ebay.
They are the majic tool for setting fuel mixtures.