If you run that 1970's 25hp on 100:1 fuel/mix, you might as well as dig a grave to dump it in. Your motor was built to run on 50:1 fuel/TWC-3 oil mix.
***Chances are you probably need a good carburetor cleaning and new carburetor kit.
Here are the settings for your carburetor.
Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
Here is a link to web site for a parts diagram for your motor and you can order parts from them as well.
https://www.marineengine.com
Your motor probably use a good decarbing.
Decarb your motor with Seafoam or comparable product very well. If you can get hold of some Seafoam, put an ounce or two in each cylinder to start dissolving that thickened carbon and coke. It will smoke like house on fire at first.
Add an ounce or two of Seafoam or comparable product to each gallon of gas (petrol) as on going maintenance as cleaning measures forever and ever
**Evinrude sells a product Engine Tuner that come in can and works well. Just follow the instructions.
Your spark plugs should be Champion J4c, but if you are going to do a lot of trolling then a hotter plug like Champion J6C.
Couple of things that could be causing your plugs to foul up.
- Too rich of fuel oil mix (either from improper mixing or improper carburetor settings.
- A lot of low trolling for long periods with improper plug.
- make sure your spark plugs are gapped to 0.030 with a gap gauge, don't trust they will come from the factory that way.
- A thermostat that is stuck in the the open position will cause the engine to run cooler and coke and carbon will build up in the cylinders, rings, and spark plugs.
A little thermostat information
If you have have stuck open thermostat or no thermostat to maintain a power head running temp between 120 to 160, then the cold waters that you are running your motor in will accelerate the carbon/coking build up process in your power head, on top of your cylinders and around the rings. Your plugs will foul up in no time causing the problems that you are having now.
The thermostat stays closed until the temperature of the power head starts reaching about 130 to140F, then the thermostats starts to open to allow some water in to circulate around the water jackets to cool off the power head to maintain a good operation, again between 120 to 160F. So, the thermostat will open and close to help maintain operation temperature. It is within the 120 to 160F range that the optimal combustion of the fuel oil mix takes place.
If you have no thermostat in or it is faulty then water will circulate around the water jackets all the time and the power head will stay much colder. Colder power head temperatures will allow the unburned fuel oil mix to build up and slug up all over instead of being burned off or exiting the exhaust/water mix going out near the foot of the motor.
Hope this helps, good luck!