""what would those undesireable symptoms be??""

I have been running manifold vacuum for about 4 months now. I have 18 initial timing (only during cranking, once the engine fires and the manifold vacuum is reached, then the extra 14 degrees is in) and 32 degrees at in-D-idle.

The only issues that I have found:
- idle circuit needs to be richer to run the extra idle timing.
- With my auto, once I had my in D rpms tuned and then I put the car into P or N, the idle rpms would climb about 200rpms and make the engine deisel during shot off. I added an idle solenoid and the problem solved.
- My vacuum can is designed for 12"hg at idle in D (or in N for a manual). I had to tune my idle to get the 12"hg in D, but once I did, the engine idles great in D. I have ran with 11 3/4"hg and had no ill affects. I also tried 11 1/2"hg and the idle started to wonder. So as long as I have 11 3/4" or more in D, then it idles great with 32 degrees of timing.