Did they sync the carbs, and properly adjust the AF mixture screws?
I'd back the AF screws out a total of 2.5 turns from 0 and see if it helps on the backfiring.
The idle is meant to be around 1,100. You can use the knob on the left of the carbs to adjust it. Clockwise ups the idle.
1986 Kawasaki Vulcan 750
NGK Iridium Plugs #7803/DPR7EIX-9
Duralast Gold ETX15 AGM Battery
Coastered & Shaved
Metzeler ME880 [110/90-19, 170/80-15]
Balance Dampers Replaced
Rebuilt Forks w/ Progressive Springs
VN750.com Grill Cover
Emgo 23-92411 Handlebars
MOSFET FH012AA R/R
1990 Kawasaki Vulcan 750
1998 Honda VFR800 FI
2014 Honda VFR800F
1989 Pontiac Firebird Formula 350