Merc 125 2-stroke not reving

[Lake Wisconsin]

I have been "borrowing" my brother’s boat the past few days and have noticed something isn't right with his motor. It is in the 2004 vintage, 2+2 2-stroke technology. This is sitting on a 17' Yar-craft. The motor dies easily at low idle, but most of these motors I have been around have been cold blooded and finicky in the lower RPM range. Mid range is good. However, once I get to around 4500 RPM it completely dogs out, like it is getting too much fuel/not enough air. After 4500 RPM, which is about 2/3 throttle, I can't get the rpm's to go any higher (even raising the lower unit out of the water at WOT), and of course, speed stays the same. We dropped the prop down 2 pitch, same exact result.

So I have done a little bit of work on motors, but haven't dug real deep into this one. My first thought would be the main jet on the carb might not be right, too big. Any other thoughts? Anyone else experience the same thing?

[cyber16]

I would start with checking the plugs & compression

[Lake Wisconsin]

On these motors - compression around 125 psi, with in 5% of each one?

[Burr]

Is your fuel flow restricted, or is your gas tank vent plugged?

You bascially have 3 things to check, or diagnose.

1. Ignition - sounds like you have spark and ignition, at least at some rpm ranges. Perhaps a shorting out coil pack could have the engine misfiring like you described. Faulty plugs fouled from fogging last fall?

2. Compression - Not sure if your response was that you had checked compression?

3. Fuel/gas - I'm thinking this is your most likely category. Plugged vent for the gas tank, dirty carb (it is spring time and your boat may be coming out of storage), faulty primer bulb that is restricting flow. Maybe a shock treatment of Seafoam might help?

The way you describe, it sounds like when you want to use more fuel, the engine can't get enough fuel to satisfy the needs.

That's a basic road map to trouble shooting your performance issues.

[cyber16]

The huge problem is, you DO NOT want to keep running it IF it is a low fuel supply problem, LEAN MEANS HOT, fast way to burn up a piston.
Always best to start with the plugs and while they are out check compression.
If you find a fouled plug replace it, if you find a cylinder way out of spec with the others, no amount of further diagnosis will fix it, needs work at this point.
As for the spec's? It may help others to know more about the motor

But normally a fuel supply problem will give you full power, then drops off as the system runs low on fuel, once the pump catches up you should then again have full power for a short bit

[toolboy]

I had this same problem with mine a couple of years ago. The mechanic changed to fuel pump and all has been great ever since. Apparently the diaphram gets week and restricts high end fuel flow.