That has more to do with the 750 kit on the long rod motor than the single carb. It's something every Triumph enthusiast should experience at least once. Like Mr. Healy states, once the cc's are up to 750 the head starts to work.
I've had a long rod 750 myself, but with twin carbs and hot cam. The throttle would not tolerate rough treatment....the front end would shoot straight up in the air.
However, having said that, the bike was near unridable because of the vibration. It's something that I began to address in my bikes a few years after I had this one. I know I could build an engine with a 750 kit that would be tolerable now, but then I hadn't discovered the secrets.
Also, the five speed isn't really necessary on these bikes either. They're just a much nicer shifting gearbox than the four speed.
Finally, Bill Getty knows how to build a bike that works. I have a Command built by him in the early 80's that still hasn't had the bottom end or gearbox apart and has been in continuous service since then.

1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Black Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.