lean misfire at idle

My johnson 1991 140hp johnson outboard keeps misfiring below 1300rpms or so. It misfires every 4 seconds or so. Other than that it runs fine at all other rpms. How do I fix this? I just decarboned it and changed out the spark plugs and changed the fuel filter. I had the dealer do the thermostats and they messed up the idle speed again. Everytime they work on something they mess around with the idle speed and it does this. They did this last season and I adjusted the idle speed and it ran fine but now this year even though I put it back to where it was it's still missing. If you take it out of gear it goes up to about 1100rpm this misses and drops down to 700 and repeats. I don't know if the only way to fix this is to clean out the carb or just leave it and see if it gets better. I always use stabilizer in the fuel.

Re: lean misfire at idle

Have you checked to make sure you're not getting air in the fuel system? Temporarily put a piece of clear fuel line from the fuel pump to the carb and start the motor. If you're getting air into the system, you'll see the bubbles in the clear line.

Re: lean misfire at idle

I put an old set of plugs back in and it ran better, still missing but not as hard. Would a bad alternator cause this problem? It only shows around 12 volts maybe 13 when cruising around 4000rpm.

Re: lean misfire at idle

Start by doing a compression test.
Then by buying an IR gun and check temps at both banks.
If OK, put in a new set of plugs, let it run at low idle for 5 minutes, pull the plugs and you may see if one plug is different from the others.
That engine is incredible sensitive for disturbance in the temps at low idle and note the difference in jets for Stb and Port carbs, and that Stb carbs feeds Port cyl banks.
The difference in low speed jets must balance the difference you have in the comp reading for stb and port banks.

Re: lean misfire at idle

Check your initial timing. To clarify, if the initial timing is set to high and you reduce your idle by adjusting the idle fuel setting, (throttle position, idle screws, etc). Not really sure of your setup but it doesn't matter. It the idle has been leaned to get it to lower you will experience a lean sneeze on new plugs that are fresh and producing a better spark than the old plugs. With the old plugs, that cylinder that was attempting to fire with new plugs, isn't firing at all. It's going to make it feel smoother even with a dead cylinder because that's what 2 strokes do. Hard to believe that you can loose a couple of cylinders and only notice it in overall power not smoothness but its true. The Merc 100 and 115 I4 2+2's are a perfect example.

Re: lean misfire at idle

how do you check the timing? Do you need a timing light? I don't have one.

Re: lean misfire at idle

You can borrow a timing light from most auto part stores for a small deposit.

Re: lean misfire at idle

I just finished with a very similar problem on my 1990 Johnson 140. It was clearly misfiring in the 1000s range on the water, making a sort of audible PUFF noise along with the combustion noise as it ran through that range before smoothing out around 2000, and on the hose was idling around 1100 for 20 seconds or so at a time and then dropping to about 800 leading off with a sneeze, then it would catch back up after about 5 seconds of misfire and run great for another 20 seconds or so.

The issue was a bad coil and it sounds like your issue too. When these solid state electronic ignition components heat up they perform differently (read: fail until they cool down). While you've got that timing light on it, and Before you make any timing adjustments, you need to spend some time on each plug wire getting used to exactly what it does when its running smoothly, when it sneezes and runs rough and starts bellowing out smoke, and when it recovers. Any small deviation or blips in the spark is your bad coil(s). You need to get familiar with what's going on with all 4 cylinders throughout that cycle.

My guess is that your "sneeze" is actually not a lean sneeze but incomplete/failed ignition on one or more cylinders. Start with that because all of your tests need to be done in the most all-things-held-constant method possible and you don't even have to pull a plug wire to just look at this one.

If you determine a coil isn't sparking correctly then swap it with a different one and see if the problem follows.

Even if your timing is off they'll still spark just the same, just not at the right time.

Re: lean misfire at idle

I have a the same problem with my 70 hp. I was convinced it was sucking air from the line but now I am seeing a small leak of oil on the head and around the gasket. I would be interested to know if yours has the same issue. I will peel it off this winter and have it milled but mot sure if it is the problem or not. I don't like the coil idea just because I went through that crud a few years ago. Things should break or work life would be a lot simpler