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

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.