In striving to get a bike running as perfectly as possible, we always hear about the importance of genuine Keihin jets because aftermarket brass is often manufactured to different specs than the real stuff.
But I never hear about the pilot screws. They do an important job, and it got me thinking about the work I've done on my carbs over the years. I have all original jets, floats, and needle hardware, but I know that I replaced the pilot screws with aftermarket stuff some time ago. The bike runs well, but I always wonder if it could run better with the genuine pilot screws.
Do the same issues plague pilot screws as other brass bits? Lengths, tolerances to taper, diameter, etc.
Well, by now the soft inner 'tunnel' (in the carb's body) for the air screw is [re]formed to match your new air screws, so they should be fine so long as you don't go back to the originals.
What I've noticed on the Keyster air screws is: sometimes they are the same tip taper as the Keihins, and sometimes they are not. (?) The carb bodies are soft enough that fully seating the screw sets the inner taper of the hole (even when brand new) so they match up well afterward.
What seems to be more important is: whether the Keihin air screws had the little hole thru their tip, and whether the new Keyster screws are like that, too. Both the kits and the carbs come both ways. The 750's 7A, 657A (almost the same) and 657B carbs had the holey-tipped air screws until about 3/1972 builds, and after that they came both ways until the 750K4. Then they became solid-tipped screws.
In actual practice, since the screw is easily adjusted, it doesn't make a lot of difference in operation. If, for example, the original air screw was solid-tipped and the new Keyster screw is 'holey', the Keyster screw will need to be turned slightly inward as compared to the setting found in the Keihin version (and vice-versa). Either way, it is about a 1/8 turn difference, so it's pretty simple to find with a few test runs.
What DOES matter more is the Keyster jets, both pilots and mains. Their stamped numbers act like: [Keihin number x 0.93]
or in other words: they run about 7% lean compared to the same number in Keihins. This can be handy if you're trying to get in-between some of the more-available Keihin numbers, like maybe you if want to have a #107 mainjet:
Keyster #115 mainjet becomes (115 x 0.93)= 106.95 (close enough).
I've used them like that when the owner sends me a set of these roundtops for rebuild that already have the Keysters aboard and we can't get a new set of Keihin needles and jets.
The Keyster needles are more problematic: they start their tapers too "late" as you move down the needle (plus, they are larger diameter throughout, so you must also use their needle jets in the carb's throat, and DON'T try using Keihin needles in those as they won't work at all, or vice-versa). This "late" situation means that at around 1200 RPM the engine starts to fall off the idle circuit's control and the needle is supposed to be open enough for the fuel to come up the mainjet's needle jet - but, the Keyster needles start to let in fuel at about 1400-1500 RPM, so there is always a 'pause' or 'hesitation' felt when in 1st or 2nd gear and rolling slowly at low speed, like in a parking lot situation, and it starts to recover with more power rather suddenly about 1500 RPM. Sometimes lifting the needle(s) by a notch or two will help smooth this over. The Keyster needles with their mainjets can be 'off' the equivalent Keihin flow as much as 15% at 6500 RPM (lean), so the bike feels low on power: using larger mainjets will solve it, but might take several tries to find the right one. What I've seen so far is: if the OEM Keihin was #110 and all the parts were changed to Keyster then the mainjet becomes about #130-#140 size for the same fuel flow in the rountops (or use Keyster's #125-#135 if your Keihin mainjet was #105).
This also carries over to the PD carbs, but the numbers and needles are even more different there.
Either way, the Keyster parts can be made to work in the end, but you will spend quite some time with their jets (and needles) to get them sorted out.
Also: all modern float valves are stiffer than the old 1970s-style Keihin parts. This means your float bowls will always run a little bit lower than they used to with the softer-sprung valves. Running the fuel level a little deeper (1-2mm) will help with this issue.
