Umm...you removed the fuel tank and Scottly mentions what I also see: it looks lean on the plugs. This could be caused by low float bowl levels of fuel. Could there be an issue with the way the fuel lines are routed? Maybe down & then back up before getting to carbs, or...?
In the 750s with the 657b series carbs and 2 fuel lines from the petcock to the carbs, it is easier to route the lines INcorrectly than correctly, with the lines going up & over something enroute to the fuel tee(s) that feed 2 carbs. Honda supplied 2 holes in the carb mounting bracket that are a guide: the more-forward petcock hose should go thru the upper one of these holes and then across to the carbs on the far side, without rising high and then back down (i.e., more horizontally). HOWEVER: with ethanol-laced fuels, a hot day of riding, and incorrectly routed lines, the fuel might not reach the far 2 carbs once in a while because of float bowl pressures from heated ethanol vapors pushing against the horizontally-routed fuel line. Here in Colorado (higher elevation than most places) this becomes an issue during steady-speed traffic, with the float bowls on the far 2 carbs running lower than on the 2 nearer carbs. It tends to 'straighten up' after pulling over (or hitting an offramp for a turn), only to return again when running at steady speeds.
One 'cure' I've used in this situation re-routed the 'far' fuel line out in front of the carbs to the 'far' side instead of thru the upper bracket hole and across behind the carbs. This reduces the length of hose a bit and tends to balance the fuel pressures in the 2 petcock spigots by a little more bit since it reduces the volume of fuel in the longer hose slightly. It doesn't take much to slow the flow, so it also doesn't take much then to restart it, either. NOTE: this method DOES NOT WORK in bikes equipped with fairings than wrap around or below the engine - like Vetter Lowers or the Vetter Phantom fairings, or roadrace fairings in slow city traffic. I have also heard (but not personally experienced) that it happens on 750s with the rolled-up sleeping bag tied across the front forks - which greatly increases engine heating, but looked cool in the movies...
The idea behind having the 2-fuel-line petcocks was to have 2 different fluid head pressures so they can 'pump' each other when the carbs on one hose open their needle valves: this drops the pressure on that line and makes the other line 'suck' a bit, helping to seal closed those other carb's fuel valves. Then when these carbs fill up and their valves close, their vacuum goes away and the other side gets the fuel. Meanwhile, the first side was lowering its fuel levels, so it trades back-and-forth like that, all the time. Trouble appears if one of the fuel lines is bent/crimped a little bit: then the switching back-and-forth becomes very sensitive to the heating of the lines because the crimp acts like a restrictor valve, unbalancing the timing and favoring the opposite carbs' flow...
