In 5th under WOT I have been having issues with what seems like backfiring (popping) and then stalling soon after, any insight would be great, possible jetting problems. Mods are as follows, Yosh arrestor, FMF Pipe Bomb, K&N and no lid.
Terry

 

Was the bike re-jetted after the mods were done?

 

Both are symptoms of lean.
Is the intake pieces snug and leak free?
If you put the lid back on....does it get better? It should, and if it does, then the main jets need to be bigger.

 

I had the same problem on a dirt bike once. It would run perfect but, above third gear it would start poping and missing. I thought it was the transmission or jetting... It ended up having a bad spark plug cable that was cutting in and out. I have no idea why it only happened in higher gears though..