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Rapid deceleration causes a large stern wave that pushes the stern forward with a lot of force. That force pushes a lot of water into the propeller hub/exhaust of any outboard. If the engine is at idle or low RPM the exhaust pressure is low. When the water pressure exceeds the exhaust pressure water is forced into the exhaust passageway. When an exhaust valve is open the water can be forced past the valve into the cylinder.
Butch. Thank you for the explanation, it certainly helped.
I stopped by the marina yesterday to check on the motor and, as luck would have it, the replacement powerhead was dangling from a lift behind my boat ready to be installed! I stuck around for the install, watching, snapping pics and picking the mechanics brain. My first question, of course, was 'how did the water get into the cylinders?' He told me that Mercury did not allow him to open the damaged powerhead so he could not be sure. He also told me that they have a client who is running the sh#* out of quad 300 with 2000+ hrs on the motors and he's personally replaced 6 warrantied powerheads (for various reasons) between them, so Mercury is clearly standing behind these motors.
I have some good pics that I'd be happy to share if anyone can suggest the best way to do that.
