1976 XS360 incessant backfire

[Pedrobabone]

Hey Guys,
I have a 1976 XS360 with a backfire that has been plaguing me for a while now. It is only on the left cylinder, here is a list of things I have done so far to try and fix the issue.
Adjusted points and timing (static and dynamic)
Replaced points (only left, I got a set and the replacement for the right didn't touch the cam)
Checked compression (good on both cylinders)
Adjusted Valves to spec (intake 0.12mm, exhaust 0.2mm)
Synced and cleaned carbs (vacuum boot was good)
Set float height to spec (26mm was too high so dropped to 25mm)
Checked for vacuum leaks (replaced intake boots and vacuum plug for petcock feed)
Replaced the coil on the right cylinder (cable was cracked and arcing)
Replaced both park plugs and boots (NGK)

Going off script,
changed float height, up and down, to see if it would fix the backfire (nothing)
adjusted timing (no better)
adjusted the syncing (didn't change)

Finally I decided to replace the coil on the left cylinder and now the right cylinder is backfiring (FML).

Does anyone have any insight to what might be happening here? Let me know if you need more info to help.
Thanks in advance.

 

[xschris]

Is the bike all stock? Post some pics. What are your pilot mix screws set at? Make sure the tips of them have not broken off into the bodies as this is common with xs360 carbs. What are your compression numbers? How are the carb diaphragms?

 

[Pedrobabone]

I knew I was forgetting something, it is completely stock.
The pilot screws are at 3, half turns out (1 and a half turns out) and the pilot screw passage is clear.
Compression is around 130.
I have added pictures of the diaphragmes.

 

[xschris]

130 is sort of low for the xs360. 160 is a fresh motor and 120-5 is worn out. Make sure the tips of the pilot mix screw are still long and pointed as the can easily wear off (post pics of them). What are you slide needles set at? Most bike are set to the middle clip. What do your plugs look like? Make sure you set the point gaps and static time them after. A strobe light will be good to set the dynamic timing and to make sure the ATU is working as it should.