1974 Johnson 50HP Outboard Compression Test

[Viking59]

Hi All,
I bought a cheap 15.5' Aluminum Boat with a 1974 Johnson 50HP Outboard. I took the boat out on the water for the first time a week ago and I was surprised about how slow it was. Just seemed like the motor didn't rev up as high as it should at top speed. The boat did plane with just my girlfriend and I sitting in the front seats. Add one person to the back of the boat and there wasn't a chance that it would plane. The motor otherwise ran well, but was a little tough to start in the morning.
I started searching for reason why the motor would be sluggish, and compression is of course one of the things that came up. I did a test and the top cylinder was about 128psi and the bottom cylinder was about 139psi. Could this be the route of the problem? Or is this difference in cylinder PSI still within the allowable range? Is there anything that can be easily done to add life to the top cylinder?
I read that prop size/pitch could also be the issue. The prop does look big for the motor, but it isn't stamped with the size/pitch, which is annoying! Would it be stamped on the inside?
I also ordered some Doul-Fins off of Amazon but I haven't been out on the water to try them out yet.

Thanks in advance!

 

[racerone]

On my compression tester a good one of those will read 150 PSI.-----Remove propeller nut and washer to see if you can find a number.---Any chance the boat / foam floatation is waterlogged ?

 

[Viking59]

So are you saying that this motor is pretty tired? I read somewhere that this motor should have 160 PSI brand new, so I have no idea what the PSI should be 40 years later.
Any advice on how to figure out the best prop size and pitch for this motor?
I don't believe that the foam under the floor is waterlogged, but I will find out soon when I redo the floor.
Thanks for your reply!

 

[w2much]

I think it would be fine at your compression . Under load be certain you are firing in both cylinders. I had one that ran fine at home had the same symptoms as your. Only one plug was firing under load. My powr pack was bad, not saying yours is. Could be bad wires also. Run it at night and see if you have an arcing plug wire.

 

[Viking59]

Thanks for the advice. Will is arch where the plug attaches to the wire? Or where the wire attaches to the power pack?

 

[Keyboardman]

My '74 50hp Johnson has 140 psi compression on both cyls. I'm running a 13 3/4 x 15 prop on it pushing an old heavy 15' tri hull. It will do 20mph wide open and gets on plane pretty quick. First thing I did was Seafoam the engine to decarb it.

 

[Viking59]

Thanks for the advice everyone. I hope to do some work on the motor this weekend and I will post the results. If anyone else has any further input, please post away!