With #8 at TDC, pressurize that cylinder with compressed air and listen for where it is escaping. Through the carb/throttle body - intake valve; though the exhaust pipe - exhaust valve; through the breather - bad/broken rings or piston; bubbles in radiator - blown head gasket. That should narrow it down Just be careful if it is not at TDC the engine might turn over (watch fan blades/pulleys) although with no compression it may not be a problem.