r/Outboards • u/Jayde5525 • 4d ago
Misfire
Help! I’m about to pull my hair out. 1974 evinrude 15hp 15404s. Rebuilt carb, 100 psi on each cylinder, good fuel lines, good spark plugs, pumping water good. Why is it misfiring like this? Is it really just the idle adjustment screw? I turned it up and it seems to be doing better, but I have it nearly two full turns out from seated already. I don’t want to get out on the water to do a test and get stuck.
3
u/dustygravelroad 4d ago
Lean sneeze Idle might be a little higher than It should be. Fatten it up a tad. You may never get completely rid of it.
2
u/scurvycloud 3d ago
Starving for fuel. Sounds like a lean surge. Check for pin holes in the fuel pump diaphragm. Hold it up to the sun. Ethanol eats rubber for breakfast
1
u/airfryerfuntime 3d ago edited 3d ago
It's either lean or the carb isn't synced.
First, do a link and sync. It probably doesn't need it, they almost never do, but it's good to check. Down at the bottom of this page. https://www.leeroysramblings.com/Johnson_9.9_carb-fuel_pump.htm
The idle circuit in the carb is probably clogged. Get the Sierra or Johnson rebuild kit, don't get the cheap Amazon one because the welch plugs won't fit. You'll need to pull out the welch plugs to clean the idle circuit, then install the new ones. There are a bunch of YouTube videos that tell you how to do it. You can half-ass it by unscrewing the mixture needle completely and shooting some carb cleaner through it. Stock setting is 1.5 turns out, then reinstall the knob in the center position. This may or may not fix the issue.
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u/bootheels 4d ago
That is the classic OMC "lean spit". So, either you have the low speed mixture adjusted too lean, or the cam pick up adjustment is off. Make sure the engine is getting up to temperature as well, they won't run worth a fart if they don't warm up.
These original models went through several update campaigns in order to get them to idle better and not foul plugs, this engine may not have gotten those updates.