what causes a knocking sound?
 

We just put a BRAND NEW Merc. motor in our boat. It starts right up, purrs like a kitten. However, when we accelerate, there's a terrible knocking sound. We haven't run any higher than about 3500-4000 RPMs since it's a new motor and we're breaking it in. Runs fine when we're just cruising around, however, when we tried to pull a tuber, the motor was cutting out or missing, and we were not able to pull a tuber or skier. The mechanic first thought it was a problem with a valve, possibly a "floating valve," but checked that out and there seems to be no problem with a valve. Any ideas of what it could be?

That sound is the mechanic's helper, still in the bilge, banging the side of the boat with a monkey wrench.

At this point I would believe anything!!

So, it seems to be knocking mostly when under load from a skiier or tuber and trying to plane? Or, is it knocking all the time?

What motor did you install? Was it a complete bobtail? What happens if you rev the motor a little in nuetral? I am just curious, how did the mechanic check for valve float?

You are omitting valuable info happyours. What is the oil pressure, water temp, what type of outdrive, what rpm is the noise first notice and when does it stop. We need more input dude.

BT :cool:

It was a Merc. 5.0 liter V8 Vortec. Did not replace the outdrive. Not sure how he checked for floating valve. We didn't try to rev in neutral.

The oil pressure was fine, water temp was fine, we've got an Alpha I outdrive. The popping noise starts at around 2700 rpm and doesn't stop after that; however, if we're not towing, the boat runs fine other than the loud knocking/popping sound.

It sounds like your Cam is going. If it is a load popping out the exhaust that is probably it. The knocking is probably the pushrod and lifter and rocker arm. and and and and and............

Also check to make sure your rockers are tight. I just had my engine rebuilt and one of my roller rockers came lose after 2 hours (don't think it was adjusted correctly to begain with). Made a real good knocking sound up around 3000.

I really appreciate all of the ideas y'all are giving me. I'm definitely going to mention them to the mechanic.

They did replace plugs when they installed the motor. The knocking/popping is worse around 2700-3300 rpm, and then it gets a little softer, but it does not go away.

My guess would be a defective lifter or a bad valve spring.... :confused:

what causes a knocking sound?
 

money trying to get out of your wallet

Boy, Luke, if you only knew how true that is!

Thanks for all the input.

A failed exhaust lobe or lobes will cause exhaust to stay trapped in the cylinder during the exhaust stroke. Then the intake valve opens and the hot exhaust comes out through the intake.

A 454 we once had a slight, somewhat random, knock when cold that cleared up when the motor warmed up. Bad exhaust lobe.

An exhaust rocker that loosened up could cause similiar symptoms- if found, don't just tighten it up!!! a bad lobe will cause clearance to open up also, so check lift afterwards.

I have also had knocking sounds I couldnt quit seem to locate turned out to be the drive .... If you do alot of ski or tube stuff and have hammered out of the hole alot your forwad gear and/or thrust bearing migt be ready to bust. If your shure sound is from the motor one thing not mentioned is it could be detonation/ timing issue.

We'll definitely tell the mechanic to check the timing. Talked to him today, and they still can't figure out problem. This is getting frustrating.

check the firing order and double check the plug wires to make sure they are going to the correct plug.

Since the boat only seems to "miss" when pulling a skier, and doesn't miss when we're not pulling anyone, does that help narrow down what the problem could be? The popping noise is there all the time, though. The boat runs smoothly when we're not towing anyone.