yep if its backfiring through the carbs, sounds like your timing may require adjustment.
BTW just saying if you order 120s or 115s without sending those carbs to PJ you are wasting your money and time and efforts.
Surely, By now you got to be asking your self...
"If its running rich at 2k rpm, How will installing larger (richer) Main Secondaries (4k + rpm) gonna help with that?"
ANSWER...ITS NOT
So, go back, click the link I posted, DO EVERYTHING ON THAT LIST.
Link to Bills timing instruction .PDF download
http://www.hondabrat.com/2013/04/manuals.html