1974 20HP Mercury suddenly dies

[BigYeti]

I have a problem with my 1974 20HP Mercury. It starts up fine, idles great and begins to run without problem. After I have it opened up for about 5-10 minutes it suddenly loses power - I mean suddenly. No warning, no sputtering - it just completely dies out. When I try to restart it won't fire. It just turns and turns. After some time it will fire up quickly and die after 2-3 seconds. After sitting a longer period of time it will fire up and run without issue for another 10 miutes opened up and then the cycle starts over again as it dies suddenly.<br /><br />Any direction is greatly appreciated

 

[BigYeti]

Re: 1974 20HP Mercury suddenly dies

Additional information to my problem. It doesn't seem to help if I pump the bulb when it fires for 2-3 seconds after the restart. It does keep it going longer on the restart if I pull out the choke before it dies out. I have also used 3 different gas tanks with the same result. I have also tried to run it with cap loosened on the gas tank with no improvement

 

[Bob H.]

Re: 1974 20HP Mercury suddenly dies

Sounds like a cracked coil. When they get hot and expand they die if there's a crack. Cool down and they fire up again.

 

[BigYeti]

Re: 1974 20HP Mercury suddenly dies

Are the cracks in the coil noticeable or are they too small to see? How difficult is the coil replacement?

 

[Motor Boater Bill]

Re: 1974 20HP Mercury suddenly dies

Since this is a two cylinder engine, it seems that it would still fire on one cylinder if it was a bad ignition coil. Since both cylinders quit, it seems you need to look further upstream in the electrical system, to the switch box, the trigger, or the stator. Or it could be bad wiring. Make sure it's not the wire going to the kill switch shorting out--had that problem on my 76 Merc 200. But since it seems to happen pretty predictably when it gets warm, I would guess it's a component in your ignition system breaking down when it warms up. I suppose that it's possible that one coil breking down could have some effect on the switch box that would keep the other coil from firing, but that is beyond my understanding of how the switch box works. I have read that a bad coil can take out the switchbox, but I can't verify that from my own knowledge. Good luck!

 

[James R]

Re: 1974 20HP Mercury suddenly dies

I agree, it sounds like stator coil, switch box or trigger coil. I have just replaced a trigger coil on this very model that exibited the same problem.
Test meter showed continuity about 80 0hms then open circuit. Flexing the coil potting revealed the break.

 

[james33]

Re: 1974 20HP Mercury suddenly dies

Hey guys...sorry to change the topic but i own the exact same motor and seem to be running into many problems....about two or three years ago my boat with the '74 mer 20 Hp on it sank. it was only submerged for a few hours and upon retrieval i was able to get the motor to start up with ease. since then i havent used the boat and the motor has developed problems....my first problem is that the impellar wont "Pee". I recently installed a new impellar and seals but came out with no luck. the motor will not circulate water. my second problem is that the motor will not advance to full throttle... I was using the motor (without water circulation for one weekend, for only about two or three minutes at a time and the next weekend when i went to try it the motor would only give me half throttle. I have checked the linkage and made sure the gas tank valve was open...Any suggestions would be greatly apreciated seeing as i would really like to use this new motor on my new hydrostream voodoo mini this summer.....

P.S this motor uses a standard Mercontrol remote system.

 

[Deanrt]

Re: 1974 20HP Mercury suddenly dies

Hi all, sorry I cant help anyone here. However not to be rude or anything James33, I think you would be best served by starting your own thread not to mention BigYeti getting the help he needs on his thread. Again im not trying to be rude it just seems like everyone would benefit. Here at home on our very cool fishing website ifish.net they call that a hijacking.

 

[james33]

Re: 1974 20HP Mercury suddenly dies

Thanks for the tip..but...i have no clue how to do that...

 

[badams270]

Re: 1974 20HP Mercury suddenly dies

Way to hijack and plug your very cool fishing site, Deanrt. Seems a bit hypocrytical.

 

[Deanrt]

Re: 1974 20HP Mercury suddenly dies

Perhaps.... if that was my intent. In retrospect I guess I should have PM'd him. My bad.

 

[scurtis76]

Re: 1974 20HP Mercury suddenly dies

Check for a small crack in the fuel line. My engine did exactly that last season then I replaced the fuel line and was fine. It wasnt the section of line from the tank to the engine but from the filter to the engine. Too much O2 and the engine dies out. Hope this helps.

SC

 

[Myrtle Beach]

Re: 1974 20HP Mercury suddenly dies

I have a simular problem with a 95, 25HP EL Mariner. I took it to a dealer for a tune up and explained the problem. He changed the pugs, adjusted the carb and changed the oil in the bottom end. I took the boat out for a spin today and ran it about 10 miles up the waterway at full throttle. It ran better untill I backed off to idle speed. Then it stalled out, it restarted ok, but when I throttled up, it kept stalling out. I could only run about 1 mph or less or it would keep stalling out After about a 1/2 hour of this, the motor started running normal. The water pump and thermostat are working ok, so I don't think the motor is over heating. The gas is fresh and the tank is clean, the battery is fully charged. I think it must be an electrical problem. Does anyone have a clue to solve this problem?

 

[25_hp_xd_man]

Re: 1974 20HP Mercury suddenly dies

I have a 1985 25hp xd mercury and had to same problem with the intermittent failure. The first time this happend (about two years ago) I found my stator was losing its ground refference and dying out. Replaced stator no more problem. Well about 6 months ago, my motor would totally die when it fully warmed up. I checked everything and found that the switchbox was opening up once it warmed up. When the motor cooled the faulty Solder joint would pull back together seeming that the motor was running fine. An intermittent problem is hard to troubleshoot. But if I were a betting man I would replace your switchbox first.
PS. This is a 2 stroke motor.

 

[8075mercury]

Re: 1974 20HP Mercury suddenly dies

I think I would work a process of elimination:
-Make sure your carb is clean and not some piece of trash is bouncing around or the float is sticking.
-Make sure your fuel diaphram is good and not pinholed.
-Make sure your fuel line and tank are good (sounds like you did this already)
-also check the small powerhead vent hoses that connect various bearings for lubrication, sometimes these deteriorate and allow air in.
-It was not clear to me if you are still getting spark during the bad episodes, but I would check that.
-you can test the trigger, coils, and stator with an ohm meter for good. However, I had a coil that was good cold but bad hot. Another post here agreed it still should be hitting on one cylinder however.
-check the wiring for good connections and bad insulation/shorts to ground.
Last, I do not think you can test the switchbox with an ohm meter. If all of the above test good, I would be guessing that the switchbox is bad. It would be good if you could borrow a working switchbox to test, 73 to 76 should match.

I have a 74 20hp I just rebuilt, these are great motors.