Optimax idles rough until it's warmed up

2010 175 hp regular optimax 1B789623. This thing idles rough and stalls when started , but lines out and idles fine after it's warmed up. It runs great at every speed above idle, but runs rough at idle when in gear, especially when putting the boat on the trailer. Over the years, I've installed fiber reeds, multiple plug sets, new air compressor, new tps, changed fuel lines, replaced low pressure fuel pump diaphrams, checked fuel/air pressures, compression, leak down rate, but this engine still has never run well when cold at idle or idling in gear. Is this just the nature of the 2.5? What else could it be?

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Originally Posted by jeffadams

2010 175 hp regular optimax 1B789623. This thing idles rough and stalls when started , but lines out and idles fine after it's warmed up. It runs great at every speed above idle, but runs rough at idle when in gear, especially when putting the boat on the trailer. Over the years, I've installed fiber reeds, multiple plug sets, new air compressor, new tps, changed fuel lines, replaced low pressure fuel pump diaphrams, checked fuel/air pressures, compression, leak down rate, but this engine still has never run well when cold at idle or idling in gear. Is this just the nature of the 2.5? What else could it be?

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Compression test first, air & fuel pressure test then pull the fuel rails for complete service including injectors
Remove plenum for inspection of reeds as well

^^^^^+1 and complete scan of all switches and sensors

In addition to checking the BASICS (ALWAYS, EVERY TIME check them FIRST):

-Belt stretched (livespan on this belt is no more than 4 years or 300 hours, whichever occurs FIRST)

-Tensioner Spring weak (needs to produce a minimum of 70 lbs/in)

-Reeds full of grit/grime/sand (may have come from adapter plate or corrosion in that area getting ingested). Even "new" gritty reeds won't idle well.

-Exhaust Tuner fell/wedged in mid

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Originally Posted by EuropeanAM

In addition to checking the BASICS (ALWAYS, EVERY TIME check them FIRST):

-Belt stretched (livespan on this belt is no more than 4 years or 300 hours, whichever occurs FIRST)

-Tensioner Spring weak (needs to produce a minimum of 70 lbs/in)

-Reeds full of grit/grime/sand (may have come from adapter plate or corrosion in that area getting ingested). Even "new" gritty reeds won't idle well.

-Exhaust Tuner fell/wedged in mid

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More detail please regarding the text in red from your reply above. I have a low hour 150 that just won't idle reliably either.

Thanks!

The exhaust tuner is the exhaust chamber in the midsection of the motor. It can come loose and will rattle around the midsection. You need to drop the lower unit and look up inside the midsection. A flashlight and a long screwdriver or something similar to see if it moves around when pushed.

the tuner is basically a tail pipe so to speak on the bottom of the adapter plate that the powerhead sits on it is a divided passage that aids in the scavenging of exhaust from the cylinder /powerhead it is bolted to the adapter plate on the smaller cu in MERC engines its inside the exhaust tube thats inside the exhaust housing it can come loose from the plate and fall down the tube a almost close of exhaust passages killing performance and causing the mtr to run extremely rich resulting in no power and poor running quality some times you can shake engine side to side and hear a clunk in mid section , getting ready to repair one next week

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Originally Posted by goinfaster

More detail please regarding the text in red from your reply above. I have a low hour 150 that just won't idle reliably either.

Thanks!

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If you need anything beyond reading the responded to the OP'ers thread, please open your own Separate thread for your engine. Thanks!

Thanks very much for the explanation both Leon and Joe. Understand perfectly.