So I used the non-vented filler cap which simulated a sealed crankcase system to experiment. With regards to idle quality it made no difference. I've also squirted carb cleaner at all the possible points of vacuum leak and haven't found any. It also sometimes idles at 800 when partially warmed up and sometimes at 500. The weather is warmish (60) and hi\umidity around 50%. Could I be seeing carb icing under these conditions? Once warmed up it's a steady 800RPM, 600 with the A/C on. Maybe I'm overthinking this and I should just drive it.

I'm tempted to leave it sealed with the PCV pulling a vacuum. That should preclude most if not all oil leaks. I've done this to my old British motorcycles and it has stopped the persistent oil leaks from the crankcase and head. Everything else still leaks (sigh).

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.