You added timing at idle speed by hooking the vacuum advance up to full-time (manifold) vacuum, so you would now have more advance at idle, compared to where you were before.My68Vette said:If my motor was tuned on a dyno using ported vacuum for the distributor vacuum, what would happen if I changed the hose to a manifold vacuum and without making any other changes or adjustments?
This could manifest itself as having an idle speed higher than before, it may exhibit "run-on" when shut off. I wouldn't expect it to suddenly start fouling plugs, or backfiring- but I could see it smoothing the lope some.
My feelings are, there's something else being overlooked here. You can check the timing chain to be sure it hasn't jumped w/o tools. Align the timing tab/damper mark to "0". Pull the cap and see where the rotor's pointing. It should be very near either terminal #1 or #6. For this check, it doesn't matter which. But it will be one or the other.
You sure the choke's not stuck closed or something simple like that?