My name is Chris and I am new to the B&S forum. I have a Craftsman 21 Hp mower about 7 years old. Recently experienced major popping and erratic running. Realized one of the pivot posts on the intake valve had backed out and bent the intake push rod (aluminum). I replaced the rod, set the valve clearances and cleaned the cooling fins on the block and head. Still had excessive back pressure and engine was slow to turn. Disconnected the exhaust manifold and engine cranked over just fine (loud). Finally, drilled some relief holes in muffler and now it runs great. Has anyone else had a similar issue with back pressure?
