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Author Topic:   Stator as Cause of Poor Acceleration Under Load
mtcop posted 12-04-2007 10:50 AM ET (US)   Profile for mtcop   Send Email to mtcop  
I've been having problems out of my 1995 Mercury outboard [accelerating under load]. Idle is fine but when I [rapidly increase the throttle the motor] has no power and dies down. I also noticed the tachometer bouncing on me a few times. Before I used the boat last week the cranking battery was dead. The boat was laid up from July to November, and thought I may have left the fish finder on. Now I'm wondering if this may be another symptom of the main problem. Will replacing the stator correct [the inability of the motor to accelerate under load]?
Chuck Tribolet posted 12-04-2007 12:51 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
I think you need to measure the voltage at the battery,
with the engine off, with the engine at idle, and with the
engine at about 2000 RPM and report back the results. That
will tell us whether there's a charging system problem.

Tach symptom could also indicate a rectifier problem.

Sitting from July to November is enough to flatten the
battery, esp. if it's not new.


jimh posted 12-04-2007 02:11 PM ET (US)     Profile for jimh  Send Email to jimh     
I believe that the coil assembly in a Mercury outboard contains several windings. When you replace the coil assembly you replace all the windings. I am not sure which winding you are referring to as the "stator."

The typical windings in a coil assembly include:

--high current winding for battery charger output, usually about 10 to 15 VAC

--trigger winding for ignition timing control, usually just a few volts

--ignition winding for primary voltage to spark coil, usually several hundred volts

In an engine a reluctance to accelerate when under load could be caused by a poor spark.

mtcop posted 12-04-2007 03:20 PM ET (US)     Profile for mtcop  Send Email to mtcop     
After charging the battery I haven't had any other problems. The [inability to accelerate under load] is still a problem. I don't know much at all about boat motors but I can do limited work on most motors. The spark plugs are brand new plugs. I did have someone hook a meter up to the stator, and it was below the limits listed in the book, but not off very far at all. I did not want to order that expensive part if that may not be my problem.
davej14 posted 12-04-2007 08:09 PM ET (US)     Profile for davej14  Send Email to davej14     
Have you eliminated the fuel source as the problem? I'm not sure about the 1995 Merc, but my 2000 Mercury has a small fuel filter under the hood. I would take a good look at it and see if it contains debris. It only costs a few $$ so if it hasn't been replaced it would be cheap enough put in a new one. Is there an external filter in the fuel line also?

Other possible problems could be caused by Ethanol blended fuels and their attack on your fuel lines. If you have an external tank with the older black rubber lines it may be time to replace them and the primer bulb. If it is an external tank, empty it and look for something blocking the fuel line inside the tank. Fill it with fresh fuel and stabilizer.

Just a few low cost suggestions to get you started.

jimh posted 12-04-2007 09:19 PM ET (US)     Profile for jimh  Send Email to jimh     
Check the gap on all the spark plugs. I notice that my outboard motor tends to stumble or misfire when accelerating under load if the spark plug gap is greater than the specified or recommended spark plug gap.

The coil assemblies of Mercury outboards (which are often referred to as "the stator") are somewhat infamous for being the source of problems.

mtcop posted 12-05-2007 08:25 AM ET (US)     Profile for mtcop  Send Email to mtcop     
Well the stator I ordered came in yesterday and I installed it but the problem wasn't corrected. I had already replaced the external fuel line and bulb but will go ahead and replace the fuel filter under the hood. I'm also going to buy four new plugs today. I bought this boat brand new back in 1995 and sold it to a friend of mine at the end of 1996. I just bought it back from him in March, and I used the boat more when I had it that he did the entire time he owned it. He had taken it to a boat mechanic to fix the problem it is having now and spent over $400 with that guy. He had the fuel pump replaced and carburetors cleaned. After taking it back to him twice I saw that I was wasting my time. After replacing the plugs and fuel filter I guess the only thing left is the carburetors. I do want to thank you guys for your help and suggestions and would like more if you have them.
jimh posted 12-05-2007 09:17 PM ET (US)     Profile for jimh  Send Email to jimh     
Based on some reading I've done (but not on actual repairs) it seems like the Mercury outboard ignition system has two sets of windings involved in generating the primary voltage for the spark coils. The two coil winds are optimized for different engine speed ranges and are often called the "low speed stator" and the "high speed stator". When generating a voltage using a permanent magnet alternator--which is what these systems use--the number of turns in the coil and the speed of rotation determine the output voltage. At some point as the engine speed increases, the magnetic flux increases in these coils, they become "saturated," and the output voltage does not increase further. I believe that with the Mercury motors the low-speed winding is set to create sufficient voltage at lower speeds, but then it become saturated as the engine speed increases. The second coil, the high-speed winding, tanks over and begins to produce proper voltage when the engine speed increases. In this way, the ignition system can be supplied with strong primary voltage throughout the engine speed range.

The other component in the Mercury ignition system is the "switchbox." This component rectifies the ignition coil output, stores the energy on a capacitor, and releases it when triggered to fire the plugs. If the switchbox is malfunctioning, you will also have ignition related problems. On some motors there are pairs of switchboxes, one for each bank or set of cylinders. Sometimes you can swap these to see if a problem follows the switchbox. But check with a good Mercury mechanic before trying that!

Let us know if you find the remedy. It is always interesting to take these problems to their ultimate resolution and have the final fix revealed, assuming there is one.

swist posted 12-07-2007 06:44 PM ET (US)     Profile for swist  Send Email to swist     
Why are you focusing on the electrical system? Bogging down during acceleration under load is a classic symptom of a lean condition in the carbs. Although it certainly could be other things, I'd look into the possibility that it might be time to rebuild the carbs.
jimh posted 12-07-2007 08:18 PM ET (US)     Profile for jimh  Send Email to jimh     
Carburetors are not much of a topic for a discussion in the SMALL BOAT ELECTRICAL area. However, I agree, lack of fuel is certainly a likely cause for poor acceleration, and probably the reason most carburetors have some sort of accelerator pump system to throw some extra fuel into the system when the throttle is suddenly increased. But we're on an electrical topic here, not fuel. If the problem is just getting an engine to run, it would be in the REPAIRS/MODS discussion. This is an electrical discussion, hence the concentration on electrical causes.
jandrewg posted 12-08-2007 01:46 PM ET (US)     Profile for jandrewg  Send Email to jandrewg     
I have a 1995 125 hp two stroke and changed the stator this year. After buying a manual to get the specified resistance readings for the low and high speed windings Jim mentions, I found I really hadn't needed to. The low speed winding had no resistance at all, meaning there was a break in the winding somewhere, an open circuit. Of course, the manual is a good thing anyway.

On a 1995 motor there is one switchbox, and four coils. In 1998 I believe, they did away with the switchbox, and instead of coils, they use CDM's (capacitive discharge modules). I think there is a test given in the manual for your switchbox, and that this could be your problem.

I also think that even very minor crud in your carbs could create the misfire you describe, and if I were to guess, it might be in the two carbs that don't have idle circuits (where the motor idles on two cylinders and tranfers to four as the throttle is advanced). You've said that they have already been cleaned, but it doesn't take much varnish or a very tiny speck to plug things up.

Either way, a good solid mechanic, with a good diagnostic ability, should be able to test and eliminate these two obvious areas. It's not rocket science, after all, just an outboard motor that needs fuel, air, and spark to run.

Finding that good mechanic just might be rocket science though...


jandrewg posted 12-08-2007 01:56 PM ET (US)     Profile for jandrewg  Send Email to jandrewg     
One more thought...

You describe your problem when "rapidly" increasing the throttle. Can you gradually accelerate to cruising RPM and does the motor seem to run fine when you do? If so, then I might suspect the accelerator pumps in those two carbs without an idle circuit.

The squirt of fuel from an accelerator pump is there to compensate for the lean condition caused by opening the butterfly when you first apply throttle, and before there is venturi vacuum to draw the extra fuel needed for the extra air.

This is an electrical thread, but these two systems can act the same sometimes, and we're trying to diagonse the problem. Maybe a diagnostic area would be a good idea?


davej14 posted 12-10-2007 12:14 PM ET (US)     Profile for davej14  Send Email to davej14     
A minor point of correction, if there is "no resistance at all" then there is continuity, perhaps a shorted coil winding. If there is a "break" in the coil winding you would have an open circuit and expect to have infinite resistance.

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