Thank you Jerry for the very detailed explanation. And Q2W, I assumed that the slide stop screw was the idle adjustment screw like it is on my keihin carbs. I am not used to dealing with mikuni carbs. All my other 2 stroke quads have keihin carbs on them. So the needle jet is a new one for me. I checked it and it has a R-4 in it. Next time I get into the local dealer I will pick up a R-6 and try it out. Also it sounds like this is normal to a certain extent. So I am ok with it. Wont be able to mess with it too much this week anymore. So when letting off the throttle at higher engine RPM's like decelerating in 5th gear, is it easier on the engine or more desirable for the motor to not fire when coasting? Right now it is just like hitting the kill switch until you pull in the clutch. I think before I could hear the engine still firing in the pipe. I just want what will make the motor live as long as possible. Thanks.. AND after tinkering around with this mikuni, I think I might try one on one of my hondas. Looks like keihin carb jetting for the zilla is pretty similar to what I have in my hondas.........
Mikuni makes R-6, R-8 for a lot of different carburetors. Make sure that you get a R-6 of the same series as your R-4. I think that your R-4 is a 389 series.
If you cannot feel the engine firing when you let off, that is usually a good sign that it is rich when the throttle is closed.
Keihin PE, PJ, and PWK carbs do not have a replaceable needle jet. All needle jets ware out making the carburetor get richer in the zero to about 1/2 throttle position. When the needle jets ware on a Keihin carb you throw it away or find another needle that has a larger diameter before the taper to compensate for the ware.
Mikuni uses anodized aluminum needles and brass/bronze needle jet. When parts have to rub each other, the least amount of ware occurs when the two parts are made of different metals. Honda has Keihin make anodized aluminum needles for all of their OEM carburetors I have seen. All of the after market Keihin carburetors use a brass/bronze needle and a brass/bronze needle jet. I think using similar metals for these two parts is very poor engineering because the Keihin needles and needle jet ware quickly as compared to anodized aluminum needles. The decision for the choice of metal for Keihin needles may have been made by the marketing department and not the engineering department.