90 Merc loses power

[shopmon2002]

Took the boat out on a river this weekend with a very swift current. She ran fine going downstream, but fighting the current back upstream, it lost power, and ran at a very low rpm. I already know that the boat has the wrong prop, because it runs 6000 WOT with a 17 pitch prop on it. But I was only running about 5000rpm at 20 mph, and never exceeded 5500 rpm. I think the boat is overheating and going into safe mode, thus limiting the rpm's. My mechanic tells me that the prop is the culprit. Could the prop cause this, even though I only ran it at 5000 rpm? It seems to me that an overpropped boat could overheat, but not an underpropped one. I am replacing the prop anyway, but am headed to the gulf this weekend and don't want it to let me down.

 

[shopmon2002]

Re: 90 Merc loses power

Or could it be that the fuel pump is going bad and losing pressure

 

[Gomer50]

Re: 90 Merc loses power

Need more info????
What year Motor/serial number???
What kind of boat????

 

[shopmon2002]

Re: 90 Merc loses power

Not sure serial no, but 2003 90 Merc 2 stroke with oil injection on a 17.5 foot sea chaser bay boat that is as light as a feather.

 

[CharlieB]

Re: 90 Merc loses power

Carb or EFI? Carb does not have a 'Safe' mode.

It is entirely possible that the fuel pump output is reduced, causing the upper carb to go lean and burn that piston.

The very first things to check are compression on each cyl, and spark. This will prove the mechanical condition of the motor and ignition system. If both check out fine then you can move onto the fuel system. Disconnect the fuel hose to the carbs, point it into a suitable container, crank the motor and observe for a strong fuel stream.

Repost your findings and we can proceed from there.

 

[shopmon2002]

Re: 90 Merc loses power

Carb engine. Mechanic is stumped, but he is the one that mentioned the safe mode, so maybe I need a new mechanic. I will check fuel pressure and reply back. Thanks

 

[shopmon2002]

Re: 90 Merc loses power

Oh yeah, compression is even and all cylinders are firing when cold.

 

[shopmon2002]

Re: 90 Merc loses power

Got it worked out. Bad gas again, second time I've drained the tank. Damned ethanol. I'm going to invest in a handful of fuel filters. Thanks again for all of your help!

 

[CharlieB]

Re: 90 Merc loses power

A water separating filter is a very good thing, you can even find them at Wally World.

 

[shopmon2002]

Re: 90 Merc loses power

Ok, I'm back. I drained the tank for the second time. I am running a water fuel seperator and an inline filter. I ran the boat on new fuel for 20 miles saturday and it ran like a dream. However, on sunday, it started cutting out on me again. It revs quickly and smoothly until it gets to 4000 rpm, then it immediately revs down. It will not get above 4000 rpm, even at full throttle. Both filters are clean, and bulb is staying hard. Boat has new fuel lines from tank to carbs, and I have cleaned tank valve. It has no antisiphon valve. If it were starving for fuel at the tank, it would be collapsing the bulb anyway, and that's not the case this time. Fuel pump shoots a very strong stream. It is almost as though it has some type of rev limiter keeping it from reaching 4000 rpm, because the engine runs well until that point. This boat is giving me fits!

 

[CharlieB]

Re: 90 Merc loses power

A 'stiffening' diaphragm in the fuel pump may seem to flow enough yet still limit WOT RPM. Caused by alcohol in the fuel.

A quick test, when the RPM 'hang's, squeeze the primer bulb to 'assist' the fuel pump, if RPM picks up at all, rebuild the pump.

 

[G8trbait]

Re: 90 Merc loses power

A 'stiffening' diaphragm in the fuel pump may seem to flow enough yet still limit WOT RPM. Caused by alcohol in the fuel.

A quick test, when the RPM 'hang's, squeeze the primer bulb to 'assist' the fuel pump, if RPM picks up at all, rebuild the pump.

Sort of the same problem mine did. It would run fine as long as I didn't go over 30 mph (no tach sorry). As soon as I did the boat would fall off a plane. Opened up the fuel pump and it was full of something. Mechanic thought it was the inside of the fuel lines. Replaced the lines and cleaned out pump and now I can run 40+