How much tourqe do u tight the head on a 2004 yz 125 2stroke

Cylinder Head Cover Bolts . . . . . . . . . . . . . . . . . . . . .10 N·m (85 in. lbs.)
85 in. lbs = 7 ft. lbs

Basically... snug, not tight. Don't want to crush the gasket.

I doubt the repair shop has done anything wrong. Yes, it sounds like an air leak between the carb and the head. Try tightening the mounting screws and bolts. Also check the tightness of carb screws on the choke etc.. Now check tightness on the head bolts and other mounting hardware on the head. Be sure the throttle is operating properly and the cable is not kinked or pinched.

A torque specification is how tight you must make the bolt by using a torque wrench which measures the tightness you apply to the bolt, ensure you are using new head bolts, a reliable torque wrench, clean the threads of the cylinder head (with tap) where head bolts screw into and when installing bolts ensure they are properly lubricated. And the sequence is the order in which you tighten each bolt.

Head Nuts: 20 ft/lb
Cylinder Nuts: 22 ft/lb

I believe you can get a copy of the complete manual from the Yamaha website:
http://www.yamahaownershandbook.com.au/lib/download.php?f=5MV-28199-30.pdf

stage one tight in correct order three or four stages to 30nm or 20 ft-lb then undo head bolts apply lube to the threads and re tighten in correct order in 3 - 4 stages to 20nm or 14 ft-lb then you mark a corner of the head bolts and turn 90 degrees this should give you a very accurate torque setting