1998 Merc 115 - small loss of power

[MercGuy]

I have 600 hours on my 98' Merc 115. I bought this engine new (on a SeaRay 175 BR), and it has always run very well.

This season it didn't seem to be running right, so I did the following:

Rebuilt the Fuel pump
Rebuilt all carbs & replaced float valves
Replaced spark plugs & wires
Checked & set linkage sync and timing
Checked compression (125 PSI each cyl).
DeCarbed with Sea Foam

The engine seems to run fine now, but I've lost about 400 rpm. It used to run right up to 5000 with my Pirhana 21" pitch prop, but now it only hits 4,600.

Any ideas on why ? is the 125 PSI compression OK ? I checked it while the engine was stone cold, and all cyl were within the width of the needle on the gage.

Any advice is appreciated !!

 

[Bob_VT]

Re: 1998 Merc 115 - small loss of power

How's the prop? Could you have bent or cracked a blade? The bottom of the boat dirty? I was told today that 125 is very acceptable across the board. Something may have changed with the boat? Water in the hull?

 

[MercGuy]

Re: 1998 Merc 115 - small loss of power

How's the prop? Could you have bent or cracked a blade? The bottom of the boat dirty? I was told today that 125 is very acceptable across the board. Something may have changed with the boat? Water in the hull?

The prop does have some very small nicks in it (it's a plastic Parana with replaceble blades). It's also very very hot & humid here in Cincinnati.

Maybe both those things combined add up to 400 RPM loss ?

 

[Bob_VT]

Re: 1998 Merc 115 - small loss of power

One other thing I thought of..... is your trim full of fluid? If it is not trimming up properly.... a tiny bit will affect the rpms.

 

[studlymandingo]

Re: 1998 Merc 115 - small loss of power

The nicked blade will cause some cavitation. The heat and humidity will cause some loss of power. As Bob said, there could be some extra weight you're carrying around. Kinda like the way you feel just after Thanksgiving dinner.

One thing to note: 5000 RPM isn't high enough for that boat anyway; you should be running closer to 5600-5800. I would try going with a 17P prop, you'd be amazed at the difference!

 

[MercGuy]

Re: 1998 Merc 115 - small loss of power

One thing to note: 5000 RPM isn't high enough for that boat anyway; you should be running closer to 5600-5800. I would try going with a 17P prop, you'd be amazed at the difference!

Actually, per Mercury the recommended WOT RPM for this engine is between 4750 and 5250...
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