Im really curious how the size of your ports compare to the size of the valve? I know from larger engines the amount of flow versus the velocity of the flow greatly determine the power output. Not only peak numbers but also the width of the curve.
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Well...this explanation will take a while, but for those who care, here goes...
The situation in the CB500/550 head is a little more complex than most, as it is fed via a long intake runner from the carb(s). This was doubtless done to improve the flat-power curve sensation of this smallbore engine, which otherwise would be very peaky, starting at 5000 RPM or more, because of the small valve area and oversquare bore.
With the long runners, we can get "free turbocharging" above 3000 RPM by making some extra volume available above the intake valve. With these engines, the intake valve's opening before BTC (5 degrees) while the exhaust is still open, the waste spark often fires the residual charge left in the chamber, especially if the mixture is a little too rich. This causes a mild backfire we dubbed 'spitback' that stalls the incoming mixture charge, causing a slight delay before this pulse wears off and the intake charge begins again. The total effect loses about 7-10 degrees of intake stroke "time" at low engine speeds (below 4000 RPM in the 500), so the engine suffers a significant 'flat spot' in the power curve. This also makes a momentary low-pressure wave toward the carbs' idle jet, which makes it 'burb' a tiny bit of extra fuel (because they mix extra-rich at lower speeds), increasing the richness problem in the 2000-4000 RPM range. This is one big reason for the D7E heatrange sparkplug in these engines, which is hot enough to burn this off IF the rider runs up to at least 5500 RPM on the shift. (and, if you tour, this is why you should switch it to something cooler, like a D8ES-L or X24ES-U).
To overcome the 'flatness' a bit, Honda installed low gearing and advanced the spark a little sooner to add some 'pep' to gears 1-2 in city-like riding.
The 500 was initially aimed at the rider who was intimidated by 1) the 750's price and 2) the 750's sudden clutch, gearbox, and [then] relentless power curve from idle to redline, compared to other bikes of the day. When it first appeared, it was GONE from every dealer as soon as it was out of the box, and much loved for the more 'sedate' personality, as most of its riders were new to the sport. Honda was real sensitive to this market, too. It wasn't until those [new] riders matured in their riding and wanted more 'snap' that the 550 came out, and I remember many riders trading their 500 for the 550 because of the bike's personality change.
Today, this becomes a challenge to 'undo' some of the tame nature that was built in on purpose. It isn't hard to do, but it takes several steps. Utilizing those long intake runners, we can make the volume over the inlet valves quite a bit larger than Honda did (they made VERY low ceilings over the valves to keep the slow-speed performance 'linear' over RPM) and capture 2 things at once: first, we allow a place for the 'spitback' to accumulate (at low engine speeds) without pushing the charge back at the carbs so much, so the carbs mix better. In effect, we are "eating" the spitback pulse right inside the head, above the intake valve, by making a pocket there which can harbor it for a moment of time. This allows the incoming air mass to more quickly turn the pulse around, back to the chamber, and all the fuel that may have started to burn up toward the intake tract at that point is now pushed out the exhaust instead. This then 'cleans up' end enrichens the next charge a little bit, resulting in more torque on the next power stroke.
As the engine speeds up, the spitback stops for lack of [burn] time. Above 5000 RPM on this engine, there is (and always has been) a distinct "switch" sensation where the power comes up a bunch, over the 5000-6000 RPM range. This is the loss of spitback as it is dying away. Above 6000 RPM the intake runners' extra length now allows another phenomenon to occur: the air moving up those pipes now has enough inertia to keep moving after the intake valve has shut. If there is more volume added above the intake valve to hold it, this airflow can pressurize this little pocket until the next time the valve opens, and this pressure provides a head-start into the chamber for the next charge cycle. Since this mass of air then does not slow down at the carbs, due to the long-distance tubes feeding the head, the carbs start mixing more effectively, too.
There is a practical limit to the size of the pocket over the valve, though. This limit is: too much pocket causes VERY poor low-speed engine control. This is why Detroit makes cars with "variable venturi manifolds" and other tricky, computer-driven gadgets, to solve this problem by shrinking the passages at low RPM and widening them as the engines winds up.
To overcome this part, there are several things we can do with wrenches, pliers, and the like. One is: don't make the pocket too big. Another is: delay the spark advance a little bit (especially since the advancer springs are now a little bit annealed, softer than they used to be) so the spitback is delayed in starting. Another is: make sure the flow into the chamber is slick as snot.
Honda's transitions across the valves are, well, conventionally manufactured, given 1970s technology. To make the valve seats, they bored a hole into the combustion chamber, using the valve guide as their alignment platform, then pressed a valve seat into it, followed by a ball-shaped cutter head that carved away the surface of the seat until the (attached) digital scale read "nnn.n" millimeters depth. This was a best-guess measurement, based on some number of head castings, so your particular head might have shallow, or very deep, seat pockets, depending on whether your head was at the maximum, minimum, or somewhere in between, as measured between the 2 dowel (jig) holes on the top of the head and the cylinder surface of the head. If the former, it looks like this CB500 head in the pix above: if more common, it looks like most of the heads we see, with the surface of the closed valve at or below the curve of the chamber's surface. This deep bowl (of the latter) causes the airflow to have to turn about 30 degrees as it enters the chamber, during the first 0.1 to 1.5mm of valve lift time.
So...if we grind away those deep bowl edges around the resulting deep-set versions of these heads, then mill the head back to remove the added chamber volume (to increase the compression back to what it was), we can change this 30-degree turn into a simple sideways flow, like in the 'perfect' heads that sometimes occurred (much like the 500 head above). This, in effect, increases the early-opening time of the cam, without modifying the cam. It can act as 3 degrees extra cam duration in a 'bad' head, or almost no change if the head was nearly perfect. To add to this, many of the inlet valves used have little lip edges on them (which were manufacturing guides) which can be polished or ground away to improve this early-start airflow even more, so I usually do that, too. And, polishing the back side of the valve ALWAYS improves the flow, another tiny bit, by thinning the laminar buildup in the valve's open area.
After you start adding [a tiny bit of flow] to [a little bit more charge] to [less spitback] to [better valve sealing] you always end up with a noticeable difference in these engines. That's always been the "secret" to making these bikes howl.
