115hp Mercury Outboard goes super slow when weight onboard.

[rrarra]

Help! I'm not sure this is normal but my 1997 115hp mere normally makes about 25 to 30knots when i'm on the boat by myself. When I add another person I am doing around 25knots. If I add another 2 persons or around 600lbs between the four passengers including myself my boat goes so slow (around 12knots), It's nearly impossible to get up to speed - much less plane. Is this normal? I know that my friends hate traveling with me when it's moving so slow and it's embarrassing.

Could this be a fuel/ethanol issue?

Boat: 19' Sea Sport

 

[Chris1956]

It is not an ethanol issue. You need to check the compression on all cylinders and see if all spark plugs have spark. That motor should not slow that much, so something is wrong.

 

[stclarl_98]

That is a heavier boat and the 115 might not be enough to get you up on plane with a bunch of heavy weights going fishing. A few things I can think of as suggestions are:

1. Change your propellor to stainless steel. If you are using aluminium prop your blades are straightening out and not giving you the thrust you need to move forward in the water.
2. You may need trim tabs to help you get up on plane.
3. Invest in a larger motor.
4. Distribute the weight forward in the boat to get up on plane.
5. Get smaller friends

If you boat in areas where there is alot of debris in the water be careful if you run the stainless prop.

Not a fuel issue.

 

[DavidMoore]

Not an ethanol problem for sure.

In addition to the above I'm wondering if your timing is advancing properly, a link and sync check might be worth it.

 

[Chris1956]

A stainless steel propeller will not solve your problem. If your compression, spark and link and synch check out, a prop with a smaller pitch might be necessary, to give some extra power. Do you have a tachometer?

 

[MH Hawker]

or it may have a bad coil

 

[rrarra]

That is a heavier boat and the 115 might not be enough to get you up on plane with a bunch of heavy weights going fishing. A few things I can think of as suggestions are:

1. Change your propellor to stainless steel. If you are using aluminium prop your blades are straightening out and not giving you the thrust you need to move forward in the water.
2. You may need trim tabs to help you get up on plane.
3. Invest in a larger motor.
4. Distribute the weight forward in the boat to get up on plane.
5. Get smaller friends

If you boat in areas where there is alot of debris in the water be careful if you run the stainless prop.

Not a fuel issue.

Thanks for your comments, lol @ #5. But in all seriousness, I think the trim tabs might be a good idea. But the best might be a newer, larger motor. Last night when I posted this I realized that I said I had a 1997 motor, it's actually a 2005 115. On the water today by myself I checked my RPM it was 5,500 and my speed was around 23knots. When my friends get onboard the motor sounds as if it's straining. My props are ss.

 

[rrarra]

It is not an ethanol issue. You need to check the compression on all cylinders and see if all spark plugs have spark. That motor should not slow that much, so something is wrong.

Thank you Chris, I'm gathering from everyone here that its not ethanol. My mechanic checked the compression & plugs and everything was good. Where do I go from here? I agree, something has to be wrong.

 

[rrarra]

Not an ethanol problem for sure.

In addition to the above I'm wondering if your timing is advancing properly, a link and sync check might be worth it.

Hey David, thanks for your reply.. What's a link & sync check & is it something I can perform myself?

 

[rrarra]

A stainless steel propeller will not solve your problem. If your compression, spark and link and synch check out, a prop with a smaller pitch might be necessary, to give some extra power. Do you have a tachometer?

Hi Chris, I just asked David above how I can perform this link & sync test, maybe you can assist. My tachometer reads around 5,500 RPMs with my speed around 23knots when I'm in the boat by myself.

 

[stclarl_98]

Rrarra, I myself have an old 18'Glaspar that I rebuilt over 20 years ago. I wanted my centre stringer to be solid so I buried the yellow cedar in 5 gallons of resin and fiberglass matt. It had a Mercury 150 in-line 6 that I broke every bolt in the head trying to remove them. Eventually finished the rebuild and it pushed the boat around nicely after launch. I eventually bought a Mercury 200 hp Black Max that pushes me around at 60 mph at 5600 RPM. My boat weighs now between 2-2500 lbs and was originally rated for a 110 hp motor. I had the boat resurveyed after rebuilding. I don't understand why we purchase boat motors to run them full out when by running my Black Max at 3600 rpm pushing me at 40 mph. Better on fuel...speed if you need and no need to get new friends. I run aluminum prop because of lumber...easier on the shafts and pocket book if and when you hit that deadhead...hopefully she's not bobbing vertical and 3 ft thick.

 

[flyingscott]

Hi Chris, I just asked David above how I can perform this link & sync test, maybe you can assist. My tachometer reads around 5,500 RPMs with my speed around 23knots when I'm in the boat by myself.

First thing you need to check is your tach. The operating range of that motor is 4750-5250 RPM so you are seriously overreving or your tach is off. If you are at 5500 RPMs at 23 knots at 30 knots you are seriously overreving. Check the tach that motor falls on it's face pretty quick when it is overreved. I am assuming it's a 4 cylinder 2+2 motor.

 

[Chris1956]

Agreed, if the tach reading is correct, the boat should easily plane with heavier weight and go faster than you report.

 

[rrarra]

Okay thanks guys. I agree that I am overreving. I'll find a tech that can check the tach - in the meantime I'm thinking that I should start to look around for a 200hp.

 

[DavidMoore]

A lync and sync is a procedure that ensures your carbs are opening at the same time and by the same amount, then adjusting the timing advance to be in sync with the opening of the carbs. You can do it yourself if you can find top dead centre and have a timing light.
​
The procedure is similar for all mercury 2 strokes but the settings might vary.

The chapter timing/synchronising/adjusting in the manual explains how to do it.

This is for my engine, http://boatinfo.no/lib/mercury/manuals/70-75-80-90-100-115.html#/58, maybe the manual for your engine can be found on this site (I would not know)

 

[rrarra]

Thank you David, I will give this a shot. I know a few people that may have a timing light.

 

[Vitamin Sea]

check your prop hub to make sure its not slipping you could have spun the hub