Hondaman, since we seem to be in a tire thread and I've learned a few things here, could you recommend tires for 3 bikes? This might apply to the majority of us who are wondering.
All cb750, the first is an original sandcast survivor which will be ridden according to it's great condition, currently has the original front tire.
The second is a K0 with an 836, roadrace cam, ported head, yoshi style pipe and stock size lester mags. I gonna flog it. currently has a K70 front and a TT on the rear
Bike 3, low mile F0, stock except for double front disc brakes. It's usually ridden when my wife wants to go along, short trips two up. It needs tires soon, the back moves around when turning and/or braking with 1 rider and pushed just a little.
Well, if they were mine...(warning: this means it will ramble a little...):
The sandcast, if the steering head bearings are still the ball-and-race type, will surely need a full-ribbed front tire like the original one. Otherwise it will track all over when it finds a crack in the pavement to follow, like rain grooves. Even so, it will still tend to follow them, because the ball-and-race steering head has little torque grip on the side-to-side movement of the steering post (i.e., not much surface area to grab!). It leads those smaller fork tubes right into the wiggle! Tapered steering bearings will reduce this tendency a lot. Also, if the swingarm bushings are still the old Zamac plastic, you will feel it when flipping from, say, a left turn to a right: the whole bike will shift its line as you 'land' into the turn. This is from the compression of the swingarm collar into the plastic, about .002" of compression. If they are the phenolic ones, this is about 1/2 that much. The bikes came with both Zamac and phenolic well into 1972: my own K2 had Zamac bushings when I took them out in the summer of 1972, because I could not hold a line at 100+ MPH on the track with the bike: grabbing a handful at the top of 3rd gear in redline would make the bike jump to the right. The phenolic made it better: bronze made it solid.
The K78 rear tire on the sandcast and 1969 K0 bikes was a short-lived disaster. It had insufficient sidewall grip and strength, so heeling over and grabbing a handful could put the bike on the pipes. They wore out quickly (4000 miles), so most of them disappeared. The K87 was built by Bridgestone just for the K0, and remained until 1972. This was a decent rear tire, if you could live with the 6000 mile life. It matched the 3.25 ribbed front tire profile very well.
Overall: if you don't mind losing the vintage look, try a set of Avon Roadriders. The Sandcast will feel like it lost 50 lbs. somewhere.
The hotrod: I suspect your Yoshi pipe on the right side has cut back your cornering clearance there to be LESS that the centerstand stub on the left side. So, the K81 (if it is an old trigonometric K81-TT100 tire, more than 7 years old, now) sidewalls probably won't get used: in that case, a metric round profile tire would be more suited to keeping the rubber footprint bigger in the realm where the (bigger) torque will meet the pavement. This would mean numbers like 120/90x18 rear with 110/90-19 or 100/90-19 front (depending on the front fender's configuration) would get along well while making more rubber meet the road at all angles it is likely to see. There are a couple of good choices here, but the main things to look for would be:
1. A linear rib, or groove, or simulated circumferential groove, MUST be there in at least the front tire. If you add it in the rear tire to match, the high-speed ride will be an easy one-hand affair, otherwise it will have a slight tendency to wander the line. The front rib is needed because the frame will torque to the right when pulling hard in lower gears with that bigger engine, and the groove will tend to keep the sideslip to a minimum. Tread patterns should be symmetric, don't get the side-slash ones that are used for monoshock frames.
2. High-traction rubber, at least the "H" speed rating.
Something to note: all of the K81/TT100 tires I have seen since 2006 are NOT trigonometric, but a simple tread pattern copy of the old K81 on a round-profile carcass. In layman's terms, this means: the modern version of the K81 is NOT a roadrace, V-profile tire. Avon does make some V-profile tires, but the ones I am familiar with are non-symmetric patterns, made for modern ultra-stiff monoshock-frame bikes. This will cause the dual-shock CB750 frame to track a little bit with the tread pattern, very unnerving at speed (to me, anyway). Most of these also do not have a circumferential rib, leaving as much rubber on the ground as possible to support the modern 100+ HP engines.
The dual-front brake bike deserves a little more of the across-the-tread ribbing patterns, so as to grip the pavement a little better if stopping hard. If you notice in the OEM tires that had the linear ribs, they added square-block cuts across those ribs starting in 1973, and made them outright blocks in the F2 tires. These facets cut straight across the tire so as to scrape away dirt on the pavement at their leading edges when the brake torque is applied, and they often wear in little "steps" right behind those edges as the result of this torque. That's where they "skid", so to speak. (You can see this on your car's tires, too...). With 2 discs, having more of these little cross-blocks in the tread will also make the bike stop straighter by holding the forces to a right angle against the gyroscopic momentum of the front wheel: or in other words, it stops the tendency of the front wheel to wash out under hard stops. There's 2 general schools of thought here: one is to use fewer cross-cuts, letting the wheel slide a bit more, while the other is to dig the tire in harder, period. Interestingly, Honda stayed with the first school of thought on their F2, which reflected the rubber-hose decision with the first-ever disc brake of the K0. They didn't want to see flying W pictures over CB750 handlebars in the media, I guess? So, even with the dual discs, the competitors could out-stop them at the time, mostly because they were using more aggressive tire profiles (like on the KZ1000 bikes).
So...for a dual-disc front, I'd recommend finding both a high-grip rubber ("H" type at least) and a blocky, cross-cut tread pattern. The K81 tread pattern, even though it is not a V-profile, works well for this. If you go K81 on the front, match it at the rear. The bike will feel much more "straight" under you then, instead of letting it find one 'rail' with the front pattern and another with the rear.
Of all the tires I have used over the years on this bike, the true trigonometric, V-carcass K81/TT100 British Dunlops were the closest to perfect (IMO). It had slightly more sidewall tread than the bike needed, put more rubber on the ground when heeled over than when straight up, could dig out of anything but pure sand under braking torque, and at 45 PSI could carry 600+ lbs of touring loads at 100+ MPH without ever breaking a sweat nor aging the rubber. The Japanese version Dunlop copy was not even close, hence the confusion that still exists today (the "legends" of the K81, true or false?). When those disappeared, it took me a long time to find anything close: today that would be the Avon Roadriders, except they are not trigonometric. More's the pity!
