Yamaha 40V '03 (2-Stroke) misfiring on cyl #3 and too rich

[mrskipper]

Hi all,
So i've been scratching my head for a while now and thought to ask for help:

I have a Yamaha 40V from 2003 that started to running roughly this summer when the boating season began. I didn't pay so much attention to that then and thought it was obvious that it needed a carb cleaning, to which i did.
Last week i did the carb rebuild, which consisted in :
- thorough cleaning
- replacing gaskets
- changing float valves and resetting float height according to the service manual of 15mm
- Resetting pilot screws
- Sync of carbs

.. and THEN my headaches began.

Started the engine and it sounded rough, turned out to be cylinder #3, bottom one (lowest(, to not be firing.
Did a compression test and they all showed equivalent values of 77 PSI, strangely good same values.

Then as i thought i flooded the engine i removed the plugs and gave it a quick cranking to empty the cylinders and found the bottom cylinder to be running really rich as it was spitting out fuel instead of a vaporized cloud of fuel like in #2 and #1.

Thinking i screwed up the carb job i then rebuilt the carbs for a total of 5 times, triple checking gaskets, float heights, float seating plus bench testing and... issue is still there, bottom cylinder spitting out fuel. On the last rebuild i set the float height to 16mm in the bottom carb to make it run a little leaner and ensure the float valve closing in time but that didn't help .

Starting to be clueless as i did everything by the book i then started to play around with the pilot screws. It appears that if i open the pilot screw on the bottom one to a higher value than the values specified on the service manual it seems to give sputters of life, but that's then way more than the other carbs and the values specified on the service manual of 1-1/2 turns.

Do anyone have any hints on what is going on? I was suspecting the carburetors but now i'm starting to wonder if there's some other issue i'm missing.
Is it normal the bottom cylinder, when running without plugs, spits out fuel unlike the other cylinders?
What else could cause those symptoms?

Thanks in advance for any replies!

 

[robert graham]

Bad fuel pump diaphragm?...

 

[99yam40]

yep need a new fuel pump
#3 is the one that provides the crankcase pulse for the fuel pump

 

[mrskipper]

Thanks 99yam40 and Robert graham!
I tried to check the fuel pump today but it was impossible to tell if it was in good working condition or not. What i atleast reacted on was that the gasket between the fuel pump and the engine block seem quite hard and dry.

But thinking logically, considering how much fuel was getting spitted out of cylinder #3, it MUST be coming from there...I'll try to find a replacement asap and keep you guys updated And also i feel stupid rebuilding the carbs 6 times if the problem really was that simple

 

[mrskipper]

Hi all,

Just replaced the fuel pump and the gasket and can confirm that fixed my issue. All cylinders are firning happily now! Never thought the fuel pump would flood cylinder #3, but that’s another proof that we never stop learning. On the other end rebuilding the carbs so many times I’m 1000% sure they’re tuned as good they can be

so thank you all

 

[robert graham]

I replace my fuel pump about every 5 or 6 years, just to be proactive....keep my old still working pump in my boat tool box for a spare. Sorta like the water pump impeller, better to replace Before they fail.....

 

[99yam40]

good to hear.

did you link and sync as the manual calls for after the last time you put the carbs back on?
needs to be done every time you put the carbs on

 

[mrskipper]

good to hear.

did you link and sync as the manual calls for after the last time you put the carbs back on?
needs to be done every time you put the carbs on

Yes i did Thanks for reminding! It's way easier on those than i thought.