Okay, you've paid off Christmas and it's been too long since it was warm enough to ride, and you're planning for next season. Wanna ride faster or get better MPG this year? Here's some [higher-dollar] tips for those of you who want to "get into it"...
1. All inline fours have this in common: crummy metal holding the cylinders together. As a result, after about 5,000 miles of running, the bores are no longer parallel, nor are they round. Power is lost, almost 10% by our 1970 rear-wheel dyno tests. To fix this, have the cylinders bored one oversize from where you are now, and tell the machine shop to make sure they are in parallel. They "wander" as much as .010", in my experience. Simply boring one oversize and refitting recovers all that power again.
2. All Honda gearboxes have this in common: their teeth are hog-cut. These tiny steps on the faces of the gear teeth rob a considerable amount of power and smoothness from the crankshaft. Aside from a LOT of hand polishing work, you can do this: remove the gearshafts and spacers, bearings and shift forks. Apply lapping compound (like for valves) to the face of the teeth and set the shafts into V-blocks that are spaced like the crankshaft. Load the mainshaft shaft with a soft piece of pine wood (clamped onto the shaft) and turn the countershaft by means of the countersprocket, sliped onto the shaft in its normal place. Do this until you're bored to tears, and you'll soon see shiny spots on the teeth of the gears where they mesh. This will transfer as much as 10% more power through the gearset to the countersprocket. Now, clean them like your life depended on it, then dip everything into 20w50 oil and reassemble.
3. In the heads: all of the inline fours except the 400-4 had breathing issues. Grinding a "pocket" above the intake valve(s) and matching the ports to the carb tubes will improve them, especially at high RPM. Polishing the intake valve(s) helps, too. Don't back-cut the intake valves or you'll pop off a valve head - an expensive day, at best. (Backcutting exhaust valves will make them burn, and quickly - the CB650 malady). Trim (narrow) the intake valve guide bosses for a smaller profile, and polish it. Smooth, but don't polish, the intake tract. If you're good at heliarcing, raise a bead about .100" high along the bottom of the intake ports (CB750 only) in a half-moon shape, then smooth it off along the sides so it "restricts" (apparently) the bottom passageway about 1/3 of the way around. The floor of the port should ramp up to it, then be fairly abrupt in fall-off on the valve side. Do this at the point where the intake track bends down toward the valve (no, it's not easy). Although not obvious, this mod creates some "free turbocharging" at RPM above 6000, all the way to 16000 (lots of work), which deals more with fluid dynamics than I will put into this post.
4. Jetting. ALL of the inline fours came rich from the factory. At sea level, we started with a 10 size less than what it came with. Here in Colorado, it's not unusual to drop a 30 size main jet. The jet needles are almost always set too high (except the 500/550, too low). The cutaways on the slides were close for sea level, too low for high altitude by about .030". My 750K2 runs at 20 less than stock for general cross-country touring.
