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Mariner 50hp Troubleshooting Question

-Marc V-

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Hi Folks,

Hoping to get a little advice to help troubleshoot a problem I'm having with my 50hp Mariner. It's a 4 cyclinder 2 stroke around 1983.

We had it out on the lake this last weekend. One night it ran fine. We put new gas in it for the next day and it ran fine wide open for about a quarter mile then it bogged way down, and wouldn't get up to top speed again.

We thought it was bad gas, so we switched out the gas (which was new from the day before) with some different gas from a different gas station, added 2oz of Sea Foam per gallon to clean it out, and ran it again with the same results.

We checked the tube in the gas tank, and that seemed to be attached fine. Checked the fuel filter, and that seemed alright. Took out all the spark plugs, and held them against the block turning the key, and they all fired fine.

Took it into a marine shop, and they thought it was the carbs, so they cleaned out the carbs with they "said" were dirty. They ran it with muffs on, and it ran alright then.

Got it out on the lake that night, and the same thing. Ran wide open fine for a little bit, then bogged way down again.

I'm wondering if it might be a bad thermostat. Maybe? The impeller spits water fine like it should, and the water coming out doesn't feel overly hot. I'm wondering if the thermostat would work like a governor in that if it's bad or gets too hot that it slows the motor down like it's doing after it's opened up at full speed, then slows way down.


Much appreciated,


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Sound to me like a fuel problem. First the obvious. You did open the vent on the tank, right? Or is it a built in tank? maybe loosen the gas cap?

Second thing I would do is replace the fuel line from the tank to the motor and to the carbs. I don't think motors that old liked ethanol very much.

Oh, you could try pumping the bulb when it starts to bog down. That might tell you something.

Next, maybe fuel pump.

Maybe someone who really knows something will chime in. Could be a heat sensitivity or something wierd in the ignition.

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I had a simlar problem with my 1984 johnson 40 hp. It turned out to be dirty carbs for me. I cleaned the carbs myself at first, but it didn't help. I evenually took it to tom, who is a small engine mechanic. He said he boiled them in some solution.

I suggest the following things first

Replace the spark plugs, even if they look good.

Buy good gas, mine is made to run on 89 oct. not 87, but I always run 91 oct. non-oxygen.

Make sure you mix the gas right. Every outboard I have ever used mixed at 50:1

replace the fuel connections to the tank and motor. To check them turn the motor all the way to the side and pump the bulb. I had this problem before, the o-ring in them starts to leak.

If it still does it, pump the bulb and see if it pulls out of it. If it does, replace the fuel pump. Its $20 for the kit, shipped and everything. Then if all else fails, take it to tom, he might seem intimidating with his dog, but he knows better than anyone, including dingmans.

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Ya we made sure the vent was open on the tank.

I had a feeling it might be something with the gas line too but its new this year. The bulb stays firm too so that seems alright. Probably take it in again. Who's Tom?

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I also like the idea of pumping the bulb to see if it pulls out if it. Id also suggest activating the choke to see if that helps to.

But before doing much more, id do a compression test, just to rule it out.

Another thing to consider on your engine is that those need/mariners have low and high speed stators. Not that I'm convinced at all that its ignition, but it cannot be overlooked. You can test for spark by cranking the engine and have nice blue spark, indicating the low speed side is working but the high speed side could still be shot. A DVA meter is the best way to test but an ohm meter can also be used to check for an open.

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does it have points or a cdi box. I dont know of any motor that new that has points, but it cant hurt to ask, as dirty points would make it run bad. If it comes down to it, I can give you directions to toms small engine repair, he works out of his house.

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After it bogs will it stay running at slow speed and are you able to build up the RPMs after a short time running it at slow speeds?

About the new gas line. If it isn't the correct diameter (too small)you'll empty the carb bowls of fuel and bog down. Also I'm not a fan of the aftermarket fuel hoses made in china with the liner as they seem to break down inside and plug.

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I had the same problem a few years ago (new after market hose) the fitting that attached to the motor didn't seal everything looked good I removed that end went to a merk dealer and spent $6 the fitting took care of the problem after tearing down the carbs and a few other things learned that I will never cut corners on hoses.

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My first Merc Classic Fifty did this and it ended up being the fuel pump diaphram kit and was back on the water.

Went thru a ton of trouble shooting along with checking alot of things and even changing out the stator only to find that it was a $17.00 fix mad

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