About This File
Original plugs in the early P series Onan's was the Champion RS14YC. Crossed to NGK 2238/TR5.
Onan changed over to the Champion RS17YX. Crossed to NGK TR4. Issues with plug fouling caused them to go with a hotter plug.
NGK 3754/TR4 is the correct plug for Horse's with P216, P218 & P220 engines.
The TR5 is ok if you run your horse under heavy loads all the time. If you run it light, heavy and everything in between, TR4 is the way to go. It's the all around plug. Attaching Onan Factory PSB concerning the issue. The gensets they refer to use the P218 and P224 engines, so info is applicable. From the "Horse's Mouth".....:)