Yamaha help

[striper77]

What could be cause of yamaha outboard knock after quick deceleration on 1 hour run @4000 rpm.Motor run fine after? thanks

 

[windknotnc]

Generally knocks are not good. Was it a knock or was it prop noise created by the rapid deceleration? I would do a compression check on each cylinder to see if there are any obvious discrepancies.

 

[striper77]

Not sure just 3 or 4 quick knocks around back boat thought motor.

 

[DaleH]

Ouch ... could be a bent connecting rod. As upon RAPID stopping an OB boat, water can enter the exhaust channels and flood the lower cylinders and whereas water can't compress ... the connecting rod to that cylinder bends. This can cause a knock. "If" this is what it is ... it will eventually lead to a failure of that cylinder, as my V6 failed this year for that very same reason. It might have taken years or months, but the results were the same.

Let's hope your knock was something quite different and intermittent.

 

[SBH2OMan]

I've heard about this problem from several sources - engines on brackets (like many of our Parkers) tend to take more wash when suddenly decelerating. As Dale wrote, water can wash back up the exhaust ports because when you're decelerating, there isn't sufficient exhaust pressure to keep the water from coming back up the exhaust and into the engine. If the water makes it past the valves, poof! you have piston, valve, ring, or rod damage (or all of the above). I heard about this happening to a local guy with a brand new motor - he got lucky and it was replaced under warranty (gotta love the 5-yr Honda warranty).

 

[gw204]

If it runs fine (w/ no knocks) before and after the decel, my guess would be the prop as windknot mentioned, or something rolling around in the aft wells and/or bilge.

 

[striper77]

Thanks,ran fine w/out knock for 10 min after.