How tight is the heads on a 2004 yz 125 2stroke

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.

Try hitting it in a clockwise motion .The bearing are very tight ,they have to be ,so that the roller work properly another trick is to run boiling water over the outside of the head stock .This will exspand the outer head stock and give you a better chance of freeing the inner bearing .

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

check the clutch adjustment also the clutch could be binding or just stuck