Craftsman LT1500 valve lash goes out of spec when blades engage

Every time I engage the blades on my Craftsman LT1500, the valve lash goes out of spec (loosens) and I get popping sounds. I adjust the engine to TDC, and re spec the lash, and tighten the lock screws down good. This happens within a minute of engaging the blades but no issues when not under load. Spindles seem to be fine and move freely. Intake and Exhaust rods don't seem to be bent either. I do have an aftermarket carb on there, could that be causing the issue? What else could be causing this and how could I diagnose/fix?
 
Craftsman Model # 247.288810
Briggs and Stratton Model 31c707 Type 3346 G2