Beware the Carbon Booger

Bart_

Active Member
Location
GTA
Ok now that I've got your attention its an oddity I encountered. I have a normal 4 stroke piece of equipment that might burn a touch of oil but I can't even see any blue. So I was a bit reticent on keeping on top of the wet oil foam air filter and was finally made attentive by loss of engine power, maybe some black smoke couldn't be sure. This power loss was a bit progressive over say a 45 minute run time. So i started worrying about tightening valves, overheating, questionable fuel diaphragms bad ignition etc.when the engine actually stopped like slow motion throttling it down. First thing I checked was for spark and there it was.

The carbon spark plug booger. Kind of grown sideways off the center electrode and sized big enough to go up and touch the side of the ground electrode. Now I've seen sooty plugs, oil crudded plugs, grey encrusted plugs, tan plugs, plugs that ate half their ground electrode, white plugs (how did the piston not melt) but never a booger before.

I wanted to suspect the mixture screw vibrating out but a little voice said look at the air filter. Last time I did it I used plenty of oil because more is always better right? But we've had record lack of rain for a month and very fine dust had made oil mud on about 50% of the intake surface. Field dress the sludge off and all was good after I chipped off the booger.

So, beware the carbon booger. Or, keep your air filters clean.

best to all, stay safe


edit - I rechecked the plug after an hour or two runtime and saw a bit of evidence of oil fouling, so it must have been the perfect combo to put it over the edge - hint of oil plus bit rich mixture
 
Last edited:

Bart_

Active Member
Location
GTA
Update - the engine puked, slowly sucked and lost power and then wouldn't restart. So I revisited the coil in true mechanic method and just shotgunned a new one in. No bueno. Plenty of spark, eliminating hot coil failure. Carb was fine, yet at failure I watched it progressively blacken the exhaust just before it stopped.

Head scratch.

Finally noticed my compression was down, figured ok what could the culprit be. Leak down made noise everywhere and was a sieve. Time to pull the head. Found the intake valve was beyond no clearance and was actually just above the seat. Exhaust was 0.005 about half spec. Cylinder looked beautiful even though I knew the rings were worn. So I ground the valves to length and got back proper seating, even though the valves were definitely pooched.

Hello compression my old friend good to see you again. Runs good.

So here's the weird part. How did the not seated intake cause a rich running condition? All I can come up with is the pulsation in the intake tract caused by the blowback made air that had passed the venturi go backwards and come through a second time picking up more gas. Seems a stretch but what else could make sense?

And I had to richen the main 1/8 turn after the fix, seems to vote for the theory.
 

96coal449

Well-Known Member
Location
earth
The oil consumption can give the cylinder a colder burn temp. This decreases the efficiency of fuel used or, in other words, how much fuel is actually burned.
 

Bart_

Active Member
Location
GTA
More clues. The exhaust valve was white, intake normal amount of carbon, after the air filter clean plug looked normal, even bit white on ground prong. Still figure on it being colder burn? Just asking'/learnin'. Wouldn't the same oil consumption after fixing cause cold burn now? But it isn't showing rich now. I'm pondering low compression showing rich - but remember this is 4 stroke without the bleeding edge temperature/jetting behaviour of a 2 stroke.
 

96coal449

Well-Known Member
Location
earth
That would indicate a hot burn condition. To hot.......

Not sure why the intake valve was running that much cooler. It's been a while since I routinely been inside an engine on a regular basis. I would think that the intake valve would also run on the hotter side. I do understand the exhaust showing signs of being hotter. It all has to exit down that avenue.

How hot was the exhaust valve? Pics?
 

Bart_

Active Member
Location
GTA
Before the fix. After fix I had to richen the main a bit because on restarting It was clearly now lean, on exact same string that had blown black during failure. Weird, huh?
Sorry, no pics.
 

Birdyman88

Well-Known Member
Location
Arlington
Before the fix
Not weird. Since its before fix, I did want ask what you're wetting your foam filter with? A tacky oil specified for wet filter, or just plain old oil. I have heard that regular oil can migrate into intake and you could see that side get carbon, especially if you have any combustion gases getting back through. Just pure speculation though.

So assume you had low pressure AND a tad of filter oil getting in. You combustion and mix would be poor. So you lean it enough to keep it running on that low pressure higher oil mix. I imagine that the temps would have to be a bit higher to run off a mix like that. Thus the hot indication on the exhaust valve. You're still burning the petro as if in lean condition. Again, pure speculation.

Hell, I don't know, but this is fun.
 

Bart_

Active Member
Location
GTA
Briggs is pretty archaic, they spec sae30 for the filter I throw on 10w30 just cause it what I've got. Another wild card, along the way I burnt out the old cheese muffler to ear discomfort levels and transplanted a newer quieter muffler that fit the head, from another similar sized engine. Maybe slight change in back pressure. But likely not significant as after the valve fix and jet tweak all my power came back - well, maybe not like new but definitely good. No oil excess was going down the carb I think because the filter element just stayed the same amount of oily - vs oil consumption where you watch the dipstick level vs tanks of gas used. Crank case breather return hose looked not oily either, so it seemed to be coping with any blow by.

I confess to leaning it in simple response to keeping the plug unfouled, which I had the theories of burning oil or clean the dust out of the air filter, stupid. Oil burning is still a grey zone in my mind, maybe it's just not enough to be visible but significant. But now maybe getting burnt off with the higher compression... on it goes... a mystery of life ...
 

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