Thanks again HondaMan. On the mixture screw, I am running it at about 2 turns out. I will see what happens if I reduce that to say 1.5 turns. How much of a difference, if any, does the ambient air temperature matter when making this adjustment? Thanks
The ambient temperature, in my experience, doesn't matter as much as the altitude. For example, when I lived in the flatlands I used Honda's "factory" settings for these bikes, but when I came to Colorado (6000 feet up here) I found that most of the bikes needed between 1/8 and 1/4 turn inward on the screws to lean them out acceptably, lest their sparkplugs foul in city traffic.
That said: the float depth in the carbs can change this, if it isn't at the Honda Manual's level(s). If deeper, the screw must go IN, if shallower, OUT to make up the difference. One really good example of this comes from the 'round top' CB750 carbs made between 1972 and 1976 "K" model bikes (of which there are MANY out there): in stock configuration with stock 26mm deep float bowls at 5000+ feet altitude, the normal idle-mix screw setting of 1.0 turns out will start fouling plugs in about 25 miles of 30 MPH-is travel. By 100 miles they will be black. Turning the screw in to 7/8 turn will reduce this to about 250 miles of such travel, while running about 10 miles on hiway at 55+ MPH will clean them back off pretty well. Going in to 3/4 turn will greatly extend the plug life, usually to a whole season, or two, of riding. The tradeoff, because these carbs are extremely simple mixers, is in low-speed performance: between 1000 and 1250 RPM (approximately) there is a 'flat spot', so keeping the revs up becomes a necessity. This one "feature" of the early 750 setup is what many riders get/got tired of, usually for not adjusting the carbs suitably, parking their bike.
One prime example of the early 750 type is my own 750K2, an early one: at 26mm float bowl levels the throttle is a little 'flat' until about 2200 RPM. At 25mm it feels more like a 750K0/1, pulling strong from 1500 RPM in comparison. At 24mm, if I can tolerate the slight fuel weeping along the front edges of the float bowls, there is no flat spot at all. But - at 25mm the plug life becomes 1 season, while at 24mm I will have to clean them at least once that season, and at 26mm they will go 2 seasons before needing attention. In typical commuter riding I would put on between 6000 and 9000 miles each year, depending on how often I got to ride into the mountains. In all cases, keeping the running RPM up improves plug life: I seldom run below 3500 RPM unless in 1st gear, and usually am at 4000+. Not everyone wants to ride these bikes that way, though.
In the era of the PD carbs (starting in 1976 model year "F" bikes and 550cc or smaller ones) the carbs appeared to meet those nasty new emissions laws (tailpipe sniffs), which brought in the 4-1 exhausts in to make this testing easier. Then, the mixture had to be "not adjustable by unqualified persons" (I would have made that "personnel", but I only completed 5 years of college...), so the PD carbs added a "non-popping" air-like-adjusting screw downstream of the fixed mixtures of the press-in pilot jets to add in a touch of extra fuel if needed to tame the NOx emissions or reduce the tendency to 'pop' in the muffler when coasting on long downhill runs. In these carbs the range of adjustment is between 1 and 2 turns (depending on which carbs, and engine size) of effective range, outside of which there is no change. The fixed (pressed-in) pilot jets are leaner than in the roundtop carbs (sizes #37.5 or #35, vs. earlier #40) so the mixture is forcibly leaner. For that reason, the camshafts had to be reduced to 0 degrees intake valve opening (instead of the 5-10 degrees of the early bikes) so the bikes wouldn't fall flat on their face when the throttle is pulled. In the 750, since Honda already had 'type acceptance' approval of the engine from before emissions rules began, they could install (and did) an accelerator pump to improve that throttle-cracking movement stumble. In the 500/550 they weren't so lucky, as the rules hadn't type-accepted the 'new' 550 engine type prior to 1971.
But...in these later bikes with PD carbs (except for the CB750F/K versions) the intake runners on the bikes also become twice as long as in the 750. So, the "rules" we all learned with the 750 went out the window, but most of us didn't realize that (unless we raced, where it mattered more), so confusion began because of the mere fact that with each intake stroke of the smaller bikes, the engine is inhaling the PREVIOUS intake stroke's carb mixture, not the present one. This makes the mixture adjustment at idle speeds always "wet" in the shop/garage, requiring that after any adjustment, the bike must be taken out and run to at least cam-efficient speed (usually 4000+ RPM) to observe how the engine reacts to the change. It is virtually impossible to set them "by ear" in the garage and hit the road with an expected change unless you're also willing to take the screwdriver along, which can be a nuisance. Yet, that is exactly how I do it: I set them at the 'nominal' setting shown in the manual (if it is even SHOWN in the manual, some are not) and go out to flog the bike. Then I come back to read the brand-new sparkplugs to tell me the real story, and adjust from there. And, I do this after the vacuum-balance, because if adjusting the idle mixture between its MIN-MAX alters the vacuum up or down by more than a 0.1 reading, something else is wrong and needs to be corrected before the vacuum readings might be trusted to REALLY show engine performance.
And yeah - this can take all of a Saturday on a freshly-rebuilt engine, pretty quickly.
