Hi,

A No. 8? I could not get the engine to run on a 6, a 7 seemed even worse?

In my case TopGun suggested I go down to a 5 or even a 4?

I thought the higher the number the colder the plug. You tend to run 7 & 8 in .21's

I've always run 6 in my .12's because they tend to need a hotter plug.

I thought the lower numbered plugs have a thinner wire which get hotter which tends to be needed in a .12 because of the small compression & most run 16% fuel.

With the .21 you have higher compression & most run 25% fuel so you don't need as hot a plug.

Have I got this right or am I completely wrong??

All the above relates to Novarossi glow plugs.

Thanks in advance