I have a John Deere LA115 that recently came back from the shop as it needed a head gasket replaced as well as diagnosis on it being hard to start when cold (but only sometimes). The mechanic said it was a dirty spark plug and replaced it. I saw the old spark plug and it did look quite dirty and gunked up. I replaced it myself I think maybe 2 seasons ago so I didn't think it would be causing my problems.
Could a bad spark plug cause sporadic hard cold starts? From reading around I believe it could - but that gives me a bigger concern in that how did it get bad in the first place? The hard start issue has been around since before the head gasket issue so I don't believe they are related. Is there a bigger issue here that caused the spark plug to go bad?
A plug that is black and oily indicates worn valve guides or piston rings.
A plug that is black and wet indicates fuel fouling that would be typical of a carb with a stuck float or jet issue.
And of course, the NGK list has other symptoms.
Your head gasket issue is also an indication of something-- a failed head gasket usually means the engine was over heated. Spark plugs can contribute to hard start conditions if they were over heated and glazed.
Can you provide any more details about the head gasket failure?
The head gasket was most likely due to my own neglect. I have read that these Briggs and Stratton engines are prone to head gasket problems but I also ran it (only a little) with a chipped flywheel fan blade. I suspect that this is at least contributed to the head gasket blowing. I have since had the flywheel fan replaced and it is no longer chipped - and I've learned my lesson.
The hard starting though started a few months before I chipped the flywheel fan. When cold and sitting for a day or 2, it would sometimes be very hard to start. I would fiddle with the choke and every time thus far, it would eventually start after I tried it a bunch over the course of maybe 10-15 minutes. The mechanic said that the carburetor looked fine though and just replaced the spark plug, which looked like "dry fouling" as per your link. I can't tell if it was wet and just dried off though.
In between the hard starting issue and before the head gasket blew, once it started it ran totally fine and I had no problems with it.
One clue to a blown head gasket is a gunked up spark plug. Whenever I do a head gasket on one of these it gets a new spark plug and I remove, clean, inspect valves for excessive yack and I reseat the valves with a lil valve grinding compound. Works like a charm.
Also make sure the choke is closing properly. Little or no choke will cause excessive cranking before it starts.
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Last edited by xcopterdoc; 06-22-2017 at 01:07 PM.
If you live in an area (like I do in California) with Ethanol in the fuel, the carb can get ruined quickly if the fuel is treated with a stabilizer. That can lead to hard starts and plug fouling and low power.
Air cooled engines operate with a *wide* range of engine temperature conditions. I'd suggest inspecting and changing plugs on a regular basis if you have an Ethanol fuel mix. Heat can cause the poor fuel here in CA to go bad quickly, untreated.
If you got the engine hot and that caused the head gasket failure, there could be other heat related damage to the ignition coil that might cause a hot start or weak spark.
But, your cold start symptoms indicate the carb.
I would suggest running the engine some more, then pulling and reading the plug again to see if it is fouled. If it is fouled after that, you've got other issues related to the overheating.
If the engine got really hot from chaff blocking the fan, etc. then it is likely you could have scored the piston(s) or burned a valve.