KZ400 W/Keihin PE28's - Stumbling at "cruise"

Jetting question!
My bike idles great, and pulls great throughout the entire rpm range. However, when the bike is cruising comfortably, it stumbles. This happens at any speed. It pulls phenomenal, but it doesn't just "cruise".
Plugs are showing a rich condition.
Carbs are Keihin PE 28's.
100 Mains
40 Pilots.
Any thoughts appreciated. This is the last demon on an otherwise phenomenal little bike!

Since you say that it happens while cruising, I'm assuming that it's happening mostly between 1/4 and 3/4 throttle opening, so if it were me, I would try lowering the needles one clip position. (Raise the clip, lower the needle) Or try bigger (leaner) needles.

I like your paint scheme by the way. Nice looking bike.

I'm sure other better qualified people will come along with better advice but I'll my hat in the ring.

My initial thought was that perhaps the pilot jet needs to be lowered a bit - maybe a 37.5 - and then retest. But you could get a little more detailed in figuring out the issue.

1. Find somewhere to add a ring of masking tape by the throttle and make marks on it for 0, 1/4, 1/2, 3/4 and full throttle positions along with a corresponding mark on the grip to point to those spots.
2. Go for a ride and determine in what throttle position you are getting the stumble.
3. Once you have the throttle position determined then do a plug chop at that specific throttle position to figure out what the plugs look like at that specific point. Put in a new set of plugs, ride in the throttle trouble area for a bit, let off the throttle, pull in the clutch and hit the kill switch and pull over somewhere safely where you can remove the plugs and check them. Based on the look of your plugs overall you may indeed be rich at that trouble point or you may be a little rich overall but lean at that specific trouble point. You will not know without a plug chop at that throttle position.

Here are a couple of images that show what carb circuits affect each throttle position. They are for a mikuni but the Keihin PE 28s are very similar and I suspect they apply to all carbs more or less.





Based on those images if your stumble is in the 1/4 to 1/2 throttle range then I'd look at clip position of the needle or trying a leaner/richer throttle valve cutaway. If the stumble is int he 1/2 to 3/4 range then you could look at a different jet needle/needle jet combinations.

So that being said I would probably take a "experiment with the simplest/cheapest thing first" approach. I would try adjusting the needle position up/down one clip and see what that gives you. I'd also go one step lower on the pilot (since you said it appears to be running rich overall), maybe to a 37.5 and see what that does. And then if I didn't fix the stumble then go through the steps I pointed out above.