Thread: 1970's Johnson 25 horse losing power

If you sprinkled water on the cylinders when it lost power and the water didn't boil, it's PROBABLY not the cause of the power loss. A new impeller is a necessity on a used motor no matter how old; one just never knows when it's been changed last or if the previous owner is one of those guys that likes to crank his outboard up on the trailer before backing it in the water (huge no-no), or if it's just been sitting unused for a long period of time (ruins the impeller).
JBJ

Scrapper - Thank you for your suggestion of the soap. I had not thought about the o rings etc. I was told by the dealer to put the engine in gear to hold everything in line while I make the exchange.

I'm thinking you have a window in the midsection for getting to the shift rod disconnect, right? If so, you'll likely need a pair of needle nose pliers to hold the rod while you slide the brass connector down. Make sure the groove in the rod is lined up with the bolt hole in the connector, or you'll wipe off the threads on your connector bolt when you tighten it. As long as you haven't moved the adjusting bolt on the shifter or the connector top bolt, you shouldn't have any problem with the shift rod. On your motor, there's no reason to put the engine in gear when dropping the lower unit. On some, particularly Mercury's, it's definitely the thing to do---they use a different shift design.

I called the repair shop today and asked the price for them to install the impeller since I have too much business on my plate. I told them about the engine concern and they advised me to hold up on the installation of the impeller until they run a compression test on the engine. I can always get the impeller refunded since I bought it from them. I will know something Monday.

The compression test was 100 psi on the top cylinder and 125psi on the bottom. My instructions are to run the motor on the lake for 30 minute or so and see if I lose power again, if so then pull out the choke and see if it returns to full power. If so then the carburetor needs an overhaul. Stay tuned. Any thoughts?

Originally Posted by cricket george

The compression test was 100 psi on the top cylinder and 125psi on the bottom. My instructions are to run the motor on the lake for 30 minute or so and see if I lose power again, if so then pull out the choke and see if it returns to full power. If so then the carburetor needs an overhaul. Stay tuned. Any thoughts?

The compression is not "good", but those numbers are likely NOT causing your problem.

The 125 is good....but not so much the 100...but probably not your problem. You would really like to see them within 10% of each other...and more towards the 125 and up. I've seen old fat four cylinder johnnyrudes in the 90's and still run good. But they were tired.

Have you done a decarb as prescribed here...

http://forums.iboats.com/engine-freq...aq-158076.html

Many times that will free up some lost compression. If its never been done there could easily be some carbon on the piston and rings causing a loss of compression. It might not help but is a good things to do anyways if its hasn't been done.


You may have done this already....but when it loses power...have you tried pumping the bulb to see if it catches back up?? Been a while since I read this thread.

Scrapper - I use Seafoam in the gas, but have done the decarb process. I have a second tank and I will mix up 3 quarts of gas to 3 ozs oil to 16 ozs of Seafoam and run this thru the 25 horse and see what happens. My 85 horse Evinrude could use this treatment too. Thanks for the post, as always.