Mercury 4hp 4 stroke w/bad plug - SailNet Community
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post #1 of 3 Old 08-13-2015 Thread Starter
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Mercury 4hp 4 stroke w/bad plug

The spark plug on my mecrury 4hp 4 stroke went bad and had to be replaced. It works great now but it is an interesting story.

We bought a nice used catalina 22 completely outfitted w/trailer from an old man who had loved the boat. It had been his baby until he got to frail to sail anymore. He kept it at his boat house on a lift when he was not sailing so needless to say it is in good shape. The motor is a 2004 model with almost no hours on it.

The before we take it out I clean the motor and drain the fuel from the tank. It runs great. We take the boat out for the first time and 100 yards from the dock the motor dies. I pull and pull while we are drifting into the shallow end of the lake so finally we begin to sail upwind. This is the first time I have ever sailed and the first time my wife has sailed in over 30 years. We learn the hard way but make it up wind about a quarter mile before it is time to return and boy is it alot easier going down wind.

I work and work on the motor. It will start and then die after about 5 seconds. I clean the fuel system. Check the ignition system. I get really good at removing the carb.

We decide to try it again without the motor and go back to our fav spot. The wind is stronger this time so we go upwind at a faster pace but we learn alot about heeling and other things. We decide it is time to head back and boy we sail back at a very fast speed.

I work on the motor alot more. Finally after a few months of working on the motor when I have time (we keep the boat here at the house) I give up and have my friend look at it. He scratches his head and does everything I did. Nothing works. He says we will have to start throwing parts at it and a spark plug is the cheapest thing to buy. So we put a new spark plug in and it runs great...

I am so happy to have the motor running but feel so stupid for not figuring it out myself.


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post #2 of 3 Old 08-14-2015
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Re: Mercury 4hp 4 stroke w/bad plug

So what exactly was wrong with the spark plug? Only time I've seen plug problems is when it's hard to start but once started they run or lag in power.
Also when sailing going with the wind (broad reach or a run) should be the slowest point of sail. I know it sounds counter intuitive. Beam reach should be the fastest. However my boat gets a slight increase mid beam reach to close reach. There may be something off with the shape of your sail. Like halyard isn't tight enough, backstay is too loose or your choking the sails or luffing.
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post #3 of 3 Old 08-14-2015 Thread Starter
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Re: Mercury 4hp 4 stroke w/bad plug

I don't know what is wrong with the plug. It looks fine. It sparks when you take it out and check it (ground it and pull rope). But it will run 5 sec and die. With the new plug it runs great. I have been working on my cars/trucks/equipment for 30 years and never had a spark plug do this.

I understand the relative speed of various points of sail even if I am new to this. We were sailing in a narrow (but deep enough) area of a large lake that is protected from any current. Unfortunately the wind was blowing exactly up the narrow lake so we had little room to maneuver. But we didn't have to worry about the current carrying us away and knew people who have cabins/docks around us so we were comfortable.

Thanks - Ace
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