Merc 115 Losing Top End Speed

[CTsalt12]

Hey All,

I have a '97 Merc 115 I've had on my 175 for the 4 years I've owned it now. 

I've noticed the last few years I've lost some top end speed.  I never take it above 4200 really, but my cruising speed used to be 33 and now it's about 26/7.  It runs fine, no issues, just doesn't have the oomph it used to.

A big about the history: I've had carbs cleaned a few times because of gas issues.  Replaced starter.  T Stats, poppets and impeller every three, all standard maintenance.  Plugs twice a season along with fuel filters.  All fuel lines replaced last year. 

I'm wondering what the best way to diagnose this is?  I've heard from a buddy it's probably time to replace the carbs.  The carbs on this engine are at minimum 8 years old, likely older.  I'm thinking a compression test it in order before anything, to rule out anything catastrophic.  Or should I just keep running it and suck it up, and chalk it off to wear and tear.  The speed loss doesn't affect my fishing much, I just want to make sure it's not a sign of something else that needs attention and could take years off the life.

Any ideas here? 

[fitz73222]

Hey Jimbo,
I may have you confused with someone else but didn't you run this engine out of oil once?

[CTsalt12]

Fitz-Nope, no confusion.  That's me.  End of 2014 that did happen. 

It wouldn't plane once and sounded off, I checked oil and sure enough it was dry.  Filled it up promptly.  Engine alarm wasn't working.  Caught it somewhat early, but it happened and certainly may be the factor. 

It did run fine all last season since then, save for the lower top end speed.

Safe to say that is the issue? 

[fitz73222]

A compression and leakdown test would be the first place to start. Running one out of oil is typically the kiss of death so it is amazing it is still alive.

[CLM65]

You mentioned that it was losing speed over the past few years.  Did this start prior to the oil incident?  Have you ever weighed the boat, or notice that it may be sitting lower in the water?  I would start with Farley's recommendation, but one thing to also consider is that it may be gaining weight due to water absorption.

[CTsalt12]

You mentioned that it was losing speed over the past few years.  Did this start prior to the oil incident?  Have you ever weighed the boat, or notice that it may be sitting lower in the water?  I would start with Farley's recommendation, but one thing to also consider is that it may be gaining weight due to water absorption.

I did notice it before the oil incident, which is why I didn't mention that originally.  I think the boat very well may have gained some weight.  I had a tiny bit of growth above the waterline where I bottom paint it the year before.  This is something I hate to think about because the only way to fix it would be a complete teardown.  The growth could also be because I became more diligent about maintaining a full tank of gas after having some phase fuel separation issues in prior seasons. 

The main suspect for water weight is the foam in the stringers, correct?  I've felt my stringers from the bilge area, and they do feel dry.  I guess this is the year I should cut a cube out and feel for moisture?  Where else could I have gained weight?

[Capt. Bob]

  Where else could I have gained weight?

Belly and butt is where most of us older guys see an increase.

[GoneFission]

  Where else could I have gained weight?

Belly and butt is where most of us older guys see an increase.

  

[akbridge]

What condition is your prop?  If you have a ragged out prop you can lose speed.  I would check the prop and have a compression check done as a place to start from.  I doubt you need new carbs.  Also, could this engine need to link'd and sync'd so it can get full timing advance and open throttles at WFO?  I don't know if Mercs of this era need to have this done. 

[CTsalt12]

Another option occurred to me today when prepping for annual bottom paint, a ritual you southern guys don't have to deal with, especially those who trailer. 

My bottom paint has started to really flake off in big chunks the last 2-3 years, the same general time range as I've lost speed.  Particularly in the stern and the keel
In the bow Is a mess.  I've sort of shrugged it off because I'm not ready to shell out over a G to have it totally stripped right, then hundreds (more) $ on paint for an old hull.  But this very well may be the issue.  Certainly has me thinking...I'm not sure it's worth the dough for a bit more speed I usually can't handle because of the chop anyway.