Model 126T02-0114-B1 on a Sears pressure washer. Engine is getting spark and fuel but will not fire. Removed plug and noticing when starter rope is pulled, instead of air being sucked into cylinder head, there is only blow back. Checked flywheel key to see if possibly out of time but there is no damage (replaced it anyway). Removed head and valves are moving as they should. Could a bad head gasket be causing these symptoms?