The CB750K weight limit is slightly conservative, and it is based on 4 major items:
1.) Front spring rate,
2.) Rear spring rate,
3.) Spoked wheels,
4.) wheel bearing torsional loads,
and, a 5th item: the frame tubing size.
I have carried as much as 640 (measured) lbs on my 750K2 on (very) long trips. The main problem becomes irregular handling in turns, as the wheels flex and side-load the frame, which also flexes.
To minimize these effects, the following upgrades will help:
1.) Tapered steering head bearings.
2.) High-quality wheel bearings (all 5 bearings). I am currently experimenting with Timken tapered Roller bearings in place of the stock-style ball bearings, and will let the forum know how it goes.
3.) Air forks up front, minimum 10 PSI.
4.) Improved shocks on the rear. Use 120 lb springs for heavy loads, 130 for REAL heavy loads. Increase rebound damping with heavier loads: some shocks let you adjust this item.
5.) Tune the spokes. First, make sure the wheels are true, then tap each one with a small wrench and listen for even pitch across all of them. They should be pretty close.
6.) Install bronze bushings in the rear swingarm. The stock Honda ones wear out in about 15000 miles and allow flex.
7.) Run Avon RoadRunner or Continental tires. They have the best payloads in the size range. Stay away from "radial" bike tires. Don't run TT100 style of any brand with heavy loads: these trigonometric tires carry only 300 lbs without distorting themselves and making the problem worse (they're good for hotrod riding, though...). Don't run low-profile (80 or 85 series) tires, they have low weight ratings.
8.) Make sure your brakes are good! Weight REALLY increases braking distance.
9.) On K0/K1 bikes, check for possible frame rusting near the lower engine mounts, inside the frame. This can be done by loosening the lower engine bolt completely, then tapping firmly with a plastic hammer along the lower frame. Listen for a sudden change in the ring of the tubes, a "dead" sound. Is it's prominent, you may have frame rust, which occurred in these models, and caused flex issues. Make sure it's not working its way out, like it's rusted a lot. Many later K0/K1 models had little holes drilled on the bottom of the tubes to drain condensation buildup, yours might have them, too. All later models had holes or treated frames.
10.) Loosen and re-torque all frame bolts, just to be sure.
I carry: Windjammer fairing, Krauser saddlebags, Bates tourPak (large), tank bag (large), fairing lowers, 2 riders and full leathers and auxilliary riding gear: heavy. I run Avon tires, 12-15 PSI forks and 120 lbs springs rear on 4th notch of B&G shocks on these excursions, TT100 tires, 10 PSI forks, 1st notch on shocks, no saddlebags or tourpak when I ride alone for fun & mountains (I live in Colorado). The difference is: 170 lbs full "light" load, 440+ lbs on a short trip, 600+ lbs on a long trip with camping gear, etc.
Remember, fairing, extra gadgets, etc., all add weight.
