Carbs Cleaned does not mean Clean Carbs!!!
Had something similar on a Yamaha sled. It had no Fuel Filter.
Alberta Cycle pulled their hair out doing repeated carb cleans with an ultra sonic tank, and compressed air.
There is something called a "Welsh Plug". It covers a "mixing chamber" for the low speed jet.
The head mechanic took over, drilled a hole in the shiny silver plate and put back pressure on it. All sorts of black debris was forced out. No amount of any solvent could take out that stuff.
After the plug was soldered closed again, it worked perfect.
Your problem may be lean / rich settings and poor jetting, but I am going to guess the carbs were the same as before the rebuild on the top end. If the problem was there before, and jetting does not fix it, then you have to look at the model of carb, whether there is such a mixing chamber where debris can collect, and consider this solution.
Just because you bought good gas does not mean that your gas was debris free. It is easy for something to get into a tank, or a jerry can, and create a lot of problems fast.
Drewski
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