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interestingly, related to last two responses regarding venting/fuel flow, i noticed something when i converted back to the Mikuni.  To get the fuel line connected to the Keihin i used two 90 deg fittings (between short sections of hose) instead of looping the fuel line.  the petcock exits out the side but the angle and location of the inlet on the carb dictated this.  What i found was a noticeable difference in the flow rate between the zig-zaged line and a short straight section of hose (even one much longer than necessary for the Mikuni).  it wasn't difficult to blow through it but definitely a difference between the two.  Not sure it makes sense, given the symptoms but still. 

IF bowl wouldn't fill fast enough (which seems unlikely at idle speed) then wouldn't rpm go up as it went lean, starving for fuel?  Also, how could it have been rideable if it wasn;t keeping up with fuel demand?  hmmm. 

not to say you are wrong but just to understand why-  if Pilot Jet (PJ) is wrong then why would it idle at correct speed at first?  then all of a sudden change?

float level being too low is something i considered as that would make sense but then wouldn't the idle go higher as it is being leaned out?  which it does do if close petcock while running (after a couple minutes).  not sure what it would do if level was high other leak out the overflows or run rich.  that wouldn't explain the sudden drop of rpm after warm-up. 

i appreciate the input.  i'm afraid i might be over thinking it OR something wrong downstream of the carb (which is why i will re-install the Mikuni).

I put a PWK on a 95 Yamaha wr250z (replaced the orig Mikuni).  bike will start and run okay (depending on exact needle and jets) for about 10 minutes.  idle speed then drops way down and at that point the idle speed screw does not change the rpm.  the air screw (AS) will affect how quickly it returns to idle but doesn't change the idle speed.  bike will die if AS all the way closed or if out 4-5 turns.  slide is open a good 1/8" compared to my KDX the slide is barely open.

I've checked the jet block gasket (which was new), the float level, and the jetting/needle is at or close to the 95 yz sizes (they came stock w/pwk). also the carb is a Power Jet model but that is blocked off.  I also have a clone carb (ChiHin i call it) that does this same this.

I'm waiting for a new cable to re-install the Mikuni until I can get some insight on this issue.
thanks for any and all input.