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Author Topic:   Mercury 90-HP Two-stroke Classic: Won't Run Above 2,600-RPM
bigmac posted 06-04-2007 02:47 PM ET (US)   Profile for bigmac   Send Email to bigmac  
I have only owned this [Mercury 90-HP Classic two-stroke outboard motor] for less than two months, and have run less than 10 gallons of gas from the installed tank over a maximum of four hours running time. She used to run fine right up to 5,000-RPM, then all of a sudden--I was not aboard the boat, I had loaned it out, I have no idea what happened, or exactly how. Boat ran fine, idling at about 800-RPM smoothly. Back to the dock with the complaint that it would not get up on plane. I had it out on plane several times that very morning and it ran fine right up to WOT.

In gear she runs fine and smooth at idle and all the way up to the brick wall of 2,600-RPM. Out of gear runs right up to 5,000-RPM. Throttle linkage feels and looks normal

I tried calling Merc customer service. They initially thought gas, pump, carburetors, etc.

The bulkhead mounted spin-on fuel filter and engine mounted cartrige were changed at commissioning two months ago and again after the problem occurred. The fuel lines were also changed. In order to fully rule out the existing fuel, and system, we used a external fuel tank with fresh gas and a fuel conditioner-gas cleaner additive. Running the half hour from the house to the launch ramp at a max of 2,600-RPM. No change, no change at all in max RPM. Stays at a max of 2,600-RPM. I would think if the carburetors were the problem there would be SOME change in running. Either better or worse but some change. Nothing.

The throttle moves through its required range but boggs down right at 2,600-RPM. Mind you it revs freely out of gear right up there to 5,000-RPM .

At Idle cold it runs SMOOTHLY, and warm it runs smooth with no missing or hesitation right up to that brick wall of 2,600-RPM. If the carburetors were a problem I don't think that the operation would be so good up to 2,600-RPM

Pretty much ruled out fuel, carburetors, or fuel pump. It runs nice and smooth at idle and all the way up to 2,600-RPM. The engine mounted fuel filter. If the fuel pump were bad or weak it would not be able to keep the engine mounted fuel filter even half-full under load, and pumping the bulb while at 2,600-RPM would provide MORE fuel to the engine if it were starved of gas due to fuel delivery system it should increase the rpm and it does not. Engine mounted fuel filter stays about half-full.

While running at 2,600-RPM if the "choke" really a enrichner is depressed it stalls out as it should. I think that pretty much puts the fuel issues to bed.

Throttle linkage was observed while running with the cowl off and appears to move normally within the stops for throttle and spark advance. The limit screws have paint still intact and if it is NOT in gear it runs right up to 5,000-RPM. I did not look into the mouth of the carbs while running to verify they were opening fully, however without the engine running they open fully at the appropriate point on the control lever movement.

Cooling: Fine, strong stream laser temp gun on heads shows they are all running about the same.

Lubrication: Fine using up the resevior oil at about the right rate and smoking at the appropiate times. Fortunate to have the identical engine but two years newer just down the dock.

SPARK. Determined by my looking under the flywheel that the stator is RED which MERC seem to think means there is both a high and low speed coil and CMDI ignition. There are four wires coming from the STATOR . Mercury could not suggest any resistance readings to be made on the coils and the cheezey CM manual I have indicates that there is NO high and low speed coils just one set. And the part number of the stator that Merc gave (832075A21) based on my serial number and the color does not match up with my cheesy CD. But there could have been changes.

STATOR WIRING: There are four wires coming from the stator, two solid yellow, one green/white tracer, and one white/green tracer. The two yellow are electrically shorted together, between them they read 0.2-ohm. From either yellow to either of the other two wires is a open. From the W/G to the G/W measures 675 ohms.

COMPRESSION: Checked today was exactly the same as two months ago, 120-PSI on each cylinder.

I ordered a set of carb rebuild kits and a manual from the local Mercury dealer on Wednesday. They assured me that they were in NJ and they would have them perhaps Thursday. I called Friday at noon and the truck had not come but they assured me they would call first thing. Now it is Saturday morining. No parts, no calls. Do ya think it would be a great idea to take my boat and motor there and let them sit on it for the summer? Right

I realize that none of the electronics packages are returnable, and that it can get difficult troubleshooting that way.

Currently the boat is in my driveway. If I take it to a dealer it will be many weeks before I see it again.


L H G posted 06-04-2007 05:28 PM ET (US)     Profile for L H G    
100 to 1 your problem is a bad stator. Those are classic symptoms.
jimh posted 06-05-2007 01:52 AM ET (US)     Profile for jimh  Send Email to jimh     
Please clarify the motor information. Is it a 90-HP or a 200-HP?

Also, what about the symptoms is a classic sign of a bad stator? The resistance readings? Or the way the engine is described as behaving?

Tom W Clark posted 06-05-2007 08:35 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
"...and if it is NOT in gear it runs right up to 5000rpm..."

If you keep doing that you will have must greater problems than a bad stator. Never exceed 2000 RPM while in neutral.

It should come as no surprise at all that a motor that is unable to generate full power can still achieve high RPM while in neutral. It is doing NO WORK, i.e. it is not under a load.

I have seen this odd troubleshooting technique (revving a motor while in neutral) so many time in my life I have to wonder if people understand how the internal combustion engine works.

jimh posted 06-05-2007 09:13 AM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the readings you have taken on the resistance of the stator coil windings:

Check to see that neither of these coils have any continuity to the engine chassis--but you have to disconnect the leads from other wiring to make that check.

The yellow leads are usually the battery charging leads and their resistance is often very low. When you read 0.2-ohms you are not reading a short, just the low resistance of the winding of the coil.

The other leads sound like they are a power coil associated with the ignition. They probably generate some signal for the ignition system.

There has been a lot of talk about Mercury motors having a so-called "low-speed stator" and a so-called "high-speed stator", but since your engine appears to have only a single stator, I don't think these designators apply to your situation.

jimh posted 06-05-2007 09:19 AM ET (US)     Profile for jimh  Send Email to jimh     
Here is the resistance reading for the stator coil on my 225-HP motor which has a 35-Ampere charging system:

Coil = 0.16 ohm plus or minus 0.05 ohm

Your value of 0.2 ohm is in that same region.

bigmac posted 06-06-2007 09:50 AM ET (US)     Profile for bigmac  Send Email to bigmac     
Jim, the motor is a 90 hp, I suspected that the problem may have been related to an unreported impact on the prop. Thought the timimg could be off. I checked the woodruff key. However the key was intact. Could the shaft have bent? It looks like fishing season is over. This will teach me not to lend out the yacht again.
jimh posted 06-06-2007 08:46 PM ET (US)     Profile for jimh  Send Email to jimh     
Do not run a motor using a propeller with bent blades. The imbalance in the propeller can damage the propeller shaft seals.

Have you corrected the problem? Please let us know the final outcome.

bigmac posted 06-07-2007 08:51 PM ET (US)     Profile for bigmac  Send Email to bigmac     
Firstly let me thank you all for your collective wisdom much appreciated. [T]his forum is great. [T]hanks. And to answer some [question]s:Tom Clark--please let me assure you I am not revving up this engine to [over 5,000-RPM] on a regular basis. I did it once VERY briefly to see if it was being limited. And I did cringe. I would not abuse a engine like that. I am ignorant in troubleshooting aspects but not stupid enough to destroy a perfectly good piece of equipment. The Prop is NOT bent. [T]here is a slight ding on one leading edge smaller than the size of a pea in the leading edge of the stainless propeller not worth mentioning cept it was not noticed prior to the incident.

Armed with three carburetor rebuilding kits and idle hands being tools of the devil I pulled one carburetor and opened the bowl. It looked pristine, as I thought. I quickly put it back together [before] the motor GODS noticed.

LHG--what are the classic symptoms of a bad stator? What I got? Under that red stator I got.

STATOR WIRING--There are four wires coming from the stator, two solid yellow, one green/white tracer, and one white/green tracer. The two yellow are electrically shorted together, between them they read 0.2-ohm. From either yellow to either of the other two wires is a open. From the W/G to the G/W measures 675 ohms. [Thanks--but I think you already gave us this information above--jimh.] And none of the leads measure anything to chassis ground,so that’s good.

TRIGGER COIL--also tucked up underneath the red stator is the output wires of a trigger circuit. They come out to bullet connectors as four wires, black, tan, brown, violet. This cheezey manual has no mention of anything like that. What might these measure? I would appreciate. I am assuming these are the low and high speed trigger windings. Measuring between each of them and chassis ground is a open. Black to brown measures 1.6-MOhm. And no other combinations yield any resistance readings. whazzupwidat

Awaiting the sage responses of the forum. Plan B is to give the beast to the local marina, cancel the insurance and buy a inner tube.

jimh posted 06-07-2007 09:28 PM ET (US)     Profile for jimh  Send Email to jimh     
Many people lump all of the coils under the flywheel under a single heading, "the stator." There usually are multiple coils. The normal set up is:

--high current coil for battery charging.It puts out about 14 to 16 volts at 20-amperes or more. This coil is often called the stator of the alternator. The battery charger coil connects to the rectifier and regulator. It is usually not a part of the spark system.

--ignition spark primary voltage coil. It puts out the low voltage for the spark coil, about 200 to 300-volts. This is what powers the ignition system. The ignition coil connects to an electronics module. The module has its own rectifier and a capacitor. A DC voltage (of around 300 volts ore more) is produced and stored on a capacitor.

--ignition timing or trigger coils. These are coils arranged to be triggered by magnets to control the spark plug firing timing and sequence. Often these coils are grouped in pairs or in triads for multiple cylinder engines. These coils generate a pulse which is used to trigger an SCR to conduct the primary coil voltage to the spark coil. When the SCR conducts the voltage is sent to the primary of the spark coil. This produces the high-voltage spark.

When someone says "You've got a bad stator" I don't know exactly what they mean. There are at least three coil assemblies under the flywheel, and sometimes four or five coil assemblies. In some motors these coils are separate and individual components, and they can be replaced individually. (Yamaha motors often have this arrangement). In other motors there are two assemblies of multiple coils, but you can still replace one without the other. (OMC motors often have this arrangement. They usually have the charging coil and the ignition coil in one assembly, and the timer base or trigger coils in a separate assemby). And finally in some motors ALL of the coils are in one assembly, and you have to replace them all, even if only one of them are bad. (This is often seen in Mercury motors).

On a Mercury motor, recommending a replacement of "the stator" is really recommending a replacement of all possible coils under the flywheel. You have to buy them all in one expensive assembly, even if only one of the many coils is bad.

Replacing a coil assembly ("the stator") is a lot of work because you have to pull the flywheel off the crankshaft. This is not a trivial task. First of all, you need an impact wrench to break the flywheel nut loose in many cases. Next, there are very powerful magnets involved and there is some danger in handling the flywheel. The magnets can cause injury if your finger gets pinched by their force. Finally, when the flywheel is removed and reinstalled you should carefully check the engine timing to be sure everything is back to normal. You may have to make adjustments. You will certainly have to make some adjustments if you put in new coils, as you cannot guarantee the coil positions will be precisely where they used to be.

So before proceeding with replacement of "the stator" I would be darn certain I had isolated the problem to be "the stator."

On Mercury motors they call the ignition electronics module the "switch box" and these can often be a source of problems, too. They are much easier to bolt on and replace than a stator.

Sal DiMercurio posted 06-07-2007 09:31 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
I must second Tom advice. Absolutely do NOT ever ever ever run a two-cycle engine over 2,000-RPM while in neutral, not even for a brief second. My thoughts on this deal is a bad power pack that only allows half speed while under a load.
jimh posted 06-08-2007 09:11 AM ET (US)     Profile for jimh  Send Email to jimh     
I am assuming these are the low and high speed trigger windings

References are continually made to "high" and "low" windings on coils associated with Mercury motors, but I do not recall anyone providing an explanation of how these function and how the ignition system switches between them. This would be helpful to know. Perhaps someone who is using the term "high speed" and "low speed" in regard to the Mercury ignition system could explain how it works.

Tohsgib posted 06-08-2007 09:23 AM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
My only question is she running on all 3 cyls at idle or up to 2600rpm? If yes then you can probably rule out coils and carbs.
Landlocked posted 06-08-2007 11:02 AM ET (US)     Profile for Landlocked  Send Email to Landlocked     
Before I did anything, I'd change out the little engine mounted fuel filter. I know you say it is OK but its a cheap check. I've had very similar problems and they were fuel filter related.


bigmac posted 06-08-2007 01:52 PM ET (US)     Profile for bigmac  Send Email to bigmac     
[Suspects that the motor is being intentionally limited by "Guardian" protection system.]

[Recounts having made] four separate calls to MERCURY OUTBOARD TECH CUST SERVICE. [Recounts having spoken to] three separate agents. [Recounts having sent] emails with Mercury manager. [Recounts having had] face to faces with five distinct Merc dealers.

[Voice surprise that after all of this there was] no mention of a particular system that is in my cheesey CD manual in pdf format on page 274-5: the GUARDIAN PROTECTION SYSTEM.

Not too detailed on exactly which sensors affect which model year(s) but the mentioned ones are low oil reservoir, engine overtemp, air compressor overheat, block water pressure low, battey voltage low or high, throttle position sensor failure. But the result is always the same: it triggers this mystery (at this point) GUARDIAN PROTECTION SYSTEM and limits the RPM to 3000. My exact symptom plus or minus a few hundred rpm! Why did no one at MERC KNOW? MY ENGINE is a 2000 model year, so it is not an OPTIMAX and the dedicated DTT troubleshooting box does not apply. I hope my hard copy $80 manual comes in from Merc and hope it is better than the CD. Chasing wires gets me dizzy. Yea I know how can I tell. I will be checking the board. Thanks for all the advice. Stay tuned. I will be "bonding" with my wiring.

And to answer some questions: yes, she runs just fine on all three right up to the 2600-RPM limit. and idles fine and smooth as well. Plugs which were changed look normal. Compression is fine at 120-PSI. The engine mounted fuel filter is staying full at 2600-RPM and has been changed 10 gallons ago at launch and last week when the troubles started and the main fileter and a aux gas tank, bulb, hoze was used. It is not fuel. This GUARDIAN explaination fits perfectly.

newt posted 06-08-2007 03:11 PM ET (US)     Profile for newt  Send Email to newt     

The guardian system is new to me also. There was no mention of it in the owner's manual when I bought my 2003 Mercury 90 hp classic! From your description though, it doesn't sound like something that applies to the classic two-stroke! What air compressor? what TPS?

Sal DiMercurio posted 06-08-2007 06:42 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
I thought about it might possible to be stuck in "SLOW" mode, but the rpms didn't seem right.
Evinrude "SLOW" mode is 1800 rpms I think & the 2600 on the merc threw me a curve.
bigmac posted 06-08-2007 08:04 PM ET (US)     Profile for bigmac  Send Email to bigmac     
Thanks all for your replies. Let us analyze this


1.What is the guardian protection system?
2.What are the years, atributes, trigger points, inputs of the guardian system?
3. Does a valid wireing diagram exist anywhere?

1.The symptoms (we need not rehash)
2.The skills of Mercury customer service
3.The "lack of committment" to anything relating to a specific engine, type, year in the cd manual
4.That you cannot fail until you quit trying. My dad was a paratrooper and D-day vet.
5.I will know every wire and circuit of this engine soon.

Don't care to talk to Mercury outboard tech cust service. Can they not use a find key on a PDF file? Stay tuned, I have nothing to loose at this point.

jimh posted 06-09-2007 08:40 AM ET (US)     Profile for jimh  Send Email to jimh     
If there is a protective circuit which has activated and is limiting the engine speed to 2,600-RPM, how were you able to operate the engine at 5,000-RPM when the engine is neutral? The protective circuit should have prevented this operation, too.
bkovak posted 06-09-2007 09:10 AM ET (US)     Profile for bkovak  Send Email to bkovak     
Why are you ruling out the carbs? I had this happen twice last summer on my evinrude 70 hp. Would run fine up to about 2500 rpms, then bog down and not go any faster. Both times it turned out to be clogged jets in the carbs due to the ethanol in the fuel. Cleaned carbs, installed a 10 micron filter, new tank and no more problems. Brian
seajet posted 06-09-2007 03:49 PM ET (US)     Profile for seajet  Send Email to seajet     
check the spark advance linkage, a friend of mine was having the same problem ,the bolt it hinges on we removed and behind it we found a steel washer all rusted we wire wheeled it up and lubed it and it was fine after ,good luck Richard
bigmac posted 06-10-2007 11:43 AM ET (US)     Profile for bigmac  Send Email to bigmac     
First let me say never buy a CD of a manual. The CD was way in the weeds as to what I should find and I think I found all of it. I will hook up the only two possible alarm--the low reservour oil and cylinder over-temp switch.
Back to the orig problem. This engine could not be simpler. There is no SWITCH BOX, no power pack, NO RPM limiter, NO GUARDIAN PROTECTION SYSTEM, no whistles, no bells. Heck the warning horn has never ever been hooked up. I am going with carburetors. I hope I don't screw them up.
bang4dabuck posted 06-10-2007 12:04 PM ET (US)     Profile for bang4dabuck  Send Email to bang4dabuck     
Sounds like carb hi speed jets are clogged. And one more important thing. Guy down at Shore Marine in Quinby, VA. seemed like a very knowledgeable and experienced guy; he was of the opinion to rule everything out first and then switch out the stator. Testing stator was useless. Stator will test out good but fail when under real use.

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