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Author Topic:   Mercury 90-HP: Replaced Stator, Won't Stay Running
soshea posted 06-11-2015 08:00 PM ET (US)   Profile for soshea   Send Email to soshea  
I replaced the stator on my c.1985 Mercury 90-HP outboard (Tallboy or Black Beauty). The engine started with the throttle [advanced], but when [the throttle lever is] dropped back to [idle], the engine dies. Any insight would be greatly appreciated. Thanks.
number9 posted 06-12-2015 03:58 AM ET (US)     Profile for number9  Send Email to number9     
Did it run OK or better prior to replacement? More information is needed to lend a hand with troubleshooting.
soshea posted 06-12-2015 07:30 AM ET (US)     Profile for soshea  Send Email to soshea     
[The c.1985 Mercury 90-HP outboard engine prior to the installation of the new stator] ran fine at low speeds, but when pushed above 3000-RPM it would run for about 45-seconds then drop to 3000-RPM. An ohms test determined the stator needed to be replaced. Before determining it was electrical, [I performed procedures to] troubleshoot the fuel related possibilities, such as carburetors, gas, fuel pump, filter, fuel lines, etc. Is [the remedy for this new problem of stalling at idle speed] something as simple as to switch the yellow wires on the rectifier?
jimh posted 06-12-2015 07:36 AM ET (US)     Profile for jimh  Send Email to jimh     
If you suspect the cause of the problem with the c.1985 Mercury 90-HP engine being unable to remain running at idle speed setting of the throttle is related to an electrical power generation problem from the stator, I recommend you insert an in-line spark checker to see there is good spark ignition voltage present for all cylinders.

I am not expert on these older Mercury engines, but I have seen many mentions of them having separate coil windings in the stator for generation of spark coil primary voltage. One winding is optimized for low engine speed and the other for higher engine speeds. Since your narrative reports you just replaced "the stator", I presume you replaced all the coils in one replacement part installation.

The next component involved in spark generation is typically an electronic device that controls the timing of the spark primary coil voltage. This is often called a switch box in Mercury lingo. The switch box may be affecting spark at low engine speeds.

Of course, there could be other causes for poor idle speed running, such as the fuel-air mixture ratio.

soshea posted 06-12-2015 09:34 AM ET (US)     Profile for soshea  Send Email to soshea     
Spark plugs are new and believed to have good spark in all cylinders. Engine starts and runs in choke position. When shifts into neutral, engine shuts off immediately. I am going to put the old stator back on to see if the problem still exists.
jimh posted 06-12-2015 01:41 PM ET (US)     Profile for jimh  Send Email to jimh     
The engine shutting off immediately is a good indicator the problem is electrical in nature.
soshea posted 06-13-2015 08:18 AM ET (US)     Profile for soshea  Send Email to soshea     
I took a picture of the wiring before removing the old stator, but used the schematics that came with the new stator and there was an error made when installing the new stator. Switched the red/white and blue/white and problem is gone. Not sure why the schematics differed from my picture, but it is fixed.

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