Knocking sound

Hello all,

First off I'm new to owning a boat and I just purchased mine 3 days ago. I bought a 1997 Maxum 20ft with a cuddy. It has a 5.7 L V-8 Mercruiser. My question is I took it out for the first time this past Saturday and got it up to about 45. It ran and sounded great, but when I gave it some more throttle the engine started making what sounded like a knocking noise. When I came off the throttle it stopped doing it. Is this something that happens if your RPM's get to high or is this something I need to get checked out? Anyone know what it could possibly be? Thanks for all your help in advance. I don't know anything about boats really so I couldn't even begin to know what it is.

Shane

Re: Knocking sound

The knocking does not sound normal. It should be checked out.

Good luck!

Re: Knocking sound

yeah I figured it wasn't normal, but it's weird that it happens only at high RPM's and near top speed. It doesn't happen any other time which is really what I don't understand. I may start it today and video it so people can hear it and maybe that will help people into giving me an idea what it may be. I'll let them hear the noise and check out my RPM's

Re: Knocking sound

Is it knocking or pinging? What do the plugs look like? How's your timing? Any misfiring? Good gas?

Re: Knocking sound

thered said:

yeah I figured it wasn't normal, but it's weird that it happens only at high RPM's and near top speed. It doesn't happen any other time which is really what I don't understand. I may start it today and video it so people can hear it and maybe that will help people into giving me an idea what it may be. I'll let them hear the noise and check out my RPM's

Click to expand...

Ayuh,... Welcome Aboard,... That's Detonation yer hearin',...

That's yer pistons eatin' themselves up, bit by bit....

Probably bad gas, or water in the fuel, causin' a lean condition....

Re: Knocking sound

+1 ^^^^^^^

Lifters, wrist pins and rod bearings make a racket at virtually all rpm's. Detonation occurs under hard acceleration and at very high heat/compression cycles. (heavy loading/high rpm)

Re: Knocking sound

How do I resolve the detonation issue? Should I use some gas/ fuel system treatment? What do I need to do? Thanks for the help

Re: Knocking sound

Not aware of any additives that will fix it & Its proabaly best to play it safe and protect your engine.

You may want to try rigging up a seperate fuel source ( make sure it does not leaf for fire safety reasons ) to verify its a fuel quality issue & replace all the old fuel if it is.

You may also want to check the engine timing as well -- as if its off it could cause some valve noise ( pinging )

Re: Knocking sound

Fuel delivery could be related to stuck anti-siphon valve in the tank (though that'd likely cause sputtering or stalling due to fuel starvation), fuel filters, clogged/dirty carb, weak fuel pump, collapsed fuel lines, leaky intake or intake-related gaskets letting in too much air after the carb, etc.

+1 on checking timing. Maybe check for adequate spark as well.

Re: Knocking sound

I'm gonna bring it into my boat mechanic next monday to check it out. He said it sounds like a timing issue, but he'll check everything so I guess I'll find out by next weekend. Thanks fella's for all the input!