Reduced speed under load

Panda809

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Hello everyone. I have a 1973 Evinrude Lark 50hp, model 50573, electric shift on an old MonArk boat. Got the motor running for the first time in a few years a month or two ago by Originally putting in new spark plugs. But I experienced the boat not getting up to speed or plane, and bogging down at full throttle. I then found that fuel pump was leaking so I replaced it along with fuel lines all the way to the tank.

After that, the engine would sound smooth and clip at a good pace under low speed but the bogging down would still occur at full throttle; but to a lesser degree but still couldn’t get to plane. I noticed the plugs were wet and blackish in color so I cleaned them up and slightly adjusted the rich/lean idle dial on the outside of the motor from “rich” about 1/8 inch to lean.

After that I let the boat sit about a day and took it out. The engine ran like a dream. It got up to plane, capped about 24-25mph and ran perfect at low speed and full throttle.

I then took the boat home, and headed out again about two hours later. This time when putting the boat in the water, the engine seemed to be flooded at first starting and took a bit to get going. Once moving, the engine wouldn’t get up to the same speed or on plane like it did a couple hours earlier. Any thoughts on what the issue could be would be greatly appreciated.
 

racerone

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The motor appears to be a 1975 model.----There was no electric shift on 1975 models.----Make sure carburetors ( high speed jets ) are clean.----
 
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airshot

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Best to get a factory rebuild kit so you will have new gaskets, new needle and seat as well as instructions to set it up for your motor. Be sure the fuel is fresh and oil mixed correctly !
 

jimmbo

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That Mixture Adjuster, only adjusts the Idle Mixture.
As other have said, it appears to be a Case of Dirty Carb(s), however.
Is the Spark on each Cylinder, strong enough to jump a 3/8" Gap?

If you clean the Carbs, once they are Reinstalled, you will have to so a Check of, and perhaps Adjust the Sync between the Carbs and Timing Advance.

Do you have a Factory Service Manual?
 

Panda809

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Hey everyone. Thank you for the information. I have not cleaned or rebuilt the carbs. I do have the service manual and parts catalog for the engine. Do you have suggestions for where to order a carb kit and if any special tools are needed?

To respond to other comments: I’m not sure about the spark being strong enough. For fuel and oil mixture, I use 6 gallons of premium, no-ethonal, gas with 16 ounces of 2-cycle oil from our local boat dealer.

Thank you!
 

Panda809

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I may be incorrect about the electric shift. But I can confirm that the motor model number is 50573R and the two manuals (service and parts catalog) say 1973 and match the model number. Thank you!
 

cyclops222

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You should be experiencing bits of old dried out gasoline varnish coming loose in the tiny passages of the carburetor.
I normally just start and slow speed run for a minute to bring in FRESH gasoline to continue softening up the old varnish in the carburetor. I do add the correct amount of Seafoam to the next time the tank is empty. After the Seafoam is in the empty tank. I then add the oil and gasoline. 1986 30 hp and a 1999 9.9 hp still strong.

Before long no running periods. Always disconnect fuel line to engine. Then restart engine and allow it to run out of gas and stop. No fuel . No starting and running problems when restarting engine. I never use E gasoline in any older engines.
NOTE NEVER put any E 10 gasoline into a empty fuel tank of a motor that was running on NON E 10 gasoline.
The E 10 is a very strong SOLVENT of old dried out REGULAR gasoline. It can soften up and break loose bits of varnish. Can drive people crazy until enough varnish is dissolved & passed thru the engine. Been there 1 time on a stand by lawn mower. I got lucky. Before I spent any money on parts.
 
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racerone

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I would say the motor is a 1975 model.-------A 1973 is listed as a 50373R-----A 1974 model is 50473M
 
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jimmbo

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A 73 has an entirely different Gearcase than a 75. What is the Diameter of the Prop?
 

Panda809

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Aug 17, 2025
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I really apologize. I just rechecked the model number and it is in fact 50373R. Again my apologies for the confusion. Thanks!
 

Panda809

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Thank you for helping me sort through the “shift” topic. Do you have suggestions to order carb rebuild kits and any needed tools?
 
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