2004 90 hp mercury won't rev above 3800 rpm

kashmir1994

Recruit
Joined
Aug 14, 2013
Messages
3
I have s 2004 mercury 90 hp two stroke on a 22 ft pontoon boat. I just purchased the boat three days ago. The engine starts and idles perfect. It seems sluggish on take off and will not get past 3800 rpm and 14 mph by GPS. I have checked the throttle/carburetor linkage and it appears to be opening fully at full throttle. The oil injection appears to be working as is the oil injection alarm. Compression test by the dealer was 110, 110 and 100. This was a cold test. I found the two yellow wires coming out of the stator to have melted together. I separated the wires and redid the connections. Up until this point the tach didn't work. It now works fine. According to the dealer the voltage regulator is new. It has a 13.5x15 prop on it. The prop has flight damage, but in my uneducated opinion, nothing that would cause such poor performance. I haven't checked the spark to each cylinder. I will be doing that today. According to the onboard voltmeter the charging system appears to be working. Could the stator be bad? I have a lot of automotive experience, but very little outboard. Any help would be appreciated. Additional info, the motor runs smooth all the time. Starts easily and there is no noticeable miss. Just a lack of power. Thanks again.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: 2004 90 hp mercury won't rev above 3800 rpm

That engine is extremely hard to determine a dropped cylinder by ear. I have one. If your tach is working the yellow wires to the regulator/rectifier module are carrying power as designed. The tach signal is the grey wire from that module and counts the pulses that the module receives and for your battery to be charging as it should.....up to 14.5 volts depending on rpm and battery charge level you have no problems there. Also this has nothing to do with performance....merely recharges the battery.

The fastest way to determine if all cylinders are functioning is with a timing light, battery operated would be best. Borrow one if you don't have one.....were a necessity back when points, coils, and condensers were the norm.

Take the cowl off and hook up power to the light. The pickup lead does not have to clip onto the sparkplug wire. Those ignition systems put out such a hot spark that you just get the tip of the lead near the plug and if it's firing it will light off and blink. Run the engine at different speeds if necessary to verify that one isn't firing.

If you have 2 that blink and one that doesn't remove the CDI (the module that the spark plug wire comes out of) from the non blinking slot and swap it with one of the others. If it fails in another position, get a new one....about $100. I think I bought my last one on here. I guarantee you that when you get all 3 cylinders lighting off, performance is greatly improved. BTDT.

Mark
 

kashmir1994

Recruit
Joined
Aug 14, 2013
Messages
3
Re: 2004 90 hp mercury won't rev above 3800 rpm

That engine is extremely hard to determine a dropped cylinder by ear. I have one. If your tach is working the yellow wires to the regulator/rectifier module are carrying power as designed. The tach signal is the grey wire from that module and counts the pulses that the module receives and for your battery to be charging as it should.....up to 14.5 volts depending on rpm and battery charge level you have no problems there. Also this has nothing to do with performance....merely recharges the battery.

The fastest way to determine if all cylinders are functioning is with a timing light, battery operated would be best. Borrow one if you don't have one.....were a necessity back when points, coils, and condensers were the norm.

Take the cowl off and hook up power to the light. The pickup lead does not have to clip onto the sparkplug wire. Those ignition systems put out such a hot spark that you just get the tip of the lead near the plug and if it's firing it will light off and blink. Run the engine at different speeds if necessary to verify that one isn't firing.

If you have 2 that blink and one that doesn't remove the CDI (the module that the spark plug wire comes out of) from the non blinking slot and swap it with one of the others. If it fails in another position, get a new one....about $100. I think I bought my last one on here. I guarantee you that when you get all 3 cylinders lighting off, performance is greatly improved. BTDT.

Mark

Mark,
Thanks for the advice. I did as you recommended and #3 CID wasn't working. Going to get a new one today. Hopefully it will make all the difference. Thanks again.
Eli
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: 2004 90 hp mercury won't rev above 3800 rpm

Mark,
Thanks for the advice. I did as you recommended and #3 CID wasn't working. Going to get a new one today. Hopefully it will make all the difference. Thanks again.
Eli

Guarantee you my man. Been There Done That (BTDT) it surely will.
Good luck,
Mark
 

kashmir1994

Recruit
Joined
Aug 14, 2013
Messages
3
Re: 2004 90 hp mercury won't rev above 3800 rpm

Mark,
Boat runs great. Thanks for the help!!
Eli
 
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