Strange power loss 1991 Yamaha 25MSHP

[halsteadmorris]

I’ve been having a mind numbingly odd issue with my Yamaha 25hp for the last month or so.

I ran out of gas at WOT about 300 yds from the boat ramp. Tried to get the motor started, failed. Got it back to the ramp, busted my bowl gasket due to it being flooded. Rebuilt the carb.

Since the incident the motor will run fine up to 1/4-1/3 throttle. After that it starts running rough. Not so rough that it’s alarming, but definitely not normal.

My WOT speed has dropped from 25-27 to 14-16.

The difference in speed between WOT and 1/4 throttle is minimal. However, if I throttle down to right before the motor runs rough, let it sit there for a second, then accelerate to WOT I get almost all of my RPM range. The motor then slowly decelerates to its typical hobble, regardless of how much throttle I give it.

I have cleaned the carbs 3 times, seafoamed the motor over 7 gallons of gas. No improvement.

New prop, new spark plugs, fuel pump rebuild. Replumbed the fuel system. New fuel quick connector. No change.

I deleted the oiler, but the motor ran perfectly fine until I ran out of gas and flooded the carb.

What problem would give these symptoms? My guess is fuel delivery, but I feel like I’ve tried everything short of purchasing a new carb, which I doubt would solve anything and is probably equal to the price of the motor.

While looking at the motor I noticed that some of these parts seem a little worn, and I can’t fully understand their function. Any chance someone could enlighten me?

Thank you all for your time and help. I’m really at wits end, but I’d rather get this motor running than purchase a new one with unknown history…

 

[99yam40]

I am not understanding how running out of gas could flood the carb or bust the gasket

 

[halsteadmorris]

They were separate occurrences that happened back to back. I was trying to be thorough about the motors behavior before the problems started

 

[99yam40]

what caused the motor to flood?
and what caused the gasket to bust?

Maybe something you did or did not do that is causing this problem, so hard to tell what the is.

Does it have good and equal compression/
does it have proper timing and advancing as it should(in spec)?

Good compression, good spark, timing in spec and the proper amount of fuel to air ratio is needed.
maybe you need a heated ultrasonic bath clean on the carb

 

[scout-j-m]

I'm curious about the busted bowl gasket. All carb bowls are vented to ensure they stay at atmospheric pressure so that the venturi can effectively pull fuel through the jets. On this particular carb, the bowl is vented to a spot just under the upper plate/gasket that covers the low speed circuit. If the float was stuck up, it wouldn't build pressure and force gas out of the bowl gasket but rather force it out the top vent as well as into the carb throat. Also, a leaking bowl gasket is going to make a mess but really shouldn't leak enough to cause a fuel delivery issue.

I'd say check compression, spark, spark plugs (look lean or rich?), and timing in that order. Post results first and lets go from there.