There are also issues with running direct MAP vacuum to the distributor, especially at idle. When out of gear, the vacuum is higher, therefore, the idle artificially increased, when placed in gear, the vacuum drops (i.e., closer to 0" mercury) therefore the timing drops, sometimes several degrees. This will often cause a rather large rpm drop when going into gear, and therefore exacerbate issues caused by timing instability. When you toss a lopey cam in the mix, this only makes things worse as vacuum signal is erratic or at least not completely steady. I found this out the hard way when I accidentally connected my vacuum advance to the wrong port on my carb. I encountered a 300rpm drop simply by putting the car in gear. Disconnecting and plugging the vacuum removed that problem altogether. I'm pretty sure there are at least a few others in here who would affirm that, if used, vacuum advance needs to be ported from a "timed" source.
