Yamaha 25 fuel probs

[firefighter_958]

I have a mid to late 80's yamaha 25hp electric start, 2cyl, single carb. It will idle all day long, but at mid to full throttle it seems to load up. I have changed plugs, ignition coil, and cleaned the carb. I have removed the air breather, and covered the carb by hand, to choke it manually, and have no vaccum or suction through the carb. Any ideas, would be greatly appreciated. Thanks

[boats.net]

Check compression and reed valves. Check fuel pump.

[firefighter_958]

Have good compression, and pressure to and from the fuel pump. As for the reed valves, I call myself checking them, and from what I could tell they were in good shape. Not the typical reed valves I am used to seeing. Im more used to the ones on motorbikes. Would it be possibly a blown seal of some sort?
Thnaks greatly

[boats.net]

Could be seals or air leak in crankcase.

[firefighter_958]

Today I replaced the water impeller and thought I would start it w/o the foot on it...just to see what it did... I run it just long enough to see that it would throttle like it was supposed to and idle back down. I placed the foot back on it, dropped it in the tank, and back to the previous probs, acts like its loading up. Seems to be anytime it picks up water it loads up. I pulled the head off, to see if it was cracked or abnormal. It had an tremendous amount of carbon built up on the piston. But didnt find any probs with the head, unless maybe the head gasket was faulty, but didnt look it.
Thanks again

[99yam40]

If the piston had that much carbon, your exhaust ports and other areas are choked off with it also keeping the motor from breathing properly. Setting into tank just increases the back pressure on the motor exhaust. May need to de-carbon motor with seafoam or some other additive or take apart and clean manually..