Uneven idle with dual Solex 35PDSITs

[Braukuche]

I am running Solex 35 PDSIT that I bought from Aircooled.net with the optional larger venturii and proper jetting. I am very happy with these carbs except for:
1) My mileage sucks, 18mpg city/freeway going average of 70mph on the freeway. The bus is a '65 sunroof standard with all the seats and the top open a bit cause of the heat, 1776 engine with a freeway flyer trans. I was getting 20mpg before with my 34PICT.
2) Probably related, but when cold the engine idles slow and tends to backfire a bit. With the engine warm and the chokes open it idles really fast, about 1400rpm, but then sometimes will idle real slow, about 400 rpm stalling out. Basically, I cannot get a steady idle. I have fiddled with the mixture (about two turns out) and the idle speed and the timing but nothing works. There are no vacuum leaks. I am using a SDVA distributor with WOT timing set at 28 degrees advance. Any ideas would be welcome on what I can do to get a more consistent idle.
Thanks,
--Dan