Someone recently reminded me that I started to do this long ago, but forgot.
(Now they are all in my book!
).
This is the list that I know of power-robbing changes that occurred, in the order I remember, to the CB750K over the years. Up to the K4, I knew these from hands-on: after that, they came from others who knew of them, but are not necessarily my own "knowledge".
K0: The very first ones had over 40 degrees of spark advance in their advancers. This dropped several times over the years, to as low as 28 degrees by K6 (more static, less total). Note the 9200 RPM redlines on the early K0, dropped to 8500 by the end of K0 production.
K0: The earliest ones had more dwell (and lift) in both cam lobes, about 5 degrees more than K1. The K1 had less exhaust dwell, and the K3 had less intake dwell, too. Each reduction "stayed" in production. The K4-5 introduced a later cam, with a sprocket that was adjusted about 3 degrees late, to cope with poor octane across the US, probably, during that gas shortage.
K0: the float levels in the carbs were 1mm higher than in late K1 engines, at 25mm. The carbs did not matter: the spec changed to 26mm for all in 1970. Torque went down with the float level, too. Roadracers generally ran them at 24mm.
K0: smaller, lighter, lower-geared sprockets (16T/45T) gave way to 17T/45T first, then 17T/48T "kits" with a sheet metal chain guard in the front cover. This became 17T/48T by late K0, which dropped again to 18T/48T on the K2 and later. The heavier sprockets and taller gearing really impacted (stock) street-strip performance.
K1: the exhaust pipes had a new baffle inside, right past the upward bend at the footpegs. They still had the fiberglass baffles at the end, though. This inner baffle collected water and made the pipes rust out quickly, though, triggering a recall at one point.
K1: the rear shocks were improved, a little larger diameter and better damping, but heavier. This extra unsprung weight costs a little more HP.
K1: the stock carb jetting was dropped from 120 mains and 40 or 42 idles to 115 mains and 38 or 40 idles. Leaner meant less torque, cleaner sparkplugs.
K1: the airbox inlets were smaller and narrower than K0.
K2: these bikes started out on Premium gas. By about 8/1972 production, they could run on regular, if not pressed, because the spark advancer had changed to the K3 spec (less advance, stronger springs).
K2: the HM341 pipes appeared. The earliest ones had 3 inner baffles, quickly replaced with 5 baffles during K2 production. I had the 3-baffle ones in my first set, destroyed in a wreck in September, 1972. The replacement pipes were already 5-baffle by that time. My original carb jetting was 110 mains, but had to drop to 105 with the new pipes, and torque went down with it. My original HP was 62 on the dyno at around 7200 RPM, went down to 60 with the pipe-jet change. The plugs did stay a little cleaner, though!
Late K2 (summer of 1972 or so): the airbox inlets got thinner, narrower, than K1.
K3 (after the headlights could be turned off again, i.e., late K3): the final countershaft bearing became a 2-row bearing. The original was a 1-row bearing, with a shoulder on the shaft that was cut back for the new 2-row unit. This change made the first owner complaints of the "high gear wall" effect show up in our dealership: the bikes would not reach 100 MPH in a high-gear roll-on test. This bearing required very careful alignment of the final drive inner (1-row) and outer (2-row) to make any improvement, but the cases were just not that accurately machined from Honda (the expensive "fix" was to heliarc it up and align-rebore with torqued cases). Roadracers usually replaced the 2-row setup with the earlier ones, to good effect.
K3: dual seals appeared on all the wheels bearings. They were one-sided seals before this change (and, the grease usually ran out into the inside of the hub, making a mess and wearing the bearings). Racers would pop off the inner seals on new bearings.
K4: these bikes ran on Regular gas, with less timing advance and leaner 100 mainjets. The HP difference with D8ES plugs (instead of D8ES-L plugs) was very noticeable, so we used a lot of the "-L' plugs to help things out. Today's equivalent: Nippon Denso's X24ES-U plug.
We often ran the mains at 105 and added 2 degrees of static spark advance, or widened the "stopping ears" on the advancers to reach for a few extra degrees on top.
K4: the rocker shafts in the heads were changed from free-rolling style to bolted-down shafts. This was a quieting measure for mechanical clatter, but significantly increased frictions and reduced rocker and shaft life. Roadracers would remove the new bolts and polish the shafts to regain the losses here.
K4: the longer, seal-less valve guides of the K0 (which lasted intermittently until now) were all replaced by this time with shorter guides with seals on them. Honda was trying for longer oil change intervals (1500 miles), to compete with Kawasaki's 900/1000cc bikes. The RPM dropped a bit from the drag of the seals. Racers just removed the seals, went back to 1000 mile oil changes.
K4: the airbox got quieter again, although I never took the time to find out where the changes were. I suspect the inlets got higher internal baffles, like the K2 late production units had.
K5: O-ring chains appeared as a dealer-installed option at extra cost. These chains deliver about 93% power, compared to 97% for a good, hard-roller plain chain like Diamond. O-ring spin friction (200+ of them) is the culprit.
K6: Although I'm not 100% certain on this point, I have it on pretty good authority that the "F" cam profile had appeared (5 degrees later, 5 degrees wider, generally) in the K6 engines as an anti-emission measure. I did not get a chance to measure this one, so you K6 guys can jump in on this one...
The overall difference: in about 1971, I tested one of my friend's K0 bikes at 65 rear wheel HP, at over 30K miles on the ticker. The last K4 I tested (1974, I think) was about 40 HP on the ground, at about 8K miles. All tests were on the same "truck" dyno, an in-the-floor type of rear-wheel dyno used for checking out diesel trucks, near Macomb, IL.
If I find or remember the other things, I'll add them later...but, as you can see, you can add 10 HP to a later "K" bike with no hotrod parts!
