Oldnwiser
XS650 Enthusiast
If you’re using the BS 38S have you checked the throttle valves for the plates for a vacuum leak? Also are the diaphragms good? And lastly are the tops over the vacuum diaphragms flat or do they show any evidence of over tightening or warpage. I hope that this helps.I was out on a little train trip.....Anyway back to your carb issue. I'm wondering if during the ultrasonic cleanings, the needle jets were removed and verified clean (perhaps ID #)?
A generally accepted - around here - source for genuine Mikuni brass is
http://www.nichecycle.com/ncs/categories/carburetor/mikuni-parts-by-category.html
They're not genuine Mikuni, but Niche does also offer a house brand of needle jet (Z6) and a needle (4JN19) that they spec as replacements for '72 (306 model) BS38 carbs that are perhaps better than other aftermarket (I have some for pending resto). However, will reuse original if possible.
http://www.nichecycle.com/ncs/catalogsearch/result/?attr=0&q=256-14941-86-00
http://www.nichecycle.com/ncs/categ...ng-tuning/needles/102-3650-needle-by-ncs.html