Several things to look at:
Air screws: 7-8 to 1-1/8 turn out on the 657 series carbs.
At 1.5 turns out they are fully as rich as they will possibly go, at 3/4 turn out they are as lean as they will get and still run.
If you notice a further difference in performance beyond these ranges, something else (like jet size or float levels) is not right. Float levels should be 25-26mm for smoothest performance, 24mm with perfect float valves and hotter plugs will raise the low-end torque a little bit.
When the low-end is very rich, as yours now is, it will stumble at the point where the slide is open too far for the engine speed at that mixture. Another thing on the K0-K2 that makes this happen is: the spark advancer springs are too soft (or even loose) and/or the shim washers underneath the points cam are gone, letting the cam wobble at low speed and then straighten up at about 1800 RPM. This sudden timing shift (backward) as it straightens up causes the stumble.
Hondaman, I thought turning the screws out let more air in, leaning the mixture. You're saying it richens the mixture. I have the 657B carbs off a '74k and the screws are on the airbox side of the carb. Prettt sure opening them up leans the mixture?
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I get this comment every Spring (and summer, winter, fall...)....no.
Here's how these carbs work, as outlined in my book (plus some of the stuff I left out at that time):
There are 3 pressure zones in these carbs. The highest pressure is outside air, pushing its way into the float bowls thru the little vent hoses. These hose must be routed to a quiet place, like up under the seat or behind the airbox, to work right above 40 MPH.
The second-highest pressure, which MUST be lower than the outside air pressure, is the air inside the emulsifier jets. This air comes into those jets on the idle side thru the air screw, and for the mainjet thru the little (brass) hole in the bell, running up to the main jet's emulsifer (some folks call this the "mainjet holder").
The lowest pressure is found under the slide of the carb, lowest at the front where it pinches off the throttle (this is called "throttling the air flow") air to the engine.
When these engines runs a single carb on a single cylinder like they do, there is a very short low-pressure "event" during the intake stroke, which is about 20% of the total time for a cylinder (I note this because it is NOT a steady vacuum, like in a car engine with one carb feeding many cylinders where the vacuum is constant, and where the metering system is VERY different for these). During the very short suction pulse at low engine speeds (below 1/4 throttle), the idle jet has to supply LOTS of gas for a short time to make up the 8:1 air-to-fuel mix needed to make these engine start, so the idle circuit has a large opening at the air screw and a separate emulsifier (right on the pilot jet), pointed directly under the highest suction point at the engine edge of the slide. When you then average out this 8:1 mixture for 20% of the run time of one engine cycle, you get close to 12:1 A:F mix ratio for the power cycle. So, they run very rich up to about 1/8 throttle, just so the engine can idle: it has to taper quickly from 8:1 at idle to about 12:1 by 1/4 throttle, so it has a natural tendency to feel slightly flatter as the throttle begins to open (i.e., without an accelerator pump, like the post-1975 engines have).
The "push" that puts the fuel up the throat is: the higher pressure inside the emulsifier tube, compared to the lower pressure in the throat above it. No, the engine isn't "sucking in the fuel" like a car engine (it's just sucking air): the carb is actually squirting fuel up into the throat AFTER the intake valve has closed, wetting the whole area. Then the NEXT intake stroke collects that wetness. I have seen hi-speed camera videos of this (in 1969-1970 in college), it really does happen this way...the "push" comes from having air pressure enough outside the emulsifier's holes to push the fuel up the center of the jet, which itself pushed up to the inside of that emulsifier from the bowl (thru the jet hole) when the previous intake stroke left a lower pressure (suction) momentarily inside the emulsifier's chamber. This "pumps" the gas from the bowl up into the center of the emulsifier, where the air pressure from the air screw then pushes a little more (and aerates it, too) up the tiny hole under the slide, just AFTER the last intake stroke happened. This makes a vapor-like blast appear above the tiny hole by the slide, which wets everything: if you take off the carbs and look, you can see this pattern where it makes a clean area in a vee shape toward the engine.
Now...about the air screw: if the air screw closes off all the air, the bowl has to try to push the fuel all the way up the jet to the throat: with the tiny hole involved, it doesn't get all the way up. The weight of the fuel makes it tend to simply block off the idle jets' metering hole, instead. As the air screw starts to open (about 1/2 turn) the fuel gets enough air to lighten (aerate) it in the emulsifier at each intake stroke to bubble up, but there is not yet enough air to help push it all up the throat. As the screw approaches 1 turn, it is close to optimized for the size of the aeration holes you see in the pilot jets, and the fuel erupts into a wet plume in a bloom of vapor as it exits the hole into the throat. Opening the screw a little more, until the most air possible is passing thru the emulsifier holes (about 1.5 turns) adds a little more lift and slightly more fuel, draining the tiny emulsifier in that one stroke: beyond that, the air becomes too much and it breaks up the vacuum in the tiny throat hole and the fuel stops moving any faster because it is limited by the size of the pilot jet's feed hole (#40). It thus reaches the rich-as-it-can-breathe point, and no further air will go thru the holes to the jets, even if the air screw is fully removed. After about 1.75 turns, this is the result. So, it gets a little richer by about 1.5 turn or so, but no richer beyond that - IF THE CARB IS WORKING CORRECTLY (i.e. it is CLEAN).
Now - IF your air screws ARE making a difference beyond 1.5 turns, then something else is wrong, which is usually blocked air passages. These can be in the carb's body between the air screw and the idle jet, or the throat's tiny hole, or the emulsifier holes in the pilot jet itself, or the pilot jet's tiny hole might be calcified, or the float bowl level might be too low. So, cleanliness in the idle circuit is VITAL to make these things behave as they are supposed to, and if not, all sorts of confusing symptoms arise. And, this circuit has more to do with the way these bikes operate than the mainjet circuit does, until 4000 RPM.
Moving onto the mainjet: I have actually seen the air passage from the bell area of these carbs to the main emulsifier COMPLETELY blocked before: even on my own bike after it sat 5 years while I beat cancer. It corroded itself closed from moisture and being "put away wet" once too often. When this happens, no amount of adjusting ANYTHING will make it run right. This passage has a small brass port in the bell area where the incoming air is the highest possible pressure available (the quiet "donut" area outside the venturi) so it can push a lot of air into the emulsifiers in the mainjet circuit. A lot more fuel must run up this circuit, so more heavy lifting is needed: this air is about 3% less than the bowl pressure, so the holes in this emulsifier are much, much bigger than in the pilot jet (where the pressure difference is closer to 6%) to help lift the fuel. Again, if these holes, especially the larger upper holes, are narrowed by calcification or just plain crud, then the fuel will not start lifting at about 2500 RPM (1/4 throttle) very well, and the bike stumbles as the idle circuit starts to fall lean and this circuit isn't doing its job yet. Aside from the afore-mentioned cleanliness "fix" for this, lighter fuels (e.g., ethanol) tend to wet the emulsifier too much, so adding slightly larger holes (or at least REALLY cleaning the existing ones) will help dry it out a little more so it will blend better with the waning idle circuit. This improves the flat spot often felt on these bikes after they have been sitting too long without some carb love.
Sometimes riders try to change the jetting to "fix" a problem that was never there. All of the 750 bikes before the ill-fated K8-F3 series were too rich, resulting in fouled plugs as a constant problem: when the carbs get a little dirty in these passages, this gets worse, quickly. In fact: when the 750 first came out, it had 120 mainjets with .033" (large size) holes in the emulsifiers: you could not ride more than 1000 miles before changing the fouled black plugs. The first "fix" for this was: Honda switched to 115 mainjets and lowered the floats from 24mm (as measured in the 'modern' manuals) to 25mm, then by the diecast 750 the floats were always staggered with one set at 24 and one at 25mm, every set. In the mid-year K1 they again lowered the mainjet to 110 size and simultaneously opened up the emulsifier holes to .035" in all 8 larger holes. This allowed the fuel to be blown out of the engine with a shot up the freeway ramps, and Honda issued a Service Bulletin suggesting the "...ride the bike more spiritedly..." technique be told to the owners. Bankers and lawyers didn't appreciate it, but we did...
At this time, the airbox became a little bit more restrictive (still had the HM300 pipes) so as to increase the operating range of the idle air screw more toward 4000 RPM, causing it to run slightly leaner in the upper range of the idle circuit, before the mainjet started its thing.
The K2 started out like the K1 with #110 mainjets and staggered-height brass floats (like mine), but the last of the K2 had #105 mainjets and plastic floats all set to 26mm height for a leaner burn with the new HM341 pipes.
When the K3 midyear changes came out (around 1/73) there was a big change in the carbs. The idle air screws lost their little air hole in their tips, which effectively pinched back the airflow for any given setting and also stretched out their effective RPM range. At this time the mainjets also dropped to 105 size, and the floats were adjusted leaner to 26mm, with both (usually plastic) floats at the same setting. This effectively stretched the idle air screw's adjustment authority to about 3500 RPM, and a flat spot developed in the performance between 3500 and 4500. BUT...the sparkplugs were finally staying a lot cleaner, especially at the 55 MPH freeway speeds we were all stuck with in those days. Some riders rode more than a whole season on one set of plugs, which up until then was unheard of in this bike! In the K4 the spark advancer was changed to enhance this effect (slower advance curve), the rest being history and legend.
These are all just notes from my old shop notebook that didn't quite make it into my book, as I thought at the time that no one would be interested. Sometimes I wish I DID put it in, to help quell this confusion that the Internet still keeps alive.
So, it boils down to this: on all of Honda's carb'd bikes before 1988 (at which time I quit keeping track), the idle air screw makes it go LEANER when turned inward, RICHER when turned outward, and they have a very limited range because the air passages they control are very small. AND...these passages get "re-tuned" by dirt, very little of it, so use a good air filter!
