[AZKen]

Pre detonation can be a hard knock sound. You can move dizzy while running to see any change. Not to say that varnish is you problem. It probably is not, but cleaning fuel lines is not the way to go after 6+ years. You could have loosened the gunk just enough to flow when under suction from pump. Who knows? But it really is not worth the risk. I am not at all a expert on the V6 (if that's what you have) but the Jolly page says 5 deg BTDC and you need to set valve lash to that page also. It's hard to say what the noise is. If it's truly a knock maybe not piston rings, be sure it's not exhaust leak. That can sound exactly like a rod knock also. Your plan with stethoscope is the answer. I'm hoping for the best. Later.