One Spark Plug Burns too cleanly - 1964 Johnson Outboard

[pen-xv]

The symptoms make me feel like fuel is not getting into the cylinder. However...

I have a 1964 Johnson 60 horse. I replaced ignition last year, all new plugs this year, new reeds, new gaskets, carb rebuild, etc. It seems to be running pretty well. Idles great, sounds great on throttle (haven't had it out, just in a bucket). When I pull plugs, three of them have black residue (the kind that would be expected from a cold 2-stroke). The spark plug in the number 2 cylinder doesn't look like it's even burning anything, or even getting anything to burn.

I am getting good spark with a spark plug tester, so the wire seems fine. Furthermore, I can see slight wear on the plug as though it's being put to work. When I pull the plug just after shutting down, exhaust comes out of the spark plug hole indicating that fuel is, indeed, burning inside. However, the spark plug is very clean. The other three plugs are all dark.

Again, the engine seems, for now, to be running well, but this inconsistency has me a bit troubled. Any thoughts on this?

 

[racerone]

Fuel pump checked ?---# 2 compression is ?----# 2 is top cylinder on port side.

 

[F_R]

First thing first. Check the compression to make sure you have something worth messing with.

It is impossible for fuel not to get to #2 without #4 also being affected. UNLESS #2 has a busted reed, or severe internal engine damage. Did I suggest compression?

 

[F_R]

UM..racerone brings up a valid question. I don't remember off hand which cylinder the pump pulse hose comes off of. But IF it is #2 and IF the pump diaphragm is ruptured, it would dump extra gas in that cylinder, making it too rich to fire.

 

[F_R]

I checked, as near as I can tell by the picture, it comes off #4. So, if that's true, that isn't the problem.

 

[oldboat1]

Maybe water washed? Might need a head gasket (should show up in a compression check).

 

[boobie]

Do a cylinder drop test on it.

 

[Vic.S]

Do a cylinder drop test on it.

+1

Then if it shows a problem swap the plugs around and do it again.

 

[pen-xv]

Thanks for all of the great replies. A compression test over the weekend reveals that I'm down about 16% - 17% in cylinder 2. I did just replace the head gasket, so I think I might try that again, and take a closer look to make sure that the head and block surface are clean; maybe I left some gunk behind that I didn't catch when I was cleaning surfaces last weekend.

 

[oldboat1]

Good idea to resurface the mating surfaces of the head and the head cover with some wet/dry sandpaper on a flat surface. Sounds like it's got a chance.

 

[Sea Rider]

Thanks for all of the great replies. A compression test over the weekend reveals that I'm down about 16% - 17% in cylinder 2. I did just replace the head gasket, so I think I might try that again, and take a closer look to make sure that the head and block surface are clean; maybe I left some gunk behind that I didn't catch when I was cleaning surfaces last weekend.

If both cylinder and crankcase mating surfaces were cleaned to immaculate perfection need to re torque head gasket to factory specs with a torque wrench. It's usually done after OB has ended its break end period when out of the box or when head gasket has been replaced with a new one soon after its first 10 worked hours.

On 2 strokes OB's won't find even darkened plugs on all cylinders, some darkens, fouls more than others. Those plugs looks relative new to be dark even.

Happy Boating

 

[pen-xv]

Welp, a new head gasket did not affect the compression. Anything else you guys might think of troubleshooting before I start pulling piston rings?

I also discovered that the spark plug in the same cylinder has an insert; someone screwed up the threads on the head long before I took possession of the engine and tried to repair it. Any chance that could play a role?

The good news, as you'll see in the photo, is that the plug is no longer burning so clean. At least something is going on in that cylinder!