Yep, the hand-worked heads, the cam changes, the carb recalibrations, the exhaust pipes, the airbox(es) and the spark advancers.
The K0, prior to about 3/1970, was a raucous, lighter, hand-made engine. That was about the time Honda discovered they were caught completely off-guard by the popularity of this ground-changing bike (which became a history-changing bike), and they had to get serious about making thousands of them instead of hundreds.
The K0 early engines had hand-ported heads, partly because their hand-shaped sand-filled socks that formed the ports were all over the place in size and shape. Looking over these heads, you will see one with the valve guides protruding as much as 14mm inside the port(s) while the next one has about 6mm of them sticking out. Their ports will look very different, too. The most common feature is: they are larger in diameter and have more headroom over the intake valves than any of the later heads, especially the late K2 and the K3 heads. But size wasn't everything: there were 'doggy' K0 bikes, too, and mostly it came from poorly-shaped intake ports, probably from the hands of the "new guy" in the production line...
The cam changed 4 [documented] times between the K0 and the K5/6, but I have (had) notes of there being at least 7 identifiable differences in both the intake and exhaust lobe timing. Generally, the intake duration that started out opening at 5-6 degrees BTDC and closing at 35 degrees ABDC in the K0 lost 7 degrees overall by the time of the K5, and the opening time got progressively later during those years. The lift dropped almost 1mm along the way (intake side), too. By the K4, which excelled in smoothness over all of the others, the 2-3 intake cam lobes have less lift by 0.25mm, with the same duration, as the 1-4 intake lobes. I believe (still) this change was aimed at increasing fuel mileage (up to 50 MPG then) for the US gasoline crisis that OPEC caused in the mid-1970s. Honda knew its largest market (USA) was limited to 55 MPH freeway speeds for bikes pushing [mostly] Vetter full fairing, saddlebags, and 2-up riding with camping gear aboard, so many of the changes were directed toward increasing midrange torque at the sacrifice of sheer top-end speed and performance, so that drove much of the change.
All of the carbs before the emission-controlled PD series of the F2/3 and K7/8 bikes were essentially the same parts, but with differing jetting to match their cams (mostly) and their pipes. The K2 pipes (HM341) were at first overestimated by Honda, and the K2 had overly-rich carbs before about 2/72 production, causing LOTS of plug-fouling troubles. This was fixed by increasing the airbox vacuum (read: smaller intake vents) and shortening the total spark advance, along with gearing the final drive up (again) over the K1 gearing. this was great for touring, not so much for street fighting and racetrack work. My original 'claim to fame' came from reverting the K2/3/4 bikes closer to K0 tuning, which added much top end and acceleration joy to the typical college kid's 750.
These changes were:
1. Increased the spring tension of the spark advancer, in those days usually by cutting 1/2 turn off one of the [then new] springs. Today this requires 1/2 turn from both springs, as the springs are annealed from engine heat much more.
2. Lenghtening the carb bowl vent tubes to reach up under the seat, or alternately to reside in 2 small holes drilled into the face of the airbox. The K0 bikes had 4 such tubes, while the K1 and later bikes had 2, or (in PD carbs) only 1, which limits fuel flow above 7000 RPM quite a bit.
3. In the [in]famous 7-chamber HM341 pipes, I drilled 4 holes 1/2" each into the last baffle plate of the pipe, then reinstalled the "plugs" on the ends and made sure the rubber couplers were snug. These matter...if the owner had installed some aftermarket pipe, this upper-midrange power was simply lost, no help there.
4. Adjusted the spark angle to reach further. This depended on the spark advancer in the bike: in most cases this can be viewed as being as far past the longest timing advance mark plus the gap between it and the earlier one, or about 3 degrees more advance. This advance must not arrive before 4000 RPM, though.
5. Switched from paper air filter in the airbox to K&N's filter, oiled lightly.
6. Changed the final-drive gearing. The K0 first started with 16/45 teeth, which broke chains and cases because the 16T front whipped most of the lube out of the chain at hiway speeds. Honda's fix was an even better 17/48 combination, which, when combined with a slightly larger rear wheel, upped the top end speed to 135 (indicated) MPH on many bikes in those days. It also increased the gear ratio, making quicker takeoffs. This engine is made to rev, don't ever lug it...
7. In the end, riding it some (hard) and then jetting the individual bike to get a light brown plug burn on premium fuel after 10 miles of riding at 70 MPH speed, after the other changes were done. This sometimes took altered float bowl levels in the carbs, maybe jetting changes, maybe new needle settings (groan...), or, very commonly, altering the size of the emulsifier's holes in the mainjet emulsifier and the pilot jet. I experimented with polishing the air screws enough to know it was a waste of time: they just need to be clean...
In all of this, I never had a set of vacuum gages for carb synch work, but then, these were stock engines with typically less than 5k miles since Honda set them up. On those bikes that got 'built' with Action Fours parts (most popular then) like cams and bigger pistons, those engines only got bench-sync jobs from my shop. Gages would have let me smooth some of those out more...
In the end, the most bang-for-the-buck in performance parts came from cam changes and [sometimes] port jobs done to the post-1972 heads, if the rest of the engine was in top condition - as most were then. The 2 cams that stood out from all the rest were one from Action Fours (no longer made, and was too soft for touring use, it wore quickly) and the still-made Megacycle 125-00 series (which is the only non-stock cam I'll install today). This cam is a copy of the so-called '90-HP' Daytona cam of the 1970 race debut of the Big Four, although in street trim with standard carbs and pipes it more closely approaches 80 HP levels with full tuning. It lasts like an OEM cam, though, and makes a good touring power curve, too.
Other things that 'happened' to the engine were: in the K4-5 early engines the cylinders were 1mm taller than the others, to lower compression slightly to allow US riders to run on Regular gas (when they could find it) during the gas shortage. It dropped the compression just a bit, but power and crispness with it. Today, when I get one of these, I fit that...today's head gaskets are all 0.2mm thicker, which acts much the same way, so you can actually over-mill the cylinders a tad to recover that, too, if you really want it. The pistons will peek over the tops of the bores when you do, so don't get too crazy with it, and measure carefully. The K4/5/6 heads also have slightly more volume (i.e., lower compression ratio) with the original swirl-directing ridges gone: this happened with their new injection-molding equipment of the K4 that removed the vagaries of the hand-finished (by then, sometimes) ports and chambers. To those who recognize "the smell" of a well-tuned SOHC4 CB750 exhaust, this changed that particular signature of this bike, making its exhaust smell just like any other Honda exhaust (or Suzy 4-stroker, or Kawi...) of the day, because it changed the engine from its original stratified-charge burn system to a more conventional combustion system, in order to increase midrange torque and improve fuel mileage at low-to mid-throttle settings, where this engine ligs along somewhat unfulfilled.
This engine is made to rev: it does not come to life until 5000 RPM when it suddenly burns the fuel more fully and the exhaust is no longer pushing back into the chamber during overlap (no, the pipes don't control this...the cam and spark timing do it). This is the "surge point" any K0/1/2 rider is well acquainted with, that simply isn't there in the later engines.
