What "OLD" says makes sense. Cracked plugs will do what you describe, too. But again I don't get why it shows mostly on slope and why all four at once?
As you know, the spark will always follow the path of least resistance. If it is easier for the spark to come out the crack and along the plug, than jumping the spark gap inside the chamber, that's where it will go. For HV sparks it is much easier to run across dirty surfaces than to jump across the air/fuel mix in that spark gap. That's why when doing a tune up I always make sure my dust boots are still whole (no missing chunks or cracks) and pushed on as best I can. I even like to use a little silicone "dielectric" grease on the inside to help make the boots seal well and helps keep them from cracking.
Same-Day Shipping! Most of our stocked parts ship the same day you order (M-F). Expedited shipping available, just call! Most prices for parts and manuals are below our competitors. Compare our super low shipping rates! We have the parts you need to repair your tractor. We are a Company you can trust and have generous return policies! Shop Online Today or call our friendly sales staff toll free (800) 853-2651. [ More Info ]
TRADEMARK DISCLAIMER: Tradenames and Trademarks referred to within Yesterday's Tractor Co. products and within the Yesterday's Tractor Co. websites are the property of their respective trademark holders. None of these trademark holders are affiliated with Yesterday's Tractor Co., our products, or our website nor are we sponsored by them. John Deere and its logos are the registered trademarks of the John Deere Corporation. Agco, Agco Allis, White, Massey Ferguson and their logos are the registered trademarks of AGCO Corporation. Case, Case-IH, Farmall, International Harvester, New Holland and their logos are registered trademarks of CNH Global N.V.