To be sure, the '75 is a mild-mannered critter. That's where the 100 MPH "wall" first appeared, when you couldn't go any faster than about 97 MPH with them. It was mostly due to Honda's trying to cheapen things, as the Kaws were killing them in the market.
That's not to say, however, that you can't wake them up! I'm putting a K3 together with a K5 top end now (it might be a K6, hard to tell, they were so much the same), and plan to have well over 55 HP on the ground (at 6000 feet altitude) for the lucky next owner, next Spring. It's a Candy Gold one, this first one.
Rule #1: evaluate whether you need to bore. Easiest way: get some new, straight, long hone stones (like the 3" ones sold at Checker) in a 3-arm hone and run it in each bore for about 1 minute. Wipe the stones with a rag after each hole. Look near the top of the bore to see if the rings have generated a slightly deeper ridge into the cylinders. If it's there, consider at least a step overbore (.25mm) before a big-bore kit, as the latter will reduce life considerably compared to a step bore. The metals in the new cylinders usually warp out-of-round in about 5000-1000 miles, then finally cure. Boring round again brings back a lot of lost power. Boring to 836cc starts the cycle over: after 5,000-10,000 miles, they need a step overbore to make them round there, too, then they settle down and stop burning oil, usually. Boring a stock cylinder set to .25mm oversize at 10,000 miles in 1973 put over 5 HP more on the ground in one test we did, bringing the engine back into the high 50's HP again. Mine went over 62 HP when I did it to my K2 (everything else stock, that is...)
Rule #2: breathing makes power, more so than boring on these engines. They are under-aspirated, not because of the carbs, but because of the situation in between the carbs and the intake valves. The back side, the stock airbox, will flow well enough to generate over 80 HP with a stock (even paper) air filter, leaving it nice and quiet. The real restrictions, especially in the later heads, are in the way they were cast and the shape and finish of the valves. Honda started out by making sure the ports were all a certain minimum size, and K0 owners have often noticed their heads look like someone had ported them: they did, at first. By the K3, this practice stopped. When I did my K2 head in 1973, I made it look like a K1 head (so I could race in Stock Production Class), which required opening up a slight globe over the intake valve and smoothing off the ridges around the intake and exhaust valve guides. The difference was dramatic and immediate, making it feel like my previous K1 had: midrange rocket! On the K3-later valves, the upper sides were no longer smoothed off, but came in as-cast condition. This made them heavier and they disrupted the flow, and the edge lips were taller (less premachining work was being done). Cutting off the lip and smoothing (polishing) the upper face of the intake valves makes another strong difference. The throat of the intake ports (right at the top of the valves) was not being opened up as much either by the K3, because it cost machinist's time to do it in production.
Rule #3: check for good fuel flow. Some later K bikes had only one outlet on the fuel tank's petcock, versus 2 on the earlier ones (a recent discovery!). On Montana's roads, this will result in low float bowl levels and a lean burn, for sure. Replace the fuel lines every few years so they don't develop kinks, too, when they get hard, and make sure the little filter screen is CLEAN.
Other things: good spark (many different ways to get this), good battery voltage (check the charging system), use ND X24ES-U sparkplugs to raise the combustion efficiency on hiway rides. Use Honda's own sprockets (others rob power from the chain) and an first-rate chain: in the U.S., it's mighty hard to beat the Diamond Powersport chain, which puts more power on the ground than the Tsubakis I tested in the past. And, it lasts longer.
All this presumes stock cams and pistons. The later "F" cam can wake up the "K", but dial it forward about 3-5 degrees, as it was made to run later for emissions reasons. They tend to blue the pipes a little, I have heard, if the jetting is too rich with these cams. This is because these cams have 5 degrees more duration, but it is also 5 degrees further "apart" between intake and exhaust, which will transfer more heat to the pipes.
Finally: the K4-later heads had the rocker shafts locked into one position with 8 little 5mm bolts, to stop a little rattle they made. This made them wear out in about 10,000 miles, and yours are probably shot. Check the rockers to make sure they are still round in their holes, then get new rocker shafts, and when you put it back together, skip the little 5mm bolts. The one longer bolt in the center will hold the shaft in place just fine, unless you're racing. The freedom to rotate will also add a touch more torque out of the engine, and MUCH longer rocker-shaft life. You can use 12mm Thomson shaft, cut to length, with a Vee notch cut in the center to clear the mounting bolt, to get the earlier style shaft. (BTW: only torque that long bolt to 7 ft-lbs on reassembly, or it will strip in the head for lack of threads - dunno why Honda did this...) Honda only sells non-rotating shafts today, for all models. I plan to put up a post this winter about how to "save the rockers" on engines where the shafts can't rotate. But, that's another day from now, as I have to go help my wife (she has a broken leg) at the moment...
