Boxerworks.com

Just guessing which carbs you might have...BING book shows needle in the 3rd notch, 135 main jet, 2.68 needle jet, and a 251 needle. There is a 'bypass' hole in the carb body that appears just after you start to open the throtle that night be clogged that could give you this stumble.
Did it do this BEFORE you changed out the parts?

I don't have Bing book with me, but 3rd notch is correct and that's where the needles are. When I replaced parts, I did clean out all the passageways and this stumbling problem went away for a few weeks, then came back. I do not recall what the numbers are on the jets, but did order new ones for my carbs - they are installed.

Since the stumble went away for awhile after you cleaned everything, it sure sounds like there are fragments of mysterious molecules getting into your carbs and blocking the little passageways again.
You comment infers the stumbling was present before you worked on the carbs tho. Did you really mean to say THAT?
AND...is that why you replaced all those parts?

One part I failed to replace was the O-rings on the idle mixture screws. Yes, the bike behaved a lot better after I put new parts in and started using mid-grade fuel. But over time, the bad behavior returned. It is curious indeed. New idle mixture screw O-rings arrived today and are installed. Will adjust the valves (for good measure - it's been 5,000 miles) and then take her for a ride.

Valves were all in spec. Went for a 20 minute ride and the stumbling is gone. There is a wee bit of hesitation when moving through 3,500 rpm under very light throttle, but cruising in the trouble range I've been aggravated by was most pleasant. I will ride some more tomorrow - see how it starts and runs when cold. First impressions are very good. $3 worth of parts - wished I had thought of them in March when I replaced the other carburetor parts. But if this does fix the problem - praise the Lord and thanks for quick shipping by Bike Bandit.