With both petcocks open, you are getting plenty gas to the carbs, no matter what. Pop the float bowls, insomuch as it takes 2 seconds and have a look. Fine sediment or water globs indicate a tank flush and need to fit filters. I always run filters...and watch them fill up with crap too. The stuff in a lot of gas pumps is crummy and you never know what the last guy got in the end of the hose. Never buy gas if there is a tanker filling the gas station tanks. If the diaphragms are bad it is bad all the time. un-mistakeable. If the jetting is off it wwill be gutless and run hot---but smooth. Air leaks at the carb or the carb-to-head rubbers will lean it out too. But simply examining the plugs will reveal this. The next time it is stumbling kill it and cheeck the plugs. I would most suspect an ignition issue---weak sparks from old coils or bad wires or old /dirty plugs. This shows up most under load and it is a stumbly sort of effect, not smooth but gutless. I'd give it a basic tuneup, plugs points, check the plug wires and also the coils before I messed with the carbs. And then don't mess with the carbs unless you have balancing issues (idle air screw does nothing for example) or plug chops reveal a mixture problem. Beware the front brake if it is damp inside and your speed is low.