Chasing a misfire.

Back those rockers off a coupla turns, start the engine, and give each rocker a 1/4 turn while running.
At one point while tightening, it'll start to run ruff, stop tightening until engine smooths out, tighten a little more, it'll run ruff again, wait till it smooths, - repeat till rockers are tight, and engine smooth.
Replace cover .