1983 Evinrude 115hp has low power, until it pops under load

I have a 1987 24' Pontoon, with a 1983115hp evinrude with a couple problems.


First some background. I'm a dieseltech, with 10 years experience with trucks, and a few with carsbefore that plus all my own stuff. I completely understand how gasand diesel engines in vehicles work, and understand the theory of2-stroke operation, but have never been trained on them. I've ownedboats since I was 12 years old, and only ever worked on my own boatmotors. My experience made rebuilding the lower unit a piece ofcake, however tuning a 2-stroke, and diagnosing a running problem isout of my area.


I bought this pontoon last year andcompletely rebuilt it. All new floor, seating, console, steeringcable, etc. Re-welded all the railings, made it like new again. Ibought the motor cheap with a boat from someone who inherited it fromhis grandpa. That boat had been sitting for at least 5 years and heknew nothing about it. The motor looked like it was in very goodcondition except wiring damage from mice. It also looks like thepowerhead had been rebuilt at some point, and the lower unit is froman '87 motor. So I believe it had been taken care of.


First problem is that the motor isn'trunning right, it has less power than expected and doesn't run assmooth as expected. I believe it is a tuning problem, but was hopingsomeone with more 2-stroke experience could help. I finally fixed myhard starting problem, found it to be the hand primer uses checkballs and only works in one orientation, (not the one I had ithanging at). So now it seems to start and run well, but not assmooth as I would expect. This is confirmed every once in a while bya pop while under a load and instantly it will have more power andsmooth out until I slow down, then back to normal. I did a tune up,including new plugs, coils, fuel filter, rebuilt 1 carb, replaced theother, rebuilt the fuel pump, replaced all fuel lines, and completelyrebuilt the lower unit. I'm hoping someone will chime in and tell mea tuning thing I should have done, or a problem that pops up fromsitting so long. I did finally order a manual for the motor toconsult with that. It is running well enough to get me up to 20mph,and I did put about 50 gallons of gas through it last year.


Second problem, I believe isventilation. Once I get going up around 17-20 mph the prop acts likeit is slipping. RPMs go up about 500 and we slow down a little. Theprop was new last year (15p) and I tried marking the hub to check forslippage, and the marks stayed aligned. At 17mph the motor issitting around 3000rpm. With more weight on board we can get up to20mph, but with just 2 people it starts at 17mph. The motor ismounted as low as it can go and I tried raising it, but that onlymade the problem worse. If I remember right it has 23” pontoons. I tried to make a bracket to lower the motor further, but thesteering cable got in the way, it is almost sitting on the mount now. I just ordered a hydrofoil and 4 blade 13p prop to try. They willbe here Wednesday, and I will take it out to test Thursday. Ifanyone has any ideas to help let me know.


I've had a very busy spring and onlygot to get it out on the water for the first time last Saturday,which just reminded me of these problems, while I had been working onthe trailer problems I had. I have family coming Wednesday to visitand we are hoping to take a ride down the North shore to Duluth towatch the fireworks on the 4[SUP]th[/SUP] from the pontoon.

I've read the entire post.

Have you performed a compression test?

I've seen similar issues be related to an owner/repair tech thinking the fuel being used was fresh and it wasn't had a lot of water in it causing low power.
I've seen where a lean carb can cause this and putting fuel in a spray bottle, spraying a little while under way if performance improves it's running lean.
If engine boggs down then it's just right.

Also you mentioned primer bulb, have you tried priming while under way? ( difference ? )

Have you checked the fuel line in the tank? I've seen where they degrade completely and screens are clogged causing lean.

I would think a 115hp would be able to use a 15p to push that 'toon just fine. Hope this helps.

I did a compression test when I first got it, before I had it running. I don't remember the numbers but I believe they were low normal and relatively close to each other. I agree that another at this point would be a good idea, and will plan on it when I get my compression tester back. I know it isn't very low on power, but some. I was thinking of possibly another problem, one that could plague me and then disappear at will. I like everything I own to run perfectly, and maintain it the best I can to achieve that goal. The fact that this will give a small pop while loaded and suddenly run smoother, with more power, and pick up on the RPM (usually pushing me up to the ventilation point) would tell me that there is possibly something sticking that loosens up at will.

I cleaned the fuel tank before using it, replaced every line, and added a moeller spin on fuel filter. I filled it directly from a well used pump, and have run a few tanks through. I also treat every tank with seafoam, and had it almost empty last fall, and filled it with a full tank of fresh gas this spring. I have used nothing but amsoil marine 2-stroke oil. Fresh fuel, and restrictions shouldn't be a problem. I will plan on priming while under way next time I have it out, and if I can spray fuel directly into the carbs I will try that too.

I did just de-carb the motor, and just got a hydrofoil to put on tomorrow. I will also be putting on a new Michigan 4 blade 13p prop tomorrow.