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Blew My Motor...

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I have been having issues with my 1979 Johnson 140 for a few months now. Sometimes it would start with no problems, other times it wouldn't start at all. I've been tinkering with it and replacing random parts here and there.... anyway Monday I was going to take it to a shop for repairs. Before I took it to the shop I decided to give it one last shot and noticed two of my spark plugs were loose.


I tightened them down out the hose to it and fired it up and it cranked up! Within a few minutes though, a spark plug goes shooting off the back of the motor, almost striking my cousin and a piston shaft went flying through the block of the motor.....no need for a shop now.


I have been looking for a new engine on craigslist and LSN and I have found two. One of them is a 115 Evinrude ($600) and the other is a 150 Mercury Power Tower ($1,000). Both engines start and run fine.


The boat is a 1990 Javelin 378 FS and its rated for a 150 HP engine. The old 140 horse engine would push the boat around 50ish mph. I am wondering how much speed/power I would lose if I went with the 115 HP engine. I would like to go with the Evinrude because I wouldn't have to change out the controls from the Johnson.....



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It really depends on what year the 115 is. I don't remember which year it was, but in the early 80's OMC made a switch in how they rated the HP of their motors to a propshaft rated HP. Since the propshaft rating was less than the previous method, the early model 140's became the new 115's and they introduced a newer bigger block for the 120/130/140. If the 115 is newer performance would be pretty much identical. If you were satisfied with the performance, and it is a newer 115 you could go for it, but If I were you I'd do some more looking and try to find a newer 150.

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As mentioned, a mid to late 80's 115 is basically the old 140.  If you look and that 115 has the same bubble back exhaust chest (looks like it has an oil pan on it), it's the down rated 140 , if it has the flat back exhaust chest, it's the early 115 and will make a considerable difference in power, they became the 88-90hp.  There will also be some performance loss because they dropped the compression from about 120psi to about 90psi so it could still run the lower grades of gas the government went to, but most of the difference in hp rating is because of the going from rating it off the crankshaft to rating it off the prop shaft.  With almost any motor made before the early-mid 80's, you can automatically take about 15% off the horse power number if comparing it to a later motor.


Not all models or manufactures made the change at the same time but pretty much all of them had made the change by 1985, but there were some that did it as early as 82.


That 150 Merc tower of power, became their 115 tower of power when Merc made the change to rating off the prop shaft and dropped the compression on it for the junk gas being produced.


I will say, be dang sure you do compression checks on those motors, those old motors lost compression pretty quickly if they did not have a high grade oil run in them, were not well maintained and decarbed regularly.

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Thanks for the advice guys!

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