Please forgive me if this has already been mentioned somewhere.
My plug cap is really hard to remove at times. I cranked up my compressor, disconnected the coil lead, and using the airblast nozzle shot a quick blurt into the head breather hole. Cap popped right out.
Good idea to cover the area with folded rag to stop any violence in case of excessive air. You don't want to send the unit into orbit.