The power question: mostly the 500/550 engine suffers from lack of breath due to (stock) a too-early spark advance curve and a slight mismatch in size of the 22mm carbs with the intake valve size. But, since the intake runners are also sized to the 22mm carbs, bolting on 24mm carbs doesn't help much because it won't then mix right in the all-important 5000-6000 RPM range where this engine first gets it legs. Those I know who solved this in the intake area bored the intake castings bigger (and thin!) to match some 24mm carbs (I have NO clue where they got those, though) and did the usual things to the intake and exhaust ports above the valves, to the point that I saw 4.00x18 tires leaving rubber and smoke on the pavement at the nightly drags on those (2) bikes. They had stock cams, though, so their ET was still not real exciting, but stoplight contests were much easier. I moved away from that area the summer they were finding new cams, never heard how it turned out for them, but I did see some 500-4 bikes roadracing here in CO when I arrived, sporting Action Fours cams and 522cc (I think that was the size) pistons at 9.4:1 CR (and some non-stock cams). They were 10K RPM bikes and did well, and heralded their front straights with a great GP sound as they headed into 5th gear, great fun! (Now that track is shut down, drat...) One had a 4-2 exhaust set with megaphones, the other had long straight pipes turned up on the back, next to the axle, with Snuff-R-Nots in the ends. Those (roadracer) guys had gone to the trouble of installing 520 chains and sprockets for the lighter weight, alloy wheels, and small (roadrace tread) tires to reduce the unsprung weight, and it paid off for them.
All the elements are there to pull 50 HP out of these engines with enough time and $$, but it takes some commitment (like BWaller does) to get it there, and no small amount of maintenance to keep it there. The gearbox is a case in point: the gears are relatively small (to make a small crankcase) and are hindered with the Hy-Vo chain (lower power transfer than regular chains in equal size, but smoother and quieter), so exceptional lubrication (no popcorn, please? I won't expound those details here...) will release some of this, and this was proven over and over by me and my friends with the 500-4 back in the 1972-3 era (before the 550 came along). I was sad to never get to dyno my 500 like I did many 750s, but I think it was a stellar 35 HP when I got it, and by the next summer was a 100+ MPH bike, sitting upright in the seat, without disassembling the engine (or modifying the carbs). Honda's state-of-tune as shipped was aimed at markets like the US East Coast, British cities and Euro burgs, who were clamoring for a Four but, "...does it have to be so big?..." and in that realm it did just what they wanted.
