Tower of Power bogging out in the water

[leirbag]

So my buddy picked up a boat with a 115 mercury on it. The motor runs strong when it's out of the water, revs quickly all the way up to 6k rpm without hesitation. When you get the motor in the water though, you can't get it over 1400 rpms, and if you go to full throttle to quickly, it'll just bog down and quit. The carb's were all recently cleaned, and we're running the motor on a 30:1 fuel ratio directly from a 5 gallon gas container to the fuel pump. Compression looks this from the top:

100
90
110
115
115
110

From my experience with two strokes, that should be enough compression to at least get around. Plugs are all new, and have plenty of spark. Any help getting us in the water this summer would be greatly appreciated! TIA!

 

[racerone]

Re: Tower of Power bogging out in the water

First of all winding this motor up in nuetral in the driveway is a mistake !--Do not do that as it proves nothing at all and can do serious damage to the motor.--------No year / serial mentioned.--The compression values are a concern.---Remove intake bypass covers on the starboard side of the motor and have a look at the pistons and rings.

 

[leirbag]

Re: Tower of Power bogging out in the water

The boat this motor came on is an 87 and the charts show this as an 85-88 so we'll go with 87... Serial is a909987.

 

[wired247]

Re: Tower of Power bogging out in the water

Its a 1986 motor. https://webapps.brunswick.com/literature/literaturePartsInfo.do?indexnbr=0000000278&docWorldFile=COMMON/&mplDescription=MARINER%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20&footType=N&serial=0A909987&mdlNbr=%20%207115746&umlHpGroup=115%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20%20&umlModelYear=1986%20%20%20%20%20%20%20%20%20%20%20&catnbr=%2013290&isisFlag=Try decarbing the motor before you get too depressed about #2. Also try another gauge. I have 4 of them and can use a different gauge to get a different reading if the numbers bum me out too much. Inlines are all about carb adjustment and ignition advance and its relationship to the throttle opening. If the carbs idle adjustment is too lean or if the passages are plugged it wont transition into the main circuit and it bogs.

Revving the motor up in the driveway is one of those thing you hear poopoo'd quite a bit with no explanation of what tragedy would actually happen if you did it. If the prop ( and thrust washer ) is removed I dont see it hurting a thing.

 

[leirbag]

Re: Tower of Power bogging out in the water

So we took it to a local mechanic who found that the plug wires for cylinder 3 and 4 were crossed. Swapped those around and now the motor will come up to 2500 rpm in the water, does that now at full throttle. Seems like it runs much better.

What rpm range should we be able to get out of this motor when it's running healthy?

 

[MikDee]

Re: Tower of Power bogging out in the water

By the way, free revving a motor leads to excess piston speed which destroys rings, & cylinder walls! It should be avoided if at all possible.

 

[racerone]

Re: Tower of Power bogging out in the water

If the motor is healthy it should run up to 5500 RPM with the correct prop for the boat.--Is the tachometer set / reading correctly??----Compression values indicate that the motor is not healthy !!

 

[leirbag]

Re: Tower of Power bogging out in the water

Thanks for the info, I realize it's not very healthy, really just trying to get an idea of where this motors performance should fall given it's health. We're not really planning on spending any money to tear into the motor.

 

[Chris1956]

Re: Tower of Power bogging out in the water

What prop and what kind of boat do you have it on? Mix the fuel at 50::1. I thought your compression was OK. It is a bit low, but serviceable.

That motor will have a high speed ignition winding on the stator and a low speed one. If your Tach is correct, and the prop is right, a bad high speed winding on the stator, could (could) be a cause of no high RPM.

 

[wired247]

Re: Tower of Power bogging out in the water

By the way, free revving a motor leads to excess piston speed which destroys rings, & cylinder walls! It should be avoided if at all possible.

A motor running in the water at 5000 RPM and a motor running out of the water at 5000 have the exact same piston speed.

 

[leirbag]

Re: Tower of Power bogging out in the water

What prop and what kind of boat do you have it on? Mix the fuel at 50::1. I thought your compression was OK. It is a bit low, but serviceable.

That motor will have a high speed ignition winding on the stator and a low speed one. If your Tach is correct, and the prop is right, a bad high speed winding on the stator, could (could) be a cause of no high RPM.

I'll find out what prop is on it, the boat is a 19' center console, not sure what model.

If it was the high speed winding on the stator, wouldn't that affect it while it's in neutral as well?

 

[canoemang]

Re: Tower of Power bogging out in the water

Check spark and use an inductive timing light to verify. Also do a link n sync just to verify timing and carb openings and settings.

90 is a bit low but maybe the rigs will loosen up a bit after its run correctly.. and yes as said above try another gauge.

 

[MikDee]

Re: Tower of Power bogging out in the water

A motor running in the water at 5000 RPM and a motor running out of the water at 5000 have the exact same piston speed.

I said free revving! Goosing the motor constantly in neutral, it's a fact!