Re: Champion UJ4J

[Pete]

Champion has a lot of lead letter designations for their plugs.

R – contains a ceramic resistor in series with the center electrode to suppress ignition noise which prevents (reduces) interference in radio, electronics, etc.

U – contains an internal auxiliary gap (also called a booster gap) in series with the center electrode to help reduce fouling by creating increased voltage to the exposed gap at the tip. They are not the same as a Resistor plug.

Scroll down to Champion for codes:

https://www.sparkplugs.co.uk/spark-plug-symbol-codes/