I have a wicked pop on the bottom when I cut the throttle off hard.

Current setup
KLX300 head/exhuast pipe
Snorkel removed, top on, stock filter
40 pilot
132.5 main
3.5 turns out
replaced needle with adjustable, third notch down from top

The pop tells me lean when I was running the stock 35, still does it with the 40 pilot, will readjust the fuel screw to see what we come up with, any other suggestions much appreciated.

 

Purely by the numbers, I doubt your lean... 132 main, 3rd notch down, and 3.5 turns out is very rich for these bikes.
I just did the mods myself, and I'm at 128 main, 40 pilot, 3rd notch down (with one washer from Dynojet kit), 2 turns out...
My bike runs pretty darn good, but I doubt it's perfect first crack... I still want to do some experimenting.
(I also have full muzzy, UNI air filter, breather mod, no smog crap, etc)

 

Found what I believe to be the problem, jetted many a four stroke and this one was baffling me(plus the carb is a pain in the **** to work on). After turning the fuel screw back to 1.5 still had the problem then the bike started dying on me, so I traced back to a fuel line I had broken and replaced with cheap rubber tubing, it was pinched and sucking in air, will need to replace with the right line tomorrow and start again .................TW