You mentioned that you replaced the carb "needles". In my experience (rebuilding hundreds of these carbs), there are NO aftermarket needles that have a correct taper, rather they are all dead lean. If you can find your original Keihin needles, maybe put those back in, as they start working right where your "bog" is appearing, at about 1/4 throttle.
Another thing I don't see in the notes above: have you changed the O-rings in those castings that bolts the carbs onto the head? They are hard like plastic now, and leak vacuum badly. Almost every single 500/550/350F/400F I see nowadays has this vacuum leak issue. On the 500, this O-ring is the same size as the ones in the valve adjuster caps.
Daiichi points: there are 2 types out there. One seems somewhat acceptable, those made in Japan. The ones made in China can very seldom be timed anywhere close to the timing marks, as their geometry is off by about 1/2 mm between the pivot dowel and the rubbing foot. I made some of the Chinese ones work for a wile by mangling the rocking arm with needlenose pliers so the contacts actually met at an angle (but was using my Transistor Ignition, which made this acceptable current-wise) to get the timing right. Eventually this caused much pivot wear, and in just 6000 miles I had to retire them.
Don't suppose the exhaust system is causing this trouble: it is not, and can not. The exhaust efficiency of the exhaust valve in this particular engine has FAR more effect than the pipe. The air filter is another story: if you have a foam-sock type, remove it while trying to get it running for now. They are too restrictive for the Fours, and cause many issues: while the engine can eventually be tuned to run with them, they don't flow well enough to breathe easily. They are good for lawnmowers and other 1-cylinder engines, if big enough, but not for 4-cylinder types (or V-8 engines, either...). The K&N filter is the very best, paper next best, pods the worst possible solution.
