You don't have a timing problem.
All your efforts are only band-aiding the real problem, which is most likely the carb. and AFAI can tell it has multiple problems. Assuming;
the intake valves are sealing and NOT sticking, and
the gas is fresh and clear, then
the carb will have to taken apart and cleaned, then, the timing returned to near stock.
In addition, you will need to;
make sure the PCV system is functioning properly, and
that the EGR system is Not faulty, and
that the only other air entering the engine, is going past the Throttle valves. and
that the crank is NOT seeing an idle load from other systems like the A/C compressor, the P/S system, a faulty convertor, etc.
AFAI can tell your carb, itself has up to three major issues, but ids not limited to just those three; Assuming that ;
A) the gas is fresh and
B) the WET fuel level is both correct and stable, and
C) most importantly that the cylinder pressure is even, stable, and it takes about the same number of compression cycles to peak the gauge; then, the issues may be/ but are not limited to;
1) the accelerator pump is lazy
2) the Transfer slot exposure under the throttles is too small, and
3) the airbleeds or emulsion tubes are restricted.
4) the fuel is contaminated.
I would start with a compression test, on account of you mentioned the vacuum gauge bouncing, which is a clear sign of something going on in the intake which should not be happening and can almost always be traced to an intake valve; simultaneously I would check every valve spring to make sure each has enough seat pressure, to be able to keep the valves closed and sealed. If you can spin an intake with just your fingers, that is NOT enough, lol.