So here’s the issue. I bought a '97 flats boat with a '97 Johnson 115 Ocean Runner. The rig was in good shape but sat for about 10 months. Got the motor running and clocked it on the GPS at 48mph. Been using it, had to swap out the fuel/water seperator cause it was clogged abnd running rough but afterwards motor ran fine. Getting good clean fuel now, but last few times out top speed was only 40 mph. Ran great, no misses, couldn’t really tell it had slowed down except for gps reading. tach doesn’t work (next project) so can’t tell what the RPM change was. Anyway, decide to check the plugs so pull the first 3 and they all look the same, little oil and discoloration but otherwise fine. Pull the last one and it had no dark discoloration or soot, but seemed “wet” vs oily and had a tan residue on it. I’m thinking water in the cylinder. Since it ran fine, just slower, I don’t think it’s serious. Sound like a leaking headgasket or could it be a cracked head or block? Not sure what to do next.
I doubt it’s water. I think your motor would probably be dead by now if it was. Sounds to me like maybe that last cylinder is not firing correctly - are you sure the “wetness” on the plug isn’t unburned fuel? Could be caused by a number of things, weak/no spark, low compression, fuel delivery problems. My theory would be you’re running rich.
But, I’m definitely not an expert on outboards, just know a little about engines in general, so hopefully someone with some experience will chime in!
Angler 204 FX
Yamaha 150
if it was water, you’d probably have rust on the plug. Do a compression test and spark check. When you do the compression test, if nothing interesting shows up, stick the lower unit in a bucket of water, spin it over and keep checking compression, if it start spitting water out of the compression tester when you hit the reset valve, you’ll know you’ve got a water problem. It could be a leaking head o-ring or corrosion in the block, but I’d guess bad spark, compression, or flooding carburetor first
The fact that there is no dark discoloration on the plug (like the other plugs)does make me think it might not be throwing a spark. Compression was tested recently and it was fine, also has a new powerpack. No spark would explain the loss of HP I guess.
if its not a compression issue, i doubt its a running rich issue, as its very tough to get to much fuel at top end, yes its possible, but pretty uncommon.
spark is most likely the issue
Would the boat still run smooth w/ no coughing/misses etc if there is no spark in 1 cylinder? If so then I think that is the issue I’m having. starts good and runs well all through power curve, just lost about 8 mph of it’s top end.
you have lost 25% of your power, but the very first thing need is a compression test, that is still my first guess
Thanks everyone Good ideas and input, much appreciated. Chris, looking forward to seeing the new shop!
Is this outboard the 60* with OIS ignition? If so good luck, I’ve been chasing my tail for two weeks, trying to locate a misfire problem, with no luck. Make sure you are using the right plugs (QL78YC Champions).
I also found this bit of info:1991 through late 1990s engines sometimes developed a crack in the water jacket allowing water into the intake at high speed. This typically resulted in #1 cylinder-ingesting water. You can usually see signs of the head looking like it has been steam-cleaned inside the combustion chamber.
Which cylinder looks different?
…Politicians aren’t the “Oldest Profession”, but the results are still the same!!!
Not a pro but all the motors I’ve been tinkering with have ran rough when lacking spark. Recently had a 115 running rough and would sporratically pick up. Found an ignition coil breaking down.
1720 KW 110 Johnson
16’ Bonito 65 Johnson
Figured I’d circle back on this issue now that is has been resolved. Turns out there was indeed water in the lower port-side cylinder. Checked compression and it was only at 85. There was also a little rust in the cylinder. Everything else looked fine though, no cracks in metal and gaskets looked OK. I had recently drained my fuel/h20 seperator and it was full of h20. We are thinking once it got full some water flowed past it and settled into that cylinder. Kept swabbing the water out as we rotated the flywheel several times until we couldn’t get any more water out. Compression got back up to 118 and in line with the rest of the cylinders. Put it all back together and seems to run fine now. Speedometer (not GPS as I forgot it) had me up to 45mph in a short burst of WOT which is close enough to my old top end of 48. Under ideal conditions I’m sure I can hit 48 again. Thanks for everyone’s help, just wanted to provide some closure and maybe some info to help out someone else in the future. Just gonna check that seperator more often!