What would cause this noise?

[Greg]

I have a Merc. 260/Alpha set up with a mild warm up from the previous owner. Getting it ready for summer I noticed a knocking coming from the bell housing area. It doesn't do it a idle, but at 1500 rpm its intermittent and at 2000 its constant. My compression is 150 and 151 in all cylenders. The boat is on the trailer so its not under any load. I realize this isn't much to go on, but any idea where to start looking? Thanks Greg

[RickR]

Might be Y pipe flappers.
Might be able tp pinpoint the noise with a screwdriver( put the handle to your ear)
Flappers are at the top of Y pipe.

Also check U joints by moving steering wheel and trim.

[Rootsy]

is the knock at or below engine speed... is it rythmic or erratic? i am suddenly VERY familiar with really looow rpm rythmic knocks that soon become something you REALLY do not want to hear... :bawling:

could be a U-joint but to be safe.. get a 3/8 12 pt socket and take the top cap off of the alpha and look at the gears CLOSELY...

JR - mr shavings...

[Audiofn]

Could it be gear lash from the lower unit not being in the water? Try and drop it in the water and see if you still have the problem.

Jon

[Formula Jr]

Put the handle to your ear? Guess I've been doing it wrong all these years:biggrin.:

A knock, a rap, a slap or a ping?

Knocks are never good sounds unless its a diesel.
A rap is a U joint.
A slap is something out of alignment.
A ping is just the flappers.

[mikev]

is the drive trimmed all the way up? if it is it is the u joints mine will make that noise if i rev it up when the drive is all the way up when its on the trailer but if i drop the drive down it goes away.

[Greg]

We listened to it again this aft. It is a sharp sounding whack whack. Its there all the time just not as loud at idle. We did the listening stick with a dowl rd and weren't able to pinpoint it. The drive unit is all the way down as I am also fixing the trim limiter switches. Tonight we are going to pull the valve covers again and just make sure that everything is moving the way it is suppposed to. No"Y" pipe on the boat, although if I have to pull the engine I will put on switchable exhaust. Thanks for all the input I'll keep you all posted. Greg

[Mr X]

My guess would be the engine to drive coupler.

[MOP]

I would pull the drive, 20 minutes tops. To get water you can shove a cut off piece of 3/4" garden hose in the water passage and run it. If the noise is gone I would first check the drive, look over the U joint if it is clean and tight then like Jamie said pop the cap off and look inside may have part of a tooth floating around. And for Christ sake don't water test it, if it is in the upper you can really do a job on it under load and bill will be much bigger. Now if the noise is still there when you run it without the drive, well let us know.

Phil


Phil

[doug hess]

Does moving the I/O from side to side with the steering wheel affect the sound?

[MOP]

If it were a valve train problem the noise would not be aft, it would be high and somewhat magnified by the vavle covers. Listen along the valve covers that should rule that out. Intermittent fairly loud clacking as you describe still sounds like the upper is going bye bye. Four bolts and you will know, I think Jamie and I have it right. Scroll down to post #23 in the link below this is what you should be looking at.

http://www.donzi.net/forums/showthread.php?t=34923&page=2

[Greg]

Rain delay, boat's outside couldn't work on it. The drive is down, and moving it from side to side made no def. Pulled the valve cover and ran it and didn't see any oil pumping out for the rockers. The oil pressure has always been erratic, but has always been there. Would an oil pump getting ready to take a dump make a whacking noise? Next step pull the drive, or just hire someone.