So I have run 4 different combos of jets on my CB750K6 now (all the same sizes) and have a little comparison for your reading enjoyment. I recently picked up this K6 with 27K miles and was last running in 2005. The carbs didn't look too bad when I cleaned them and all the original jets and needles looked serviceable.
After the first clean I reused all old seals and jets and the bike started 3rd kick for the first time. It ran pretty well but would not idle well below about 1200 rpm and the throttle just didn't seem as smooth and it should be.
Next I got 4 Keyster carb kits and used everything in the kit including needle and jet, slow jet and float valve except for the seat for the float valve since it looked a little different from the original. With this set up the throttle seemed a little better but would still float a little and was just not right. Still would not idle well below about 1200 rpm.
At this point I pulled the carbs and installed the stock needle and needle jet with everything else as in the last step. Now the idle was about the same but there was better throttle response especially around about 3-5K rpm.
The bike ran pretty well with every set up but I want it to be right on so I pulled out my wallet and got new Honda slow jet, needle and needle jets. I purchased new main jets but they are not the right jets for these carbs.
Now after installing these and re-syncing the carbs for the 5th or 6th time this summer, the bike will idle pretty well around 1000 rpm and the response seems about right. Runs real strong and smooth. However, now I have either the #3 or #4 cylinder that is missing a little at idle and really low throttle but not above that. Tuning the carbs the idle seemed highest with the air screw at about 1/2 turn out (was at about 3/4 to 1 turn out with other set ups). Looking at the plugs after idle they seem a little white when they always looked perfect with every other set up.
I'm a little confused why I have a slight miss at idle and it seems to be running a little lean. I'm going to go through the #3 and #4 carbs again and see if there is just something dirty.
So I guess in summary, my experience is that with old needle and needle jets my bike didn't idle very well and the keyster carb kit wasn't much better. The Honda needle and jet seemed to clean that up.
