Chasing m25xp smoke

[Patches]

Sounds like an issue I just went through a few days ago. 

Had motored for 11 hours, full load, from Seattle to Anacortes.  No smoke.  Sailed around for a couple of days in the San Juan Islands, and then when motoring on a longer passage noticed smoke coming out of exhaust, and some slight increase in water temp from the normal 160 to 165-170.  Color:  white, gray, dark gray...

Shut everything down mid-channel.  Checked raw water strainer: clear.  Probed raw water through hull with a long piece of thin dowel:  clear.  Whats up?

Starts normally, no smoke.  Run it up to 2000-2200, and after a while smoke.  Hmmm.

Called a mechanic in Friday Harbor and explained.  He came over and thought was carbon build up in the exhaust riser.  I told him it was new 18 months ago and is stainless.  Tells me to start the motor in gear, run it hard, purge the carbon, blah, blah, blah...

Watched it for a few more days and experimented.  Running continuously at 2000-2200:  smoke.  Run it over 2500 (1) smoke and (2) jump in water temp to 165-170.

Back down to the raw water strainer.  Took off bowl:  clear.  Re-attached, tightened (not all the way) and watched the bowl while I opened the raw water through hull.  Ah, slow to fill--too slow.  Probed the 90 degree inlet elbow to water strainer from raw water through hull.  Had to remove hose connecting the two first.  Voila:  eel grass creating a blockage in the elbow before the raw water strainer/basket.

Put it back together.  Ran it for 10.5 hours yesterday from Anacortes back to Seattle at 2500-2700 rpms.  No smoke.  None.

Problem (smoke/sooty stern) appeared to be caused by reduced water being pumped because of the blockage in the inlet elbow to water strainer and then injected at the exhaust riser before the water lift muffler.  More hot dirty exhaust, and not enough water, being spewed out the back.

Not sure this helps, but thought I would describe what I found.

Patches

[Ron Hill]

Patches : Been thru that a number of times, but mine were complete clogs!! 
That's why I "caulked" a round bronze strainer onto the hull to cover the raw water inlet.  That was in 1996 - never had a problem sense!!

"Caulked on", because if it ever presented a problem - all I had to do was take wooden dowel and punch it off from the inside!!
(which has never happened!!!)   

A thought