z400 jetting

[fmftrx300ex]

my 2003 z400 has a motoworks slippon a uni air filter and an open box currently i have a 25 pilot and a 157.5 main jet it idels a lil choppy and after a couple good runs up and down the street after it got nice and warmed up it started to idel kinda lightly and low and stalls on a hard jab of throtel (this is in nutral) it will rev up if ur easy but seems week off idel am i to lean? my piolet screw is only 1 1/2 turns out let me know what uthink

[LPR]

You are surely lean on the pilot. First, get a NEW sparkplug.
Start the z and let it warm up. Next, turn the screw to the left 1/4 turn. Wait 10 seconds for the engine to catch up to the change. Continue this 1/4 turn at a time, until the motor starts to run rough again. Turn the screw IN one 1/4 turn (it SHOULD be at its smoothest idle) and you are done.
If you get more than 2.5 turns out you should increase the pilot one size, rinse and repeat.
Does the z backfire when you let off the throttle (engine braking)?
If it does, it is surely lean on the pilot. Only other thing would be an airleak somewhere.