Yamaha 40YETOL random stalling at speed, need help

[Yamaha40]

Hi there,
I have 2000 Yamaha 40HP YETOL two stroke engine on 5.0m RIB, which mostly runs normaly, but on some random occurrences at planing speed it will stall, drop RPM-s, but not completly to idle and will not turn itself off and after some period of time it will regain RPM (I'm not touching throttle, so throttle input stays the same)

Most times this will happen when accelerating, it will not hesitate however, but when it reaches 3.000 RPM+ it will start to stall and drop RPM-s for few moments and then sudenly regain RPM-s and continue to accelerate. It is random if it will do it again on same acceleration (throttle lever stays at same position whole time).

In other situations same thing will happen when I'm cruising at cruise speed, around 4.200 RPM (25-27mph). It will sudenly and randomly stall, drop RPM slowly to about 3000 RPM (about minumum planing speed), and suddenly it will regain RPM-s and come back up to 4.200. This also happens at WOT (5300 RPM)

Few times I noticed when running at aprox. 4.600 the engine will vary the throttle +- 100-200 RPM. RPM will drop gently to 4.400 and after few seconds go back up to 4.600 some times to 4.700

It generaly sort itself after this and RPM stabilize, only once it boged down on me three or four times in a row in 15 minutes long run. I found it most stable to run at 3.800 RPM, but it will stall when I try to plane it out of the hole.

It idles fine and below 2.500 RPM this problem does not occur

What i did so far to solve this problem is I tried different fuel tanks (Yamaha 25 liter) and two different fuel hoses. Also to mention primer bulb feels normal on both fuel hoses when this stalling occurs. All with no luck. Then i cleanded carbs, also with no improvement.

Next what i was thinking to do is swap fuel pump from my 2003 Yamaha 25HP BMHS two stroke (that engine runs normaly) because they are the same model.

I'm posting this here to ask if somebody expeirenced the same problem, or knows what generaly went wrong on my engine. Also could it be ignition system? Some timing issues?

Anyway, sorry for my english, and greetings from Adriatic Sea, Croatia

[Yamaha40]

Status report...

So far i have done this to try fix my problem:

1) replaced fuel pump with the 100% working one of my other outboard and new fuel filter. No luck, same problem as in original post
2) replaced cylinder number 1 coil because the one on the engine was a little bit dodgy. It had electrical tape over it and under the electrical tape ere signs of some kind of damage (plastic swelling). After that engine run for 9 hours with no problems, but it come back
3) cleaned carbs one more time and replaced all internal fuel hoses and secured them and replaced fuel connector on engine. Carbs are synced, and idle fuel mixture is set.

Also at idle i would get excessive fuel leak in the water (like in this photos in link below), but with no apparent source... carbs are not the source, nor the pump or fuel lines, so i was thinking could this be bad lower crank seal? Could bad crank seal result in random power loses, and rough running. My engine will start fine and run about 15 minutes at idle and then die, and starting up again would be a little bit hard, but it will run again normaly. other simptoms are exactly the same as in the first post.

Also when i tilt my engine up i will get large fuel spill with some other creamy stuff, and i dont see where this is coming from.

Images: https://drive.google.com/folderview?...kU&usp=sharing