25hp merc slowing down and speeding up at full throttle by itself.

[Philippe12]

Bought a 2009 merc seapro 25hp last spring and ever since i've owned it when holding it at full throttle after about 20 seconds it slows down to 3/4 throttle and speeds back up in about 5 second intervals all by itself. If i back it off for 10-20 seconds and go back to full throttle it's okay for another 20 second then starts to slowing and speeding up again on it's own.

Could this be a fuel pump problem? or maybe it's sucking in hot air from the cowl?
The water coming out isn't burning hot at full throttle so i don't think it's a overheating problem.

 

[rlb9844]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

Sounds llike a fuel problem to me. Could be a weak pump or something stuck in the fuel line. Is you tank vent clear?

 

[roscoe]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

More than likely its a carb issue.
Specifically the needle valve/float setting.

Time to clean/adjust the carb.

 

[Philippe12]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

Ya but the motor is brand new!! It did this out of the box. Maybe i can start at the fuel pump and get a rebuild kit for it. the carb is definatly not dirty, maybe misadjusted? i guess it's possible a piece of dirty got into something, not the highspeed jet though as i had that out a couple of weekends ago.

 

[Philippe12]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

should the gas tank bulb be hard all the time? I noticed at full throttle that the bulb will go completly soft, i will pump it and it will go hard again for 4-5 seconds then go soft again, is that normal?

 

[kellmike626]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

If it's brand new, and your warranty hasn't expired, let the dealer fix the problem. Otherwise, start with the carb like others have been saying.

 

[rwidman]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

If it's brand new, and your warranty hasn't expired, let the dealer fix the problem. .

Of course. Why put time and money into when the dealer will fix it for free? And what if there's an expensive defect?

 

[H20Rat]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

an engine doing that at WOT is a sure sign of it running lean. VERY dangerous if you value pistons...

 

[Philippe12]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

Thats what we thought too because the water comming out would get very hot and it would backfire when you come down quick from full throttle and shut it down. My next door neighbor is a retired mercury mechanic and told us mercury tunes them VERY lean, right on the line of blowing up to meet emission requirments. So he gave me a drill set and i upsized the highspeed jet .004" . I gained a bunch of power! which is awsome! motor runs alot quieter too for some reason. Made it idle rougher in gear though but thats okay. All 2 stroke mercs idle rough lol like a vibrating harley, it's their trade mark.

But jetting didn't fix the original problem.

 

[rwidman]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

Thats what we thought too because the water comming out would get very hot and it would backfire when you come down quick from full throttle and shut it down. My next door neighbor is a retired mercury mechanic and told us mercury tunes them VERY lean, right on the line of blowing up to meet emission requirments. So he gave me a drill set and i upsized the highspeed jet .004" . I gained a bunch of power! which is awsome! motor runs alot quieter too for some reason. Made it idle rougher in gear though but thats okay. All 2 stroke mercs idle rough lol like a vibrating harley, it's their trade mark.

But jetting didn't fix the original problem.

That pretty much takes care of getting it repaired for free under warranty.

 

[kellmike626]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

That pretty much takes care of getting it repaired for free under warranty.

I still think taking it to the dealer was the best idea. I stand behind it

You might want to fix the lean condition still. ...like rwidman said, if you tinkered with the engine, your warranty is gone, so now you might want to pull out the wallet

 

[Philippe12]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

its not running lean! i jetted it up richer. and the plugs are nice golden brown.

 

[AVNTech]

Re: 25hp merc slowing down and speeding up at full throttle by itself.

Philippe12
You said the fuel line bulb was going soft during WOT at the time the engine started to bog down. This would indicate a restriction in the fuel delivery system, junk in the tank pick-up, a collapsing fuel hose, or a restricted tank vent. Engine starves for fuel, slows down, allowing fuel to "catch up", eng speed picks back up. Jet diameter would have no effect on this.

Bob