Increased main jet to #110 as part of long-term tuning for altitude and K&N air filter, and did frequent road testing.
Replaced steering bearings. Long overdue as evidenced by the classic self-centering notchiness. Used all factory parts and factory special tools.
Began engine oil experimentation with goal of determining effect on shifting. Started with a multivis synthetic. After that went to single grade 50 conventional. And I'll be trying others. The synthetic surprisingly did not affect shifting. The 50 grade greatly improved hot engine shifting (together with the synthetic non-result this shows that viscosity is more effective than anti-friction) but at the expense of notchy cold engine shifting and much longer warmup time. Learning and experimenting. One thing I learned is that the choice of oil has a huge effect on warmup, and this made me revise my thinking about the value of a carburetor fast idle system. So I put one together.
Revised hazard lights wiring. Among other things, installed a dedicated switch. For more than a half-century I have been making this mod using the factory kill switch, but decided this year I wanted my kill switch back. Some measure of prudence seems to have come with old age.
Diagnosed possible left side oil leak. This model has something of a history with oil leaks on the side of the engine that contains the countershaft seal, pushrod seal, shift shaft seal, shift drum seal, oil pressure switch, oil pump, etc. There was no leak however. Discovered was actually a minor fuel leak that was running down side of engine. Replaced one of the carbs' float valves (I didn't replace all because they are only a little over five years old and are high quality--and uber-expensive--OEM).
Revised clutch cable routing.
Installed LED headlight bulb. Interestingly, this did not reduce the headlight's draw on the system. This however is not a problem and the LED is a better headlight. I have no plans to convert the tail and turnsignals.
Installed a CB650 primary chain tensioner. This includes the needful changing out of the original oil pan for the deeper 650 pan. The engine was removed from the frame and a hole drilled and threaded in the crankcase.
Rebuilt engine top end. This included using a 550 cylinder assembly and boring it to fit 61mm pistons. The sleeves were also machined to fit the 500 crankcase. The result is 591.5cc and 11.5:1 compression (which suits my altitude very well). See my 2025 file for the later, planned second stage for the top end.
Replaced cracked intake manifold rubbers and all clamps. Used all OEM parts.
Modded carbs with CB350F fast idle system. Minimal carb body mods were necessary, but all of the choke parts from the 350 carbs were needed except for (the obviously smaller) choke plates, making this a pretty involved (i.e., expensive) mod for the average person. The CB500 doesn't really need a fast idle system. Good tuning makes it not necessary--this engine will happily idle at 500 rpm cold. However, my experimentations with different oil viscosities has shown that heavier oils significantly affect warmup time, hence the fast idle mod.
Bored out carbs 0.5mm. This was done by burring out 0.5mm from the top of the oval ("keyhole") bore. Boring the venturi the conventional way is a big mistake on keyhole venturi Honda carbs.
Installed one of those throwaway stick-on clocks. I like it.
Replaced original petcock with NOS factory part. Surprisingly, they're still available. I like and believe in virtually everything concerning the CB500 four. The petcock isn't however my favorite part. It's so poorly made it's surprising it came from the early CB750. I may eventually put on a Pingel.
Replaced cam chain and cam chain tensioner slipper.
Feb 2025. Results of the top end rebuild: Nice! 175 psi compression. Improved midrange. And a surprising, new, high-rpm lunge. This is at a 10 degree intake timing. On the schedule will be a little more cam advance to 12-15 degrees intake. Want to move that power lower in the rev range. Haven't put the modified carbs on yet. Still comparing apples to apples.
May 2025. Decided the extra torque warranted a gearing change. Optimal gearing (via final drive sprockets) is a balance between acceleration and top speed, or technically, a matter of making best use of torque (the name we give combustion's force). Said another way, a bike comes with the gearing that best matches engine power and the loads of the road. If the engine has the power, tall gearing is best. Torque-emphasizing gearing isn't needed. Lacking power however, gearing that makes use of torque (via rpm) is chosen. The rebuilt engine's newfound power should permit me to re-gear for top speed without significantly losing acceleration. The "new" engine should have torque to spare.
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