Yamaha 250 Possible Limp Mode?

[Andrew93]

I have a 1996 Yamaha 250hp that was in storage for 6-8 years on the Seafarer I picked up. I have a newer OX66 to hang on the boat but would also like to figure this motor out since it will serve someone well. It has good compression, new fuel off an external tank, new fuel pumps and filters, and cleaned carbs. Motor starts and idles, but will no go above 2800rpms, its like someone hits a switch, but accelerates to that point. I at first thought it was the TPS with no luck and also the temperature sensor tested ok following the steps in the book.

Someone told me it sounded like it was in limp mode. No alarms, motor is not overheating, main oil tank if full and the motor is getting oil (with the motor sitting for so long I have 50:1 in the external tank also but I can see the oil injection in the intake). Is there a separate sensor for the oil system to place it in limp mode that is not the float? The lights on a gauge read normal.

Thanks for the help

Andrew

 

[Harpoon]

It sounds like your high speed jets on the carbs may be clogged up. Easy fix. Beyond that there is the TPS, Knock Sensor, or the diaphragm fuel pumps (could have pinholes). Fuel pump would be my second guess after carbs.

 

[Andrew93]

It sounds like your high speed jets on the carbs may be clogged up. Easy fix. Beyond that there is the TPS, Knock Sensor, or the diaphragm fuel pumps (could have pinholes). Fuel pump would be my second guess after carbs.

Thanks, that makes the most sense to me, especially where the motor was in storage for so long. The fuel pumps are brand new. I have a new-to-me OX66 for the boat I want to hang anyways, but it would be easier to sell this one running right.

 

[seasick]

It sounds like your high speed jets on the carbs may be clogged up. Easy fix. Beyond that there is the TPS, Knock Sensor, or the diaphragm fuel pumps (could have pinholes). Fuel pump would be my second guess after carbs.

Thanks, that makes the most sense to me, especially where the motor was in storage for so long. The fuel pumps are brand new. I have a new-to-me OX66 for the boat I want to hang anyways, but it would be easier to sell this one running right.

It's not limp mode. That would slow to about 1100 or so revs. It's probably fuel related and could be a lot of things. Carb jets yes but also bad pumps/pressure, clogged filter, dunked up or corroding fuel lines , bad anti-siphon valve, clogged tank pickup tube/screen etc.
One un-scientific test is to run at speed till you reach the limit on revs and then see if the revs stay the same or slowly decrease. If they decrease yo may have a restriction in the fuel system. If they stay relatively stable, you could have a carb jet issue. In all likely-hood after that amount of time, the carbs are probably gunked up.
Avoid the temptation to try and adjust the individual carb adjustment mixture screws or the throttle plate adjustment screws. If you do, you will have to sync the carbs and that will require a special tool (multi-port manometer).
Although it is a long shot, sometimes running a bit of carb cleaner will help clean up the gunk.
Let us know what you find out.

 

[Andrew93]

Thanks for the info, someone suggested limp mode, and that didn't seem likely at first either and figured I would ask here. I am going to have to lean to clogged jets, the motor has new fuel pumps and I have been running it off a 5 gallon external tank to rule anything on the boat out.

I have an OX66 I am hanging on the boat as soon as this one sells, I wanted to be able to provide as much info as possible to a potential buyer.