You can check timing with a flash timing light while running.
You can also check it roughly using a 12V test light, a tail lamp bulb and some wire is sufficient. Connect the light from ground to the points contact (the moving contact or the terminal for the wire up to the coil). With the ignition ON turn the crankshaft slowly via the big nut on the points shaft. The bulb will go on and off, when it goes OFF is when the plugs will fire. The light should go off just as the F mark passes the pointer mark. Check both points sets.
For a real timing adjustment you should have a flash timing light, and a dwell meter is a nice bonus. Set the timing at the advance mark, the F mark is not important if the timing is correct at the advance mark (2000 RPM plus, approximately). For the best performance use a dwell meter to set the dwell equal on both points sets. Dwell should be 195° ±5° for the 750. You have to play with the gap adjustments and the timing plate position repeatedly to get the timing and dwell correct, it's a bit of an art.
If you plan on keeping the bike a long time and have a few $$$ to spend, install an electronic ignition like the Dyna S.
With the carbs, check the emulsion tube above the main jet. After removing the jet you can push it down from above, push a paper clip oir something similar down where the needle goes (with throttle wide open of course). There are very very small holes cross-drilled in these emulsion tubes that tend to clog. Clean them out with copper wire strands from extension cord wire or similar, don't use steel tools to poke at them. Any clogs will have a large effect on fuel mixture.
And try new plugs, bad running at higher RPM and throttle is often just a plug problem. Any problem with the spark plug wires or caps often shows up just like what you describe - the plugs take more voltage to fire at higher pressure and if there's a leakage path in the cable or cap the spark will gladly take it. If you have stock resistor caps you should use non-resistor plugs - there must be no R in the plug number. Non-resistor plugs are a bit hard to find and it may be best to get non-resistor caps (any bike shop can get NGK caps) and just use resistor plugs. You can check the caps with a cheapo electrical meter, unscrew them from the cables and check resistance from the plug contact to the cable contact screw. You should get either zero ohms or 5000 ohms depending on if they are non-resistor or resistor type. If they read 10,000 ohms I would get 5000 ohm ones - 10K are too high for the dual-fire system.
