I'm throwing this out to the diagnostic Gods. Merc 350 Mag, Alpha
Drive. Motor starts perfect, and revs out of gear perfect. Once I
put her in gear, I get a very noticeable knocking/clunking (from the
motor or drive they sit deap so its tough to say). I'm ok with the
outdrive being shot, its old, but the other symptom that has me
stumped is when I put it in gear and the knocking starts I get what
seems to be exhaust coming out of the PCV hoses from the valve covers
to the breather. It's white smoke not blue. So at first I thought it
was a problem in the heads but the fact it runs perfect with no smoke
out of gear makes me think the drive. Could the exhaust that goes
through the drive be backing up because of whatever damage is in
there? If something in the heads was shot it would be making noise
even out of gear correct? No matter what, I'm pulling the boat out
and having the outdrive checked out, but this smoke coming from the
valve covers has me stumped and wondering if anyone has ever seen such
a thing. Thanks, KD

wrote:
I'm throwing this out to the diagnostic Gods. Merc 350 Mag, Alpha
Drive. Motor starts perfect, and revs out of gear perfect. Once I
put her in gear, I get a very noticeable knocking/clunking (from the
motor or drive they sit deap so its tough to say). I'm ok with the
outdrive being shot, its old, but the other symptom that has me
stumped is when I put it in gear and the knocking starts I get what
seems to be exhaust coming out of the PCV hoses from the valve covers
to the breather. It's white smoke not blue. So at first I thought it
was a problem in the heads but the fact it runs perfect with no smoke
out of gear makes me think the drive. Could the exhaust that goes
through the drive be backing up because of whatever damage is in
there? If something in the heads was shot it would be making noise
even out of gear correct? No matter what, I'm pulling the boat out
and having the outdrive checked out, but this smoke coming from the
valve covers has me stumped and wondering if anyone has ever seen such
a thing. Thanks, KD
You have stripped or broken gears in your upper gearcase. A broken shaft
wouldn't make a racket. Confirm this by removing the top cover and
taking a peek. Sorry, it's rebuild or replace time.