Home › Forum › Ask A Member › 1981 Mercury 18hp.
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shane-r13.
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May 17, 2018 at 1:39 pm #9951
Hey Everyone, Got an issue here with a 1980 Mercury 18hp. I bought the motor used. tested well at 125psi each cylinder. Ran good in barrel, took it onto the water and it would bog above half throttle. Brought it home cleaned carbs etc. Ran good once again in barrel, took it onto the water same bog above half throttle. After a good inspection I realized that the high speed jet was missing completely and if i covered it slightly with my thumb it ran perfect. Came home ordered the standard size jet as there was 3 options but i bought the factory one. Put it in motor ran great in barrel, brought to the lake and it had the exact same issue bog above half throttle, covered the jet slightly with thumb and it runs great. Of all the carb diagrams i’ve seen it doesn’t show anything but the jet but it almost seems like a screw should go in and cover it or something. Any ways i’m lost, any ideas here or anyone with experience with this motor?
Thanks,
ShaneMay 17, 2018 at 5:25 pm #76189Can you post a picture of the carb, or post the serial number of the motor? I am confused how you could cover the main jet with your thumb? It should be screwed into the float bowl and sealed with an o-ring. If it was missing,all the fuel would pour out of the carb and it wouldn’t run at all.
May 17, 2018 at 5:42 pm #76191That’s that funky, NLA carb. Most parts are now no longer available for those. In any case, the main jet IS the bolt in the bottom of the carb bowl, if I recall correctly. Not sure what you’re putting your thumb over to get it to pick up, though…
May 17, 2018 at 6:53 pm #76192the throat of the carb I would think.
May 17, 2018 at 9:34 pm #76196Sounds like you are describing the Back Drag Jet in the throat of the carburetor. And yes that needs to be in there. The carb kit # is 1399-8153, I believe, and you will also need a 1399-8154 fuel pump rebuild kit. But I would need your serial # to be sure. One thing I can tell you is you probably need a good carb rebuild, along with a fuel pump diaphragm and the gaskets replaced. Pay close attention to the fuel pump parts and how they go back together… they are super easy to mix up. That and the fact that there are different style gaskets and check valves that must be replaced withe the correct parts. Also, check the fuel filter bowl gasket (O-ring) for vacume leaks. It may not leak fuel, but can have a vacume leak present. That would cause a bog problem.. Is your fuel line from the tank to the motor a bright silver color with red printing on the side?? If so, put it in the trash, cause if it hasn’t already failed, it will! Those hoses are notorious for falling apart inside, thus causing a fuel restriction. Just a few things you might think about.
May 18, 2018 at 11:44 am #76228quote Jerry Ahrens:Sounds like you are describing the Back Drag Jet in the throat of the carburetor. And yes that needs to be in there. The carb kit # is 1399-8153, I believe, and you will also need a 1399-8154 fuel pump rebuild kit. But I would need your serial # to be sure. One thing I can tell you is you probably need a good carb rebuild, along with a fuel pump diaphragm and the gaskets replaced. Pay close attention to the fuel pump parts and how they go back together… they are super easy to mix up. That and the fact that there are different style gaskets and check valves that must be replaced withe the correct parts. Also, check the fuel filter bowl gasket (O-ring) for vacume leaks. It may not leak fuel, but can have a vacume leak present. That would cause a bog problem.. Is your fuel line from the tank to the motor a bright silver color with red printing on the side?? If so, put it in the trash, cause if it hasn’t already failed, it will! Those hoses are notorious for falling apart inside, thus causing a fuel restriction. Just a few things you might think about.Just curious Jerry as to why you would suggest a fuel pump kit? I seems to idle and run fine when I cover that back drag jet slightly. To my untrained self it seems like the fuel pump works fine. Just curious to your rationale before i start ordering parts. Thats all. I have had the fuel filter bowl off multiple times so thats likely is a wise idea to replace the or ring in it. No the fuel line isn’t silver and I’ve tired 3 tanks and 3 sets of lines thinking maybe it was a tank issue and no change in the motors behaviour.
May 18, 2018 at 11:45 am #76229The serial number for this motor as people asked is 7178421. And it is a back drag jet not the main jet. My mistake.
May 18, 2018 at 6:32 pm #76253Can you install a newer style carb on these motors in place of the back drag ones?
May 18, 2018 at 9:24 pm #76277quote Shane.R13:quote Jerry Ahrens:Sounds like you are describing the Back Drag Jet in the throat of the carburetor. And yes that needs to be in there. The carb kit # is 1399-8153, I believe, and you will also need a 1399-8154 fuel pump rebuild kit. But I would need your serial # to be sure. One thing I can tell you is you probably need a good carb rebuild, along with a fuel pump diaphragm and the gaskets replaced. Pay close attention to the fuel pump parts and how they go back together… they are super easy to mix up. That and the fact that there are different style gaskets and check valves that must be replaced withe the correct parts. Also, check the fuel filter bowl gasket (O-ring) for vacume leaks. It may not leak fuel, but can have a vacume leak present. That would cause a bog problem.. Is your fuel line from the tank to the motor a bright silver color with red printing on the side?? If so, put it in the trash, cause if it hasn’t already failed, it will! Those hoses are notorious for falling apart inside, thus causing a fuel restriction. Just a few things you might think about.Just curious Jerry as to why you would suggest a fuel pump kit? I seems to idle and run fine when I cover that back drag jet slightly. To my untrained self it seems like the fuel pump works fine. Just curious to your rationale before i start ordering parts. Thats all. I have had the fuel filter bowl off multiple times so thats likely is a wise idea to replace the or ring in it. No the fuel line isn’t silver and I’ve tired 3 tanks and 3 sets of lines thinking maybe it was a tank issue and no change in the motors behaviour.
To answer your question on the fuel pump…. those fuel pumps are tiny and don’t pull all that much fuel anyway, compared to a larger say, OMC one. If the diaphragm or gaskets are deteriorated, you could experience high speed bogging, or even problems at idle. On an engine that old, I replace all that stuff, along with a carb cleaning and a new impeller. That way your motor will be dependable. Fix it right and be done. If you do a carb rebuild, it will be easy to go ahead and do the fuel pump. BTW, I just checked your serial # on the Mercury Marine EPC….. parts look up. The part #s I gave you are correct for your motor.
Another question for you… How far away from the outboard is your fuel tank? If you are running this motor in a test tank or barrel, you need to get the fuel tank up off the floor, closer to the motor. Those little fuel pumps won’t lift fuel that high.May 19, 2018 at 12:39 am #76296In my barrel it is on the floor but while running in the boat its on the floor of the boat too but that is obviously closer to the motor than during a barrel run. Thats for the tips on fuel pump and such. I’ll look into ordering all those darn parts. Thanks.
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