1974 20HP merc suddenly stalls and won't restart

Please help! I owned a 1976 20HP Mercury for many years and it ran without problem. I loaned it to my brother and he left it behind at the bottom of a lake. I purchased a 1974 20 HP Mercury (reconditioned) to replace it and have had nothing but problems.<br /><br />I use the motor (or would like to) on the weekends only.<br /><br />I squeeze bulb and choke at first start. It seems to run fine - idles a little high but not too bad.<br />I leave without problem. I run wide open for about 10 minutes then the motor suddenly loses power or will die as I slow down. When I try to restart I have to choke - sometimes it fires and others it won't for awhile. when it does restart and idles down and I try to put it into gear it dies. This repeats many times until I am lucky enough to get it moving. Sometimes it will only run at low speeds and others the dying repeats itself. But as soon as I stop again the restarting issues repeat themselves.<br /><br />Please offer me any advice you have as I am at wits end. I have had it back to the place I purchased it 3 times now with no help. He always gets it running in the tank.

Re: 1974 20HP merc suddenly stalls and won't restart

Any guesses?

Re: 1974 20HP merc suddenly stalls and won't restart

Bump

Re: 1974 20HP merc suddenly stalls and won't restart

Bump

Re: 1974 20HP merc suddenly stalls and won't restart

The electronic ignition on these motors is a bit of a mixed blessing. It is nice to have a high energy ignition system for easier starts, less plug fouling etc. But when it stops working the parts are expensive. I suspect you have an intermittent problem in the ignition system. Maybe the power pack or stator. The problem could be in the fuel system too. You didn't mention anything so I assume that it is not overheating.

Re: 1974 20HP merc suddenly stalls and won't restart

Mrmerc - it doesn't seem to overheat. Plenty of water out the peehole. The engine turns over after it dies but can't sustain itself. Anything I can try?

Re: 1974 20HP merc suddenly stalls and won't restart

I have a 75,20HP that acted similar to what you discribe. Truned out to be the Diaphragms (fuel Pump)in the Carb. After rebuild it is Ok.

Re: 1974 20HP merc suddenly stalls and won't restart

Jango - Thanks - How difficult is this rebuild for a novice?

Re: 1974 20HP merc suddenly stalls and won't restart

Yeti, you will need the shop manual. Here is a link that may help further. http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=17;t=013170

Re: 1974 20HP merc suddenly stalls and won't restart

Sounds like a fuel problem.When it falters try squeezing the primer bulb. if it picks up then the fuel pump is the most likely culprit. But seeing as how it runs in the tank and probably on a different fuel source, check that the tank vent is clear and that your fuel hose is in good shape.

Re: 1974 20HP merc suddenly stalls and won't restart

The following Mercury Service Advisory describes a very effective way to test the fuel system using a vacuum gauge. If your test shows that the vacuum in the fuel line exceeds specs, you still have to figure out what specific component is causing the problem, but at least you narrowed it down. If the vacuum is OK, you can then test the pressure in the fuel line between the fuel pump and the carbs using a similar connection as the vacuum test. Your shop manual should specify what the normal pressure level should be during idle and WOT.

Another way to troubleshoot is to connect a portable fuel tank to the motor and see if the problem goes away. If it does, you have narrowed it down to a small group of possible causes. If the problem does not go away, you have at least eliminated a small group of possible causes.

http://www.marinepartsman.com/Mercury-Marine-service-bulletins/Mercury-Mariner/2001/EN_17.PDF


If you want to determine if you have an electrical problem, a quick and easy way to do this is to use a SmarTach meter to monitor secondary voltage while the motor is running and bogging/stalling. The meter can be connected to the spark plug wires using an induction connections and you can observe what is going on with the voltage when the motor is bogging or stalling. If you determine that the voltage is dropping when the motor boggs/stalls, you then have to find out what component is going bad using a DVA meter and an ohmmeter to perform various tests as described in your shop manual.

There are other ways to troubleshoot but I find the above tests very effective in narrowing down the problem area quickly. Others may have additional or better advise than this.

Re: 1974 20HP merc suddenly stalls and won't restart

I think hkeiner has some very good suggestions above. I'd add to that a couple very simple first steps to try to sort out whether this is fuel or spark, and narrow down from there. If squeezing the bulb helps, looks like it's fuel. For spark, I love my $5.99 spark tester from Harbor Freight. It connects between the plug wire and the plug, so you can run the motor with it in place. When the motor bogs, see if you have fire on both plugs. If not, troubleshoot spark. Until you narrow it down to either fuel or spark, you are shooting in the dark. Once you do narrow it down, the further diagnostic steps above will help you zero in on the culprit. Just my two cents.

Re: 1974 20HP merc suddenly stalls and won't restart

I had a similar motor with Thunderbolt CDI type ignition.

I replaced the CDI box and all was well again.

Re: 1974 20HP merc suddenly stalls and won't restart

I should have also mentioned that these motors (mid-1970's Mercs) are notorious for bad wires--the insluation crumbles off and the wire corrodes. On my 1976 Merc 200 the kill switch wire was shorting to ground under a wire clamp on the motor pan when I got it--no start. I disconnected the kill switch and it fired right up.

In any case, I'd strongly recommend diagnosis before surgery! CDI boxes aren't cheap...

Re: 1974 20HP merc suddenly stalls and won't restart

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.