25hp Johnson won't idle and fouls plugs

Hi all.

Just purchased my first boat, an '81 14ft Gregor with a 25hp Johnson and 7.5hp Chrysler kicker. The motors do run but the Johnson will not idle and fouls the plugs within a few hours. I have no idea how old the motor is but was told mid '70s. Can anyone point me in the right direction to resolve these two problems?

Also, I was advised to replace the lower end oil in both motors. A friend suggested that the Johnson would require 2 quarts. However when I drained and refilled it only took less than half a quart. Did I do something wrong? It looks pretty simple to me. The 7.5 took even less oil.

Thanks,
BentStick

BentStick said:

Hi all.

Just purchased my first boat, an '81 14ft Gregor with a 25hp Johnson and 7.5hp Chrysler kicker. The motors do run but the Johnson will not idle and fouls the plugs within a few hours. I have no idea how old the motor is but was told mid '70s. Can anyone point me in the right direction to resolve these two problems?

Also, I was advised to replace the lower end oil in both motors. A friend suggested that the Johnson would require 2 quarts. However when I drained and refilled it only took less than half a quart. Did I do something wrong? It looks pretty simple to me. The 7.5 took even less oil.

Thanks,
BentStick

Click to expand...

Congrats on the new rig.
Yea, changing the lower unit oil is pretty easy. Your friend was wrong on the amount of oil needed though. But it is cheaper to buy it by the quart rather than those 10 oz tubes. Just fill until it overflows the top hole.

Regarding your fouling of spark plugs, start with the basics. Here's a few things to check...
1) Make sure your fuel mix is 50:1. More than that could cause your plugs to foul.
2) Check the spark plugs, and make sure they're the correct plugs and gaped according to the spec.
3) The carb slow speed needle could be adjusted too rich and cause plugs to foul.

If the engine has been sitting for an extended period, could need to have the varnish and garbage cleaned out of the carburetor....old spark plugs...wrong mixture (the tag on my 25 Johnny says run 100:1, so take that as you will)....

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.

Click to expand...

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!

WOW,

Thanks for the advice guys, much appreciated! This gets me off and runnin'. Finally located the serial number, the motor is a 1980.

One of the biggest items that has not been mentioned so far is the need to check the thermostat on that engine. That engine needs to run nice and warm at idle. Should be around 135f at idle. You should be able to place your hand on the top of the head while idling and only hold it there for 3-5 seconds.

To the fellow that has the 100:1 engine. There was a service bulletin that went out years ago retracting that ratio. You should be running at 50:1 in that engine. Off season storage and lack of usage promoted internal rust due to the very thin film of protection on that ratio and generated higher wear when the engine was put back into service.