Please help having trouble with 2003 Mercury 60/40

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I have a 2003 Merc 60/40 2 stroke it has lean sneeze issue idles fine and runs wot great only sneezes when slowing down and about 1/4 throttle while scanning about 3.5-4mph carbs have been cleaned and all gaskets replaced idle mixture screws are set at 1 1/2turns out factory manual says 1 1/4 turns but idles and starts better at 1 1/2 checked for air leaks and synced the carbs also replaced the reeds with Carson fiber reeds problem was the same before changing the reeds also upper and lower crankshaft seals have been replaced bottom one was definitely bad after changing them the problem got much better but it's still there also replaced the fuel lines and rebuilt the fuel pump checking the spark plugs after idling for 10min shows it being rich so I'm lost I considered the timing but service manual says idle timing is 2°BTDC and it's spot on hope this is posted in the right place any help would be greatly appreciated

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Wow, sounds like you have been doing your homework!! I am assuming that compression on this motor is 120 on all 3 or better??

How do plugs look after a WOT run and shutdown? Literally run it WOT for a few minutes and then back her right down and turn the key off. Is there water on your plug/s? Do the faces of the pistons look steam cleaned at all? What you are describing is a classic water into the cylinder issue.

These blocks are pressed together, as they have no head gasket. (Pain in the arse), and what I have heard is they are notorious for hairline cracks in the waterjacket, allowing a very fine mist of water into the cylinder.
 
Do like rotus is suggesting, also run a fuel water filter separator if you aren't running one it's worth it to know your engine is getting clean fuel. If you have a little water in the fuel it can cause similar issues. Sounds like you've done/fixed and checked everything that could have caused the issue. I think rotus is on to something either water is getting in from the fuel or the water jacket possibly. Water jacket would suck cause as he said those mercs are a blind bore no head gasket if it's getting water in from the water jacket most likely a crack somewhere.
 
It has great compression 135-135-136 plugs are perfect after WOT run for several miles it has no signs of water but mabey I need to look alittle harder

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I know you said you cleaned the carbs, but did they get cleaned really well? Sometimes it doesn't doesn't take much to partially plug a passage or jet to cause some weird symptoms.
 
The sneeze has usually been the idle air adjustment for me. I had to tweak my Mercury 60/45 in the fall when it got colder and now with the warmer weather it's starting to sneeze again so I have to find that smooth spot for it.
 
Yes i did clean the carbs very well but I think I'm gonna pull them back off again and run them through my ultrasonic cleaner because Im still questioning myself on them and yes I know what you mean by it doesn't take much had a machine at work once that wouldn't idle and it had an eyelash in the pilot jet haha

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