Diagnosing Cause of Misfire and Vibration in Mercury Outboard

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Author Topic:   Diagnosing Cause of Misfire and Vibration in Mercury Outboard

mccrickard posted 10-02-2014 09:59 AM ET (US)
I have a Mercury 75-HP outboard, on a used boat I bought a few months ago. The engine ran like a top until a few weeks ago. The Mercury 75-HP outboard started to vibrate at idle and at higher speeds; there is a very noticeable miss when at idle, as well. I have changed the fuel filter due to the old one looking very bad. Any help is welcome, thanks
adlert posted 10-03-2014 11:46 AM ET (US)
Sounds like you've got a cylinder (or more than one) not doing it's job. You're going to need to do the usual first step diagnostic sort of things. Each cylinder within your engine needs 3 basic things to run: appropriate compression, an appropriate fuel/air mixture, and an appropriate spark to ignite that mixture.
I suggest checking for spark and compression first and before running the engine any more to avoid possible serious damage. Spark can be checked via a number of ways; inline spark checkers are good; a timing light may be used; spark plugs grounded against the block... the ideal tool is an adjustable air gap spark checker adjusted to the factory spec air gap but I suspect that may not be handy to you.
Regardless whether or not you isolate a sparking problem it's always a good idea to check compression on all cylinders. Compression gauges are quite cheap and available at most auto parts stores. It is wise to you know what you've got to work with overall engine mechanical health-wise. For instance, even if you isolate a no spark situation on a cylinder that is causing your worst symptoms, you may or may not want to fix that particular engine if in fact a compression problem is looming.
If compression and spark are good, likely fuel delivery is your problem.
jimh posted 10-03-2014 12:23 PM ET (US)
It is unlikely that a problem in the main fuel supply line would cause only one cylinder to misfire. Replacing the fuel filter, if it was in poor condition, was necessary, but it is unlikely to remedy the misfire. Generally a vibration or misfire occurs in a cylinder because of lack of combustion of the fuel.
Perform a spark check test with an inline spark tester. This is an easy and simple test. Perform it first. If all cylinders have spark, check the fuel induction for each cylinder.
Is this engine a two-cycle engine or a four-cycle engine? What sort of fuel induction system does it have? Carburetor fuel induction, or simple throttle-body electronic fuel injection? A misfire can be due to poor fuel induction into the combustion chamber.
mccrickard posted 10-06-2014 07:41 AM ET (US)
[The engine] is a Mercury two-stroke-power-cycle three-cylinder 75-HP. I believe the oil mixes with the fuel before it enters the carburetors. There is an oil tank fill within the housing of the engine. I [will] check the spark and compression.
The fuel filter that I replaced seemed old, but the marine parts supply store did not have the skinny in-line one I needed. They gave me one that would fit the hoses, and it is see-through but larger, with a micron filter inside the casing. Is it okay to have a larger filter on that engine? It primes well, fills up, but when I start and run the engine it does not stay full, The engine still stays running, just the same. Thank you for all your help and I will keep you all posted with the progress of the repairs.
jimh posted 10-06-2014 08:01 AM ET (US)
Information about the method used on Mercury two-cycle outboards to provide automatic mixing of the gasoline and oil can be found at
http://continuouswave.com/whaler/reference/mercuryOilInjection.html
Don't be confused by Mercury's use of the term injection. There is no oil injection in the modern sense of an oiling system for a two-cycle engine. The Mercury system just mixes the oil into the gasoline line with a T-connection on the suction side of the fuel pump.
The physical size of a filter element in the fuel supply line to an engine is NOT an important parameter. The real concern for a fuel filter is in two realms:
--how much vacuum or suction occurs across the filter element; that is, to what extent does the filter restrict fuel flow; and,
--what is the size of the smallest particles the filter element will block, usually measured in microns. Older filters tended to be about 30-micron filters. Newer filters tend to have 10-micron elements.
For more about fuel filters, see the article at
http://continuouswave.com/whaler/reference/mercuryOilInjection.html
It is unlikely that the in-line fuel filter you have installed will be found to be a cause of the misfire. A misfire occurs usually in one cylinder. Because the in-line filter is filtering the fuel for all cylinders, it is not likely it is the cause of a misfire in one cylinder.
mccrickard posted 10-06-2014 10:13 AM ET (US)
I will check the spark and compression on all cylinders later today. When running it has a very distinctive misfire--smooth, then bog on misfire, smooth, then bog on misfire, and so on. The [tachometer] even drops from to 900-RPM or 850-RPM from 1,000-RPM on the misfire.
I am not sure how many hours on the engine.
jimh posted 10-06-2014 10:00 PM ET (US)
I wanted to let you know that I did not know, either, how many hours were on the engine when I made some recommendations of how investigate this problem. Check for spark. Report your findings.
jimh posted 10-06-2014 10:17 PM ET (US)
ASIDE: I don't see any possible connection between the misfire of the outboard engine and the model year, model name, and manufacturer of the boat it is sitting on, so please stop insisting on mentioning that data. It won't help cure the misfire
If you think it essential that readers be able to see a recording you have made of your engine misfiring, you might consider uploading the recording to a website that hosts amateur recordings. Have you heard of youTube.com? They host amateur recordings and provide very good bandwidth for playing them back.
CONTINUOUSWAVE is not able to host your home-made recordings. We just don't provide that sort of service. CONTINUOUSWAVE tries to collect, organize, and distribute good information, mostly in the form of text and occasionally in small images.

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Author	Topic: Diagnosing Cause of Misfire and Vibration in Mercury Outboard
mccrickard	posted 10-02-2014 09:59 AM ET (US) I have a Mercury 75-HP outboard, on a used boat I bought a few months ago. The engine ran like a top until a few weeks ago. The Mercury 75-HP outboard started to vibrate at idle and at higher speeds; there is a very noticeable miss when at idle, as well. I have changed the fuel filter due to the old one looking very bad. Any help is welcome, thanks
adlert	posted 10-03-2014 11:46 AM ET (US) Sounds like you've got a cylinder (or more than one) not doing it's job. You're going to need to do the usual first step diagnostic sort of things. Each cylinder within your engine needs 3 basic things to run: appropriate compression, an appropriate fuel/air mixture, and an appropriate spark to ignite that mixture. I suggest checking for spark and compression first and before running the engine any more to avoid possible serious damage. Spark can be checked via a number of ways; inline spark checkers are good; a timing light may be used; spark plugs grounded against the block... the ideal tool is an adjustable air gap spark checker adjusted to the factory spec air gap but I suspect that may not be handy to you. Regardless whether or not you isolate a sparking problem it's always a good idea to check compression on all cylinders. Compression gauges are quite cheap and available at most auto parts stores. It is wise to you know what you've got to work with overall engine mechanical health-wise. For instance, even if you isolate a no spark situation on a cylinder that is causing your worst symptoms, you may or may not want to fix that particular engine if in fact a compression problem is looming. If compression and spark are good, likely fuel delivery is your problem.
jimh	posted 10-03-2014 12:23 PM ET (US) It is unlikely that a problem in the main fuel supply line would cause only one cylinder to misfire. Replacing the fuel filter, if it was in poor condition, was necessary, but it is unlikely to remedy the misfire. Generally a vibration or misfire occurs in a cylinder because of lack of combustion of the fuel. Perform a spark check test with an inline spark tester. This is an easy and simple test. Perform it first. If all cylinders have spark, check the fuel induction for each cylinder. Is this engine a two-cycle engine or a four-cycle engine? What sort of fuel induction system does it have? Carburetor fuel induction, or simple throttle-body electronic fuel injection? A misfire can be due to poor fuel induction into the combustion chamber.
mccrickard	posted 10-06-2014 07:41 AM ET (US) [The engine] is a Mercury two-stroke-power-cycle three-cylinder 75-HP. I believe the oil mixes with the fuel before it enters the carburetors. There is an oil tank fill within the housing of the engine. I [will] check the spark and compression. The fuel filter that I replaced seemed old, but the marine parts supply store did not have the skinny in-line one I needed. They gave me one that would fit the hoses, and it is see-through but larger, with a micron filter inside the casing. Is it okay to have a larger filter on that engine? It primes well, fills up, but when I start and run the engine it does not stay full, The engine still stays running, just the same. Thank you for all your help and I will keep you all posted with the progress of the repairs.
jimh	posted 10-06-2014 08:01 AM ET (US) Information about the method used on Mercury two-cycle outboards to provide automatic mixing of the gasoline and oil can be found at http://continuouswave.com/whaler/reference/mercuryOilInjection.html Don't be confused by Mercury's use of the term injection. There is no oil injection in the modern sense of an oiling system for a two-cycle engine. The Mercury system just mixes the oil into the gasoline line with a T-connection on the suction side of the fuel pump. The physical size of a filter element in the fuel supply line to an engine is NOT an important parameter. The real concern for a fuel filter is in two realms: --how much vacuum or suction occurs across the filter element; that is, to what extent does the filter restrict fuel flow; and, --what is the size of the smallest particles the filter element will block, usually measured in microns. Older filters tended to be about 30-micron filters. Newer filters tend to have 10-micron elements. For more about fuel filters, see the article at http://continuouswave.com/whaler/reference/mercuryOilInjection.html It is unlikely that the in-line fuel filter you have installed will be found to be a cause of the misfire. A misfire occurs usually in one cylinder. Because the in-line filter is filtering the fuel for all cylinders, it is not likely it is the cause of a misfire in one cylinder.
mccrickard	posted 10-06-2014 10:13 AM ET (US) I will check the spark and compression on all cylinders later today. When running it has a very distinctive misfire--smooth, then bog on misfire, smooth, then bog on misfire, and so on. The [tachometer] even drops from to 900-RPM or 850-RPM from 1,000-RPM on the misfire. I am not sure how many hours on the engine.
jimh	posted 10-06-2014 10:00 PM ET (US) I wanted to let you know that I did not know, either, how many hours were on the engine when I made some recommendations of how investigate this problem. Check for spark. Report your findings.
jimh	posted 10-06-2014 10:17 PM ET (US) ASIDE: I don't see any possible connection between the misfire of the outboard engine and the model year, model name, and manufacturer of the boat it is sitting on, so please stop insisting on mentioning that data. It won't help cure the misfire If you think it essential that readers be able to see a recording you have made of your engine misfiring, you might consider uploading the recording to a website that hosts amateur recordings. Have you heard of youTube.com? They host amateur recordings and provide very good bandwidth for playing them back. CONTINUOUSWAVE is not able to host your home-made recordings. We just don't provide that sort of service. CONTINUOUSWAVE tries to collect, organize, and distribute good information, mostly in the form of text and occasionally in small images.