1998 Mercury 75hp 3-Cylinder 2-Stroke Problem

[hawgcaller]

I've got a 1998 75hp 3-Cylinder Mercury 2-Stroke (modeled by Tracker) on a Bass Tracker Pro Team 175. The motor has never given any major issues in the past. The only thing to fail has been a CDM for the #1 cylinder about 2 years ago. I originally thought it might have been the stator and/or voltage regulator, so I replaced both before I found out it was the CDM that was the problem. I have changed the plugs yearly, lower unit gear oil frequently, and it's been taken care of. I use the boat at least once a week for fishing local tournaments.

I'll try and be as detailed to what happened, and hopefully that'll help with the diagnosis. I started the tournament and the boat cranked fine, ran fine, giving no issues all morning. However, when I cranked up to go to one of my last holes, the boat wouldn't get on plane. The motors max RPMs are usually around 5100-5200 at WOT, but it wouldn't come past 3200 RPMs. The motor would get louder the more I pushed the throttle, but the power never increased. The fuel in the tank was maybe a week old. I added SeaFoam to the fuel, changed the spark plugs, added another new CDM b/c I thought it may have not been firing on the #3 cylinder the way it wouldn't gain power. I also pulled the brass drain plugs on all three carbs and they were clean, and was able to spray carb/choke cleaner in the drain hole and it sprayed back out the top port. I had my dad turn the ignition switch and I got fire on all plugs (However, I didnt see fire on the bottom plug, so I put the old CDM back on and got fire back). The motor cranks up fine, but at idle the motor doesn't sound like it used to, it's not a healthy sounding idle, and I can feel vibration through the steering wheel. I have read a lot of threads in the forum and can't seem to find anyone who has this similar problem. My problem is consistent, it's not an intermittent problem that comes and goes.

Does this sound like a fuel/fire problem? Could the big red Stator I replaced on top of the motor two years ago have prematurely failed? Are there two stators on the motor, a high end and low end? Any help is appreciated.

 

[CharlieB]

Re: 1998 Mercury 75hp 3-Cylinder 2-Stroke Problem

There is only one stator which contains both the low speed, high speed, and charging windings.

Limited RPM at WOT can be caused by one of three things;
1 limited fuel flow - squeeze the primer bulb to see if RPM increases, if so, rebuild the fuel pump
2 defective rectifier - disconnect both yellow wires from between the stator and rectifier, if the motor now performs, replace the rectifier
3 low output from the high speed windings of the stator - DVA test the stator
4 Weak or no spark on on or more cyls - test all plug wires for at least a hot 7/16 inch spark

Remember, there is NO RETURN on electrical parts, test, and retest to PROVE any fault before ordering any parts.

CDIElectronics wrote an excellent Ignition Troubleshooting Guide available for FREE download at their Support page. The Guide spells out every ignition and details which wire and how to test to positively determine any ignition problem, get it, read it, follow their directions.

 

[hawgcaller]

Re: 1998 Mercury 75hp 3-Cylinder 2-Stroke Problem

I have also replaced the fuel line with the primer bulb because the other one was about two years old and hard as a rock from the ethanol.

 

[hawgcaller]

Re: 1998 Mercury 75hp 3-Cylinder 2-Stroke Problem

I have squeezed the primer bulb while in gear and it doesn't make a difference in the performance, so I'm guessing the fuel pump is functioning. I'm getting a 7/16" spark on the plugs. I'll have to try unplugging the rectifier and see if it makes a difference. If it doesn't then I'll test the stator. Thanks Charlie for pointing me in the right direction.