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Author Topic:   Mercury 140-HP: Ignition System: Spark Misfires
haze posted 12-31-2007 07:49 PM ET (US)   Profile for haze   Send Email to haze  
My 2002 Merc 140 has been perfect since new until this week. Now it misfires under load to the point of not useable. It seems to cross fire and won't [run at] cruise [speed]. I changed to new plugs. Same. I changed to new fuel and tank. Same. I suspect saltwater conductivity across and at coil connections. Yesterday it went perfectly for five minutes when accelerating but then started misfiring again. I plan to wash salt out. Any other ideas? Paul
joeyuniz posted 01-05-2008 01:36 PM ET (US)     Profile for joeyuniz  Send Email to joeyuniz     
It could be a carburetor problem or a coil plug wire. Try to locate the cylinder or possibly cylinders that are not firing or going on and off intermittently. Also check to see if one side gets hotter than the other.
Tohsgib posted 01-07-2008 02:21 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
I would also suggest coils, seems like something electronic is heating up then misfiring.
haze posted 01-08-2008 06:03 AM ET (US)     Profile for haze  Send Email to haze     
So far I have checked compression. All about 120-PSI and washed off salt. Yesterday the motor started and accelerated twice no missing all power was there. After five minutes the missing returned and I just got back to the ramp. I then drained all carburtor bowls but no water or trash in the fuel. I started the motor with the ears and it ran perfectly. I pulled the leads off one at a time and the spark jumped the 1/2 inch gap to keep firing. I am now suspecting a [Capacitor Discharge Ignition] breakdown once it warms up. I check all vacuum leads and fuel flow. I run an electric pump as well. Filters are good.
jimh posted 01-08-2008 09:34 AM ET (US)     Profile for jimh  Send Email to jimh     
Electrical components often react to heat.

Use an in-line spark tester to observe which cylinder is experiencing the misfire. Often there are duplicate sets of ignition components, one for each bank of cylinders. If you can isolate the problem to a bank of cylinders, it may help you deduce the source of the problem.

Intermittent ignition problems can be hard to resolve. If you cannot definitively isolate the fault, I suggest replacing components of the ignition system in order of their cost. That usually means in this order: spark plug, ignition coil, ignition control module, stator. Note that you do not have to replace components with new parts; you can swap out components which appear to work properly with ones which appear to have malfunctioned.

L H G posted 01-08-2008 12:07 PM ET (US)     Profile for L H G    
To the best of my knowledge, the last 140HP Merc was made in 1980. It was an in-line 6 "Tower of Power". Which 2002 engine is this thread about?
haze posted 01-09-2008 12:35 AM ET (US)     Profile for haze  Send Email to haze     
This is a v6 2.5 litre carb engine. same block is used for 140hp through to 200hp some with efi and optimax options. All black case with red stripes etc.
L H G posted 01-09-2008 01:36 PM ET (US)     Profile for L H G    
In the States, that block has never been sold as a 140.
Here it's 135 Optimax, or 150,175 & 200 carb, EFI's or Optimax. I'd take the engine to a dealer for repair. Sounds like a trigger or stator to me.
jimh posted 01-09-2008 10:28 PM ET (US)     Profile for jimh  Send Email to jimh     
Larry always thinks it's the stator. That may be because on a Mercury outboard it often is a failure in one of the windings of the stator which causes ignition problems. Mercury stators are somewhat infamous in that regard.

The ignition system of these older outboard motors is not particularly sophisticated, and anyone with a relatively basic grasp of electrical theory can understand, diagnose, and repair it. Heck, even people who have no particular grasp of electricity can make repairs by following a few diagnostic tests, and in many outboard repair shops, that is what you get for your $95/hour in cost.

There is a great reference available on-line which gives concise diagnostic procedures. Look into this resource before giving up:

CDI Electronics

Their website has a great deal of information, including this nice guide to troubleshooting outboard engine electrical systems: Troubleshooting%20Guide%202006.pdf

jimh posted 01-09-2008 10:32 PM ET (US)     Profile for jimh  Send Email to jimh     
Also see:

Stator as Cause of Poor Acceleration Under Load

L H G posted 01-10-2008 04:40 PM ET (US)     Profile for L H G    
I forgot to add that:


I have had about a hundred of them replaced on my various V-6 Mercs. And I can't do it myself. If you have a V-6 2-stroke, non-Optimax Merc, you WILL have a stator failure sooner, if not later.

Now, with OMC's or Yamahas, the stator will NEVER fail.

Tohsgib posted 01-11-2008 02:11 PM ET (US)     Profile for Tohsgib  Send Email to Tohsgib     
Did you try the gas vent? It would take a few minutes for the vacuum totake effect and starve engine. What does gas bulb do when it starts to misfire? If it collapses or gets soft, you have a fuel problem.
haze posted 02-09-2008 06:26 PM ET (US)     Profile for haze  Send Email to haze     
So far the dealer has identified the problem to be the cdi or trigger/stator so he wil replace each one and test. He said coil 2 was firing at random when tested and also causing other 2 coils on the same wiring loop. I will let you know.
jimh posted 02-10-2008 10:42 AM ET (US)     Profile for jimh  Send Email to jimh     
Haze--Thanks for the follow up on your Mercury motor's problem with the ignition system. It sound like the capacitor discharge ignition (CDI) or the trigger coil portion of the "stator" coil assembly are being considered as the culprits.

Also, thanks for mentioning that the malfunction was causing errant firing of other cylinders. That is good information.

I hope your Mercury outboard ignition problem can be remedied by replacement of the stator or other components. Please let us know the final outcome.

The biggest problem with repair of these electrical devices is not electrical but mechanical. Usually the stator assembly is located under the flywheel of the outboard. Depending on how the flywheel is fastened to the crankshaft, removal of the flywheel can be difficult to accomplish without specialized tools. On some motors, a large single nut is used to retain the flywheel. This large single nut is often tightened to very high torque. Loosening that nut can be difficult. Generally an impact wrench is needed, as well as a specialized tool to retain the flywheel so that the torque can be applied against the nut.

Contrary to some advice offered above, I would not characterize the Yamaha outboard has having a stator assembly which is immune to failure. What is somewhat different about the Yamaha motor is the separation of the several coils of the "stator" into individual assemblies. You can purchase the individual coils, as needed, instead of having to buy a complete assembly of all the coils. This may reduce the cost of repairs.

haze posted 02-18-2008 09:02 PM ET (US)     Profile for haze  Send Email to haze     
Today the dealer confirmed it was a faulty trigger and that the CDI had survived. He replaced the trigger coils and it runs fine again. The next interesting item of information was that only Verado and Optimax are now made in USA and the carburetor two-cycle motors are outsourced to Tohatsu with some quality [problems] now arising and more of his customers moving to Suzuki and E-TEC. Thanks to everyone for their comments, ideas and help. Certainly isolated the problem to the ignition timing system. Mercury Stators? No doubt about them. Regards Paul

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