Figured I'd update my results on this common CB750 issue for those who find this thread down the road.
I ended up raising my needle clip one notch (so I raised the clip from the 4th to the 3rd/middle position, resulting in the needling dropping and leaning out the mixture). My carbs now have a 40 pilot, 105 main, with needle in the middle clip. All genuine Keihin jets.
My airbox is a K&N with Hondaman's airbox mod, which basically is drilling about a dozen 1/4" holes around the airbox intake to increase breathing. Exhaust is a Hindle 4-2-1 with a pretty open design
My stumble is now completely gone. I was getting an off idle stumble, but also a consistent gurgling sensation when cruising just off idle. Imagine riding at about 25-35mph in the city. Your throttle position is only open a little past closed. My bike sounded really garbled, even though I was running stock jetting with the mixture screws set to the stock 1 1/8 turns out, and a stock Honda air filter with unmodified airbox at the time. Basically a fully stock K4, except the pipe. At highway speeds with a more open throttle position I had no issues, but my pipe was getting sooty around the tip, and my highway mileage was around 30-32mpg.
The bike sounds smooth now, and still pulls hard up to 100mph. I might try jumping up the main jet to 110, as that was the stock jetting for earlier K4s - 40 pilot, 110 main, middle clip. Reason being that I don't feel much change in power between 3/4 open and fully open throttle position. It doesn't fall flat like when it's overly lean, just kinda feels the same. I've read this is a characteristic of these carbs as they 'run out of the ability to mix additional fuel' because the fuel level drops to the bottom of the emulsifier or something, but worth a try anyway.
I went on a 40 mile ride and my MPG increased by 3. The ride was a mix of city and hard/fast highway, and I ended up at 35mpg. I'm thinking longer highway cruising will bring that up to 38-40mpg. Before I'd usually get around 30 for a ride like this. Highest I ever hit before was 35mpg on a long highway ride.
PROBLEM:
The only issue I have now is some minor popping at idle. It's not a loud bang like when you are really lean and engine brake. Just a subtle blubber. I set the screws down to 7/8 turns or a hair less and the idle popping goes away, but the engine idles rougher. It drops down every few seconds and then picks back up. If I open the screws up further, the idle stays at a steady RPM, but it pops a bit. So I'm hunting for that sweet spot I guess.
Hope this helps people down the road!
This one sounds like one of two (or maybe both?) things is happening:
The variable idle suggests the fuel drain rate is just slightly hydraulically 'locking' momentarily when one of the float bowls is low. I only mentioned it about 1/2 page in my book (end of Carbs chapter, last 2 pages) because it only usually happens when the fuel lines from the dual-petcock tanks become slightly too long to the carbs. This makes a slight 'air dam' or 'bubble' in the top of any bend in the fuel line, usually the one furthest from the petcock, and at low-feed rates like idle, this can lower the fuel level in the bowls about 2mm before it gets enough head (or 'suction') to move the fuel past this bubble. Clear fuel lines, while a poor choice in the long term (no one makes them in 5.5mm), show this situation pretty well. This causes momentarily 'slow-idle blips' as I used to call it, and can make for popping during long decel runs, like down a mountainside.
The other culprit can be calcification in the air holes of the idle jet's emulsifier. These are impossible for older guys like me to see now, but you can feel them with a pointy tool like a thick needle. I use just such a gizmo to push the crud into the tube, then clean the tube out. I find this in 90% or more of the carbs I clean, even though they were previously "ultrasonically" cleaned. This, and the larger holes in the mainjet emulsifier tubes, are most frequently affected now because of the switch from older MTBE fuels to the newer ethanol-laced fuels: the MTBE etched the once-sharp (rounded would be better, BTW) edges of these little holes and made places for water in ethanol to corrode them and start the calcification process that slowly narrows them. So, when sticking the needle thru the holes, wiggle the outer end of the needle in a large circle to nudge back the sharp (or etched) edges of the emulsifier holes, rounding them on both inside-outside edges, this prevents the bubbles that form during operation from having something to cling to and speeds up their departure into the fuel stream, making for a more even fuel mix - and a smoother idle.
The phenomenon of "no further speed response past 3/4 throttle" is common after the K0 version of the round-top carbs. This situation was common in all Hondas of the era, except for the CB/CL72 and some of the CB/CL77 bikes with the 3-jet "power jet" carbs. These other bikes had a third jet that fed the upper area of the venturi when the throttle passed the 3/4 mark, because the airflow over the lower needle jet does not change much after passing the wasp-waist middle of the throat opening. This wasp-waist was introduced in the late K0 carbs to try to extend the throttle response with a leaner mixture, as the K0 bikes were REALLY hard on sparkplugs if not ridden hard. Those bikes had 120 mainjets and small emulsifier holes to make the upper 1/3 throttle feel more responsive, but it made the lower 1/2 throttle very wet in the process. For a VERY brief while, some K3 carbs came with a little lifting collar on the intake side of the needle's jet: these were like the ones seen in Mikunis for 2-stroke engines, which suffered the same flow issues. They vanished very quickly and are seldom even seen anymore: I don't know if they were recalled or something, but I haven't seen one since 1973, and then only on a bike or two that came thru Illinois (my shop) in that summer, on tours to somewhere else. They were Keihin carbs, though, and looked otherwise normal. The bikes were pure stock (Vetters on both), so I don't think the owners were super-techs or anything like that. I happened to be changing air filters on both of those bikes (at different times of that year) when I noticed the different needle jets. Overall, I suspect this was why no one really griped about the long throttle roll on this bike's dual-cable carbs: on the K0 you can quickly pull the throttle to 100% opening with the 1-cable design, but this became impossible on the "Grand Prix" pull-push throttle, which was designed to prevent lawsuits from the #4 carb's cable hanging the throttle partway open in the early bikes, following a hard-right handlebar turn. On the dual-cable system, the full engine/carb power is felt at just about the point where the carbs no longer change mixture rate, or just above 3/4 throttle.
BTW: this is where the main difference is felt in the CR29 cabs: they mix all the way up to 90% throttle because of the very different (and expensive) design. It's not the 1mm bigger throat, but the jetting design difference, that makes this change happen. In fact, they strongly resemble (...drum roll...) the CL72 carbs, bolted to a single flange.
