Modern Vespa

I was overdue for a spark plug change: 4+ years, 7000 miles on an NGK 7784 CR8EB in hot Texas weather.

As I was removing the old plug, I found out it was a bit loose, which meant it got hotter, and thus was badly worn.

So I put in a "hotter weather" colder range NGK CR9EIX and torqued it properly. I even used dielectric grease in the cap.

(Here's an excellent explanation of what plug heat ranges mean, from NGK)

The result with the new plug was that my GTS 250ie would start but not idle properly when cold and just die. This was bugging me for two days and then I realized the ECU probably needed resetting.

So removing the + terminal from the battery for 1/2 an hour reset the ECU, and now she's idling and running smoother than before.

The colder plug will probably work just fine, but could foul during engine warmup.

Guys with the 2 stroke race bikes will sometimes use a hot plug to warm up the engine and then swap to a colder plug.