Ok, so I finally figured out (after process of elimination) that my problems stem from a lean AFR, not from a vacuum leak. I observed the following conditions and drawn a few tentative conclusions:
1. I have 2 fouled plugs, which aren't fouled enough to prevent them from firing effectively, even marginally. I realized this when I swapped the #1 and #2 plugs with brand new ones, and the problem still persisted.
2. I have eliminated all possibility of any vacuum leaks in the system.
3. I retarded my timing about 2-3" Hg, and the problem of my bubbly exhaust has largely in part been almost eliminated. There is an almost undetectable popping at idle, indicative of a lean AFR, and this idea is reinforced by the that my mixture screws are at 2 turns out on my Weber 38/38.
4. A check of the nose cones on the plugs revealed that only part of the plug was colored chocolate brown, while the rest of the cone remains clean. This tells me that the either the timing is advanced too much, or there is insufficient fuel to burn for the timing.
5. To reinforce this above assertions, it should be known that I tune with a vacuum gauge which has had an intermittent fluctuation of only 1"Hg. I am also running stock 3.56 gears and 31" tires, which is a little high for the tire size, and I rarely see 3000 RPMs.
I've already ordered some 0.47 idle jets and some in-between air corrector jets that I didn't have (185's and 195's). I opted for 47's because 50's are slightly too rich as I have engaged part of the progression circuit by turning the idle screw in further than what is recommended. This is a necessary condition because I noticed on a 200 that I had, that even with the correct idle jets, the motor wouldn't idle anywhere near 750 RPMs. Keeping this in mind, I have come to the conclusion that the smallest idle jet that I can use without exceeding 2 turns out on each mixture screw represent the most ideal conditions at idle. Any further insight or advice is welcome... Tim.