Yes I believe too lean a mixture will contribute to the popping..Just keep track of your adjustments and do a little experimenting.
But incase I was misunderstood, disabling the TBV may increase the chances of the popping continuing or increasing.
You might also consider disabling the Vacuum retard by removing the vacuum tube from the bottom of the rear carb and capping the same nipple.
That same high manifold vacuum that activates the TBV, will also activate the Vacuum Retard..
The retarded timing doesn't exactly provide ideal conditions for combustion, either, as evidence by the engine picking up idle speed when the retard is disabled and the timing allowed to advance.
You'll have to adjust the idle speed down, though..if the retard was working properly in the first place.
By doing a little experimenting, I think you'll quiet things down.