Here's a tip for those of you who like the stock pipe style (4-pipe models), but want a little more midrange pep: find the pipes from the CB750K and CB750K1 models. They had a straight-thru design with glass-packed baffles, needed repacking about every 10k miles or so. These will add almost 10 ft-lbs of torque at 5000 RPM if you rejet properly, which is: whatever size mainjet you now have, raise it by 20 and lift the needle 1 notch, unless you're running K&N air filters, then leave the needle alone.
If you have the K2-K3 pipes, these have 5 chambers in them. Drilling 4 holes of 1/2" diameter in the last baffle (remove the plug piece at the end first, then reinsert the plug piece after the mod), will raise almost 2 HP at 6500 RPM, no other changes needed. Make sure to keep the rubber interconnects between the pipes in place, these help more than you realize at higher RPM.
If you have the K4-K5 pipes, these have 7 chambers in them. They can be modified as the K2-K3 pipes, but the holes must be drilled through the last 2 chambers, which can be tricky because the exit in each chamber is spaced 180 degrees from the previous chamber. There is slightly less to be gained from the K4-K5 pipes because the chambers are shorter.
I like the stock, voluptuous look, myself...I like the quiet, too!
Modification 5/2020: the modern-day gasolines burn much slower than the old-time 1970s fuel: there was another "notch down" of the burn rate around 2015 or so, nationwide (it's all for catalytic convertors). If you raise the mainjet by this much today, it will probably foul the plugs(!). So, if you started with the typical #105 mainjet of the post-K1 era carbs (starting about 3/72), raise it to only #115 and switch the needle to the middle notch instead of the #4 where it probably is now. Drill out the 8 larger emulsifier holes to 0.039" and open the smaller ones to 0.027" or so. This will keep the plug fouling to a minimum with these pipes. Also: don't let the spark advancer over-advance: instead you might even think of backing it off a bit to reach only the lower of the 2 full advance marks at 2500 RPM. It should not reach full advance before 3000 RPM with modern fuels, or it starts spitting back at the carbs, making it run richer than it should.
