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

[Zigblazer]

I have a 1987 24' Pontoon, with a 1983 115hp evinrude with a couple problems.

First some background. I'm a diesel tech, with 10 years experience with trucks, and a few with cars before that plus all my own stuff. I completely understand how gas and diesel engines in vehicles work, and understand the theory of 2-stroke operation, but have never been trained on them. I've owned boats since I was 12 years old, and only ever worked on my own boat motors. My experience made rebuilding the lower unit a piece of cake, however tuning a 2-stroke, and diagnosing a running problem is out of my area.

I bought this pontoon last year and completely rebuilt it. All new floor, seating, console, steering cable, etc. Re-welded all the railings, made it like new again. I bought the motor cheap with a boat from someone who inherited it from his grandpa. That boat had been sitting for at least 5 years and he knew nothing about it. The motor looked like it was in very good condition except wiring damage from mice. It also looks like the powerhead had been rebuilt at some point, and the lower unit is from an '87 motor. So I believe it had been taken care of.

First problem is that the motor isn't running right, it has less power than expected and doesn't run as smooth as expected. I believe it is a tuning problem, but was hoping someone with more 2-stroke experience could help. I finally fixed my hard starting problem, found it to be the hand primer uses check balls and only works in one orientation, (not the one I had it hanging at). So now it seems to start and run well, but not as smooth as I would expect. This is confirmed every once in a while by a pop while under a load and instantly it will have more power and smooth out until I slow down, then back to normal. I did a tune up, including new plugs, coils, fuel filter, rebuilt 1 carb, replaced the other, rebuilt the fuel pump, replaced all fuel lines, and completely rebuilt the lower unit. I'm hoping someone will chime in and tell me a tuning thing I should have done, or a problem that pops up from sitting so long. I did finally order a manual for the motor to consult 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 is ventilation. Once I get going up around 17-20 mph the prop acts like it is slipping. RPMs go up about 500 and we slow down a little. The prop was new last year (15p) and I tried marking the hub to check for slippage, and the marks stayed aligned. At 17mph the motor is sitting around 3000rpm. With more weight on board we can get up to 20mph, but with just 2 people it starts at 17mph. The motor is mounted as low as it can go and I tried raising it, but that only made the problem worse. If I remember right it has 23” pontoons. I tried to make a bracket to lower the motor further, but the steering 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 will be here Wednesday, and I will take it out to test Thursday. If anyone has any ideas to help let me know.

I've had a very busy spring and only got 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 on the trailer problems I had. I have family coming Wednesday to visit and we are hoping to take a ride down the North shore to Duluth to watch the fireworks on the 4[SUP]th[/SUP] from the pontoon.

 

[emdsapmgr]

That engine can't be running all that badly if you can get the pontoon up to 20 mph. Yet, I am sure there is still room for improvement. I'd probably start by doing a compression test on the engine. That engine should have compression in the 100+ lbs range on each cyl. Validate that all 4 have compression. If so, then it's worth putting more time and $ into it. Next, I'd check the spark on each plug. You can do it two ways: a spark-gap tester works well. After warmed up well, the spark should jump the 7/16" gap on each cyl. Or you can do the cyl-drop test. When the engine is running, pull one plugwire off the plug with a set of insulated pliers. The engine rpm's should drop. Reconnect the plug, the rpm's should return to normal. If you pull one plugwire off and the rpm's don't drop, that cyl is not firing. If you find any unusual spark activity, you can easily test on this engine. The dual identical power packs can be swapped. One controls the spark to each head. If the problem spark condition moves to the other head, then the pack could be faulty. Your ventilation problem is well described. Would be nice to know if that larger pontoon was originally manufactured with an engine pod that was meant to handle 25" long engines. Yours is probably a 20" engine. Check these two to make sure. That water flowing from under the engine pod should flow just under the anti-ventilation plate of the engine-the one just above the prop. A 5" difference in the engine's mounting could easily explain the ventilation problem.

 

[Tim Frank]

+1 ^

Also, prop might not suit your application. Post details of your rig in the prop section and you'll get advice on whether you are propped OK.

 

[Zigblazer]

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 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.

I have not checked spark strength, mainly because I replaced all four coils before I even ran it. Two had the wires chewed off by mice when I got it. I will see if I can did up my old spark tester and check that just to be sure. I don't think I have a dead cylinder, it isn't that low on power, but I do think I may have one or two weaker cylinders. I guess I'll have to wait on the repeat compression test to find out if it is mechanical. Again I was thinking that a sticking read valve or something would be more likely since it will occasionally give a small pop and run smoother with more power.

I know the motor I have has a 20" shaft. I do not know about what the pontoon was made for. It is a Maurell Crest III pontoon from the 80's. The anti-cavitation plate is well below the bottom of the motor mount tunnel, and a few inches above the bottom of the pontoons. I've looked at it while under way, and it doesn't look like the plate is out of the water at all, but it is complete chaos back there. All I could find when I looked up info on this pontoon is that it was rated for 2900lb carrying capacity and 115hp motor. It had a 1974 135hp evinrude on it when I bought it. I chose to go within it's intended range, a little newer, and with power tilt/trim.

 

[emdsapmgr]

If I guessed, that age of pontoon probably came with a standard 20" transom. Likely a good match for your engine. The new prop choice should be in the ballpark in terms of pitch. The fact that you detect a sudden improvement in engine performance makes me think you have an ignition problem. The coils are new, so you still have these electrical components: two power packs, a timer base and a stator. All are expensive. Considering that, it's worth your time spent to do some ignition troubleshooting-rather than just buying replacement parts and installing them. You may find that the excellent ignition troubleshooting information at this website is helpful: cdielectronics.com. They have specific ignition diagnostic failure criteria for your engine. Keep in mind that weak ignition components tend to fail one the engine warms up. May run fine while cold, then start to act up once it warms up.