I HAVE A FOULED MULE

FOULED SPARK PLUG QUESTION

On Mar 12, 2025

A DIRT WHEELS FAN WRITES: “I am having a problem with our 2016 Kawasaki Mule 610 Special Edition. Most of its life we have been having a problem with the engine fouling spark plugs. The oil level is correct, the air cleaner element is clean, the carburetor has been cleaned and a wet/dry compression test is within 15 psi of each other. We do not detect any oil burning at all. It seems like almost every month of use on the farm we need to replace the spark plug due to hard starting due to the spark plug being black and fouled. Dealer says the Mule is operating correctly?? Really? Here’s hoping you can provide some answers to our problem.” – Brad Nelson, Edgemont, SD

OUR PRO ANSWER: Your problem is the spark plug tip is operating at a lower temperature than it should be. From NGK Spark Plugs: For a spark plug to function properly, it must have a tip temperature high enough to burn off carbon deposits (self-cleaning) and avoid fouling, while remaining low enough to avoid overheating the ceramic firing end and pre-ignition. A hotter heat range spark plug has an insulator design with a longer heat flow path to the metal shell of the plug. As a result, more heat stays in the ceramic firing end and less is dissipated to the engine. A colder heat range spark plug has an insulator design with a shorter heat flow path to the metal shell of the plug. As a result, less heat stays in the ceramic firing end and more is dissipated to the engine. Depending on what source you talk to, the factory recommended spark plug was either an NGK BPR6ES or a BPR5ES. It appears the 6’s were originally spec’d but was changed to the 5’s because of plug fouling like you are experiencing. NGK’s spark plug heat ratings go hotter as the numbers go smaller. So my recommendation is if you are currently running a BPR6ES, go one level hotter to a BPR5ES. If you currently are running a BPR5ES, go one level hotter again to a BPR4ES.