1990 Yamaha 40hp rough idle

[Lit549]

My Yamaha cv40eld 40hp outboard idles rough.Ive rebuilt the carbs although they looked new
new fuel filter and pump and line.
Compression is perfect
New plugs
It's at a boat mechanic now and he can't figure it out either.anyome ever have this issue?Engine runs great across lake just idles extremely rough .
I've also played with the premix.ive tried everything from 50:1-90:1

 

[99yam40]

Have you checked the timing with a light to see if it is in spec?

have you made sure the compression, spark, and timing are all correct and in spec.on all cylinders?

did you change plugs?

did you follow link and sync procedure when putting carbs back on?

there are recirculation check valves on the crankcase that can fail along with seals between the cylinders and top and bottom seals too that could be leaking

 

[Lit549]

I changd the plugs,rebuilt carbs even though they looked new.Changed fuel pump and hose,check compression (perfect ) and checked for hot spark.Took it to a mechanic and he synched carbs after disassembling and cleaning again,he checked compression,timing.So far he hasn't fixed it.So what's the procedure to check these seals and valves you speak of ??Thanks

 

[99yam40]

reeds is something else to look at.

check valves should only let something thru in one direction.
I used a syringe to see if they checked or not.
I even went as far as using 2 stroke mix in the syringe to make sure 3 of the 4 were really bad on my C40TLRX

have no idea how to test crankcase seal between cylinders or on the ends

 

[robert graham]

Any adjustments with those idle mixture screws?....maybe a speck of dust/trash in an idle mixture circuit?....

 

[Lit549]

I rebuilt the carbs and they look NEW on the inside,mechanic took them apart and cleaned them too.So far same rough idle ..