The K7/8 speed limitation is largely a function of that one's cam. Honda was faced with meeting strict (California) emissions standards, using an existing engine design, starting in 1976. They used lean-burn carbs and extra exhaust scavenging to meet the standards.
If you do nothing else but drop in the Megacycle 125-00 (or 04 hardweld) cam with the existing PD carbs and jetting, you will be surprised...you might wish to then raise the mainjets in the PD to about #115 and drop the needles a notch (if yours have the adjustable needles, many do not), or else just check to make sure the carb float bowls are at the right level. If they happen to be the later PD42b version of the carbs (mostly found on F3/K8), the float level is set at a lean 12.5mm, so altering that to 14mm like the early K7 will also help.
The PD carbs have an upper fuel-flow rate restriction, which was accomplished by using a single hose on one side of the rack to feed the whole rack AND a single bowl vent hose for all 4 carbs. This makes it VITAL that the vent hose breathes from still air. In the past I have altered the bowl vent hoses on the PD carbs to have them connected 2x2 like the earlier K1-K6 setup (keep in mind, the K0 had all 4 hoses), then run the 2 breather hoses up under the seat, and tied it down next to the tools in the tool tray. This is one of the quietest air spots on the bike (unless it has a full fairing like Vetters with lowers, then there are better places), and this will help ensure the fuel is pushed up the jets to a higher RPM rate. This was common with hi-speed roadracing back in the day: some riders made "breather boxes" to receive these hoses, which were small vented boxes placed in any site where the fast-moving air would not hit them, and the breather hoses were glued into the box. This worked well, too.
On my own 750 (Vetter equipped, and lowers when on long tours) I have the 2 hoses poked into 2 holes in the lower front of the airbox (see my book) to pick up the quietest air possible with stock parts. Don't open up the vents in the airbox: this will only disturb the air as it heads into the carb bells, and this, too, will lower the emulsifier pressures, which is exactly what you DO NOT want to do for higher speeds.
