F20leha high rpm knocking

[Gunflint]

I have a rental boat with a f20leha on it. A customer hit a rock pretty bad and then ran it for 7 miles back to the dock with the prop shaft bent and the prop all folded over. It vibrated the anode, trim tab, and water screens off the motor! I changed the prop shaft and prop, gears all looked good. Now at wide open throttle it has a bad knocking noise from the engine. If I back off it a little bit the noise goes away. Any ideas? Thanks

[rodbolt17]

carefully measure the distance from the plug holes to the piston at TDC.
its possible water was injested and lightly bent a rod.

[Gunflint]

carefully measure the distance from the plug holes to the piston at TDC.
its possible water was injested and lightly bent a rod.

How would that cause water to get into the cylinders? Could the driveshaft possibly cause a knocking if it was bent a little? It doesn't vibrate badly. ran it with a couple of people in it and it didn't knock, only does it with light load in the boat.(higher rpm) Thanks again

[99yam40]

if the motor sits low and the boat stops suddenly, there is a chance water will get in through exhaust when the wave hits from behind

[Gunflint]

I do not have the tools to measure piston height. It doesn't knock in neutral when throttled up but I'm probably not revving it as high as when its loaded. Thanks for the insight on the wave through the exhaust, I wouldn't have thought of that one.

[Gunflint]

Could changing the prop from a factory Yamaha 9 1/4X 10 to a solas 9 1/4 X 10 cause it to rev a little higher? I don't have a tachometer but it seems to rev higher than the boats with the stock prop.

[TownsendsFJR1300]

Just a thought, are you positive the noise is from the engine OR could it be from the lower unit.

It sounds as hard as it hit, was able to bend the hardened prop shaft, etc, could you have missed a cracked lower unit?? Specifically where the case holds the main prop shaft bearings?

If the bore is slightly larger than the bearing (due to damage), that could explain, especially under load, the lower unit innerds knocking about...

[Gunflint]

I took the motor out of service today and needed the solas prop for another motor that ran fine. I installed the prop and test drove it and it makes the exact same knocking noise now. When I first installed the prop the paint needed to be removed from the base of the prop in order to get it to fit into the lower unit without sticking. When I removed it today it had some rub marks around the base of the prop. Hard to believe that it would cause that loud of a knocking noise but I am 99% sure that it is the prop causing it. Probably should have paid the extra 30$ for a Yamaha prop. Thanks for the help.

[TownsendsFJR1300]

Thanks for the update, could have been alot more expensive fix!!

[retaunton]

I have 15hp four stroke that sounds like the motor is about to blow up when I run a Solas Prop with a 9 pitch, but when I put the Yamaha prop(10 or 11 pitch) on It runs smooth, could it be the exhaust difference causing the racket?