No, to the first question. The total size of all output orifices (LJ included) vs. the size and stroke of the diaphragm, vs. the stiffness of the AP plunger drive spring is what combine to give you the duration of the squirt. The AP discharge nozzles are produced in various sizes, but because of how they are installed (pressed) and their lack of general availability, they aren't a practical tuning tool at all. And as to the second part of that, yes, of course LJ's do affect the total output, as I just outlined.
Is diaphragm stud length the only method of adjusting duration? All else equal, will different LJ's not also have an effect on duration?
Keihin builds carbs to the specs of their customers who have presumably spent a long time in figuring out how they want the carbs delivered. They don't offer the springs as a tuning tool, true. But many other carb manufacturers do, or offer a way to adjust the preload on the spring. Holley, Carter, and Weber all offer selective jets to control not only the leak circuit, but also the output circuit. I know these are automotive carbs, but the car guys have had far more sophisticated carburetors avilable to them for decades than the newest FCR is. Often when a company doesn't offer a part for tuning, it's because they don't see the profit in making them available, or because they just know better than you, so you don't need it.
I’m not aware of any alternative OEM springs for the AP lever. If this was the design intent, it seems they did not intend on making the volume adjustable this way. Besides, Keihin designed the carb, not Yamaha, right? They did, however, offer various LJ's and diaphragms for adjustment.
I'll half agree with this. However, the O-ring or stiffer springs do not prevent the use of leak jet selection to tune the AP. In fact, because these two approaches allow you to use a diaphragm with a shorter travel, they actually allow more tuning latitude than wiring does. With wired linkages, the pump rod must move through the entire throttle opening, and changing the leak jet is the only adjustment you have left that you can still make.
I re-read your post and think this is where we may have a misunderstanding. I agree that hard wiring and grinding off the diaphragm stud is "caveman" (no offense to any real cavemen:smirk: ). I think, though, that there is merit in the O-ring mod and in adjusting the duration and volume via the available range of diaphragms. Based on this, I also think it is perfectly acceptable to further tune your AP with LJ's, especially when you tune for various seasons, something the O-ring or stiffer spring still do not allow.
Not semantics at all, they are two different things. The first modern motorcycle engines to have LJ's were the bikes with smaller, high strung engines with hugely oversized carbs, i.e., the YZ250F's. These engines were had such aggressive cam timing and big carbs that they had to have a really big boost from the AP. (BTW, you know that prior to the YZF and the FCR, there were no AP's on any thumper, and the big XR's didn't have one for years, if it even does now?) Such a massive dose of gas being dumped into the engine every time the throttle is bumped a little, the extra fuel became a problem, and so the solution of a leak circuit was brought over from the car world, where it had existed for years, to dump the smaller, slower deliveries before they reached the air stream.
Whether the LJ is used to reduce or eliminate (or even increase) pump shot or to alter squirt volume and/or duration is semantics.
The YZ450 did not even have a leak circuit until '05, nor did any previous model of the big blue bike, but I'm sure you could tune the AP on one if need be. My point in this is that the leak jet is not a cure-all for everything that ails your throttle response, and a perfectly tuned AP will not make up for incorrect jetting throughout the rest of the carb, as some people (not you, I would think) are now starting to believe.
What I would really like to see is an adjustable stop screw for the diaphragm, and a way to change the tension on the AP linkage spring. Then, the LJ could be used to do the final trim, and would rarely need to be changed more than one size.