Yes, a higher idle speed will certainly raise the vacuum levels. Your more recent descriptions may be pointing to a carb setup issue. If you got a more stable idle with the choke plate 1/3 closed, then that is a sign.
Thanks - but the excessive initial timing it wants has me stumped. The carb is a new eldy 600. I set it up by adjusting both screws to 1.5 turns; then adjusted each (turning both the same amount in 1/4 turn increments while also adjusting idle screw) until I got max vac. Then tweaked both separate from each other a minor amount looking for stability of needle and/or additional increase.
Even when I increased idle screw to say ~1500 RPM and pull the advance down to below 30, it starts backfiring. So based on what you folks have taught me so far, even if the carb wasn't perfect, it looks like the cam in this engine is probably working against it - correct? Or do you think the timing and backfiring could be all carb related. I'll do a compression check this evening. Thanks again for the help!