Thanks for the info. Paint thinner is not "strong" enough to dissolve the coatings that todays fuel leaves behind. Plus - Your symptoms are indicative of un-dissolved debris remaining in the carb galleries. I've never used Ultra-Sonic cleaning but have read enough threads to understand it's not the final solution. Some have suggested that this method can not work effectively in the small gallery areas.
It may "pain" you to have to go through this procedure again but I'm suggesting you do; but this time use Lacquer-thinner. You may be equally surprised at "the color" of the discharge by doing this. And, you may/should get some solid debris flushed out with this solvent. It would be very interesting to know what comes out so try to capture the discharge in some form of filter - even if you simply use a piece of paper-towel to trap the junk.
Do not under estimate the "shit" that new fuel will leave behind within minutes of running through a cleaned set of carbs. It's disgusting to see such heavy "staining" in the venturi's on the bottom of the bores coming from the main-jet as well as on the top, from the idle-ports. Nothing we can do about the quality of fuel. Can't speak of other territories but various Canadian fuels I've tried leave the same shit-stains. regardless of "winter" or "summer" fuel or brand. Our sources here are limited to 3 refineries.
It sounds like you used the Cleaning Tool correctly; aside from not using Lacquer-thinner. "Regular" paint thinner just doesn't cut-it (pun intended!)
When you think you're done..............do it once more ! That should solve your problem.
1 final comment: I've done a couple of successful "Quick-Flushes" using the Cleaning Tool. I did this as much as a "Test" as anything. Had a fuel-filter "Issue" that gave me an idle-"pop". I removed the carbs, drained the bowls and flushed each venturi several times to ensure that what ever was being flushed into the bowls would be drained out. This is a short-cut but does work if the carbs are otherwise clean, with the exception of "that one piece of debris".
Don't worry about the lacquer thinner damaging the rubber "bits" The throttle shaft-seals are isolated by the Cleaning Tools" design. Just don't "soak" the rubber parts for any length of time. Lacquer-thinner evaporates very quickly on it's own and almost immediately with some compressed air.
This should help ! Let us know please.
I'm "outta here" for a couple of weeks to go soak up some Cuban sun, drink some great "Sugar-cane" beverages and sniff some MI-7 turbine gases ! I'll look forward (?) to reading your results IF I decide to come home !
cheers, scotch-on-rum