I am currently having the same problems with my 421 cub, last ride was almost perfect but my WOT plug checks were showing lean. I have pwk39's with 170mains, CEL needle on 3rd clip, and 55 pilots. sea level, 50-70 degrees air temp. I called up cascade innovations and they said they set up their cubs with- 160 mains, EEJ or EEK needles on 4th clip, and 50 pilots. They said this needle richens up 1/4 to 3/4 allowing use of smaller mains, and they run the 50 pilots to keep off idle crisp. I haven't had a chance to try this yet. But Cascade has never steered me wrong. Also, I am not sure what jetting differences there are between the powervalve cheeta and the cub.
I'm pretty sure others have got the cel to work with 38's and 39's. I'm still researching previous posts.