Hondaman have you ever done any modifications to the accelerator pump?
Mostly, after replacing the diaphragm with a new one, I find uneven squirts happening. I usually find the little hoses between the carbs to be either unhappy (i.e., loose from age), weeping slightly, or the wrong kind of hose. Sometimes the act of removing the parts causes a tiny (and I do mean TINY) grit bit to move to the tiny jet tube(s), and then that one barely dribbles when the others squirt.
Not to worry excessively over it, though: more than half of the decently-running 750s with these carbs work fine so long as at least 2 of them still squirt. This usually ends up being 2 and 3, for some reason? Heck, the earlier bikes didn't even HAVE them!
The reason for the squirt is simply because the DOT/EPA required Honda to meet car-emission rules with the 550/750/Goldwing bikes, starting in 1976. To do it, the carbs got leaned out and the cams changed to have 5 degrees later intake valve opening (exhaust timing stayed the same) for more scavenging to occur. This made the bikes cold-blooded, and pushed the RPM power curve upward about 800 RPM overall. Since the emission rules required testing at idle (less than 1000 RPM) and 2500 RPM (typically 55 MPH on an old Detroit V-8 engine of the day), Keihin cleverly changed the needle tapers to be thicker (leaner) until 3000 RPM (1/8 throttle), and added slip-on plastic caps to the idle air screws (and wrote (DO NOT REMOVE CAPS in the Owner's Manuals) to, IMHO, poke fun back at the DOT...but, it made the bikes meet the rules. In the 1977 era, California's CARB bunch decided to join the fray, and Honda had to come out with...your carbs. First it was the PD42a, which mixed more accurately-per-RPM range than than the old roundtops, and the idle jet was first #40, then #38 partway through its tenure: in the PD42b this was changed to #35 on many of the California bikes (#38 on the others) and the floats were lowered to lean out the upper midrange, too. These carbs EAT the exhaust valve guides from lean heating, which makes low compression when cold because of poor exhaust sealing, and causes much shorter engine life on the K7/8 and F2/3 later bikes.
So, all of this known...to improve the PD42b carb performance with today's fuels - if you can get some, change the idle jets upward. If yours has #35, get the #38 (they are actually #37.5, but that's trivia..), if yours are the #38, get the #40 size instead. These are available in used sets of K7 PD42a carbs, if nowhere else (I might have some?). If your floats can be adjusted, make them 1.5mm richer (deeper) than they are now, and then use K7 (PD42a) settings, like 1.5 - 2.0 turns out on the idle air screws. This will stop the cold-bloodedness and make the bike more driveable.
If yours has the #35 pilot (idle) jet, don't jump up to #40 right away (stay with just the #37.5/#38), because the air port to the #35 jet is smaller than the ones for the #37 jet. This was done to let the air screw setting remain the same during this change, so the manuals did not have to be re-printed, and the DOT then get too nosy...
The air screw will not work properly if turned in past 1.0 turn, nor out past 2.5 turns: it just acts rich, in both directions, then. This is because less than 1.0 turns pinches off too much air to let the fuel get pushed up the jet (so it doesn't mix at all, and won't burn) and more than 2.5 turns lets in so much air that it floods the idle area under the slide and makes everything too wet. Both burn poorly, and foul plugs.
