weston magnum plug
You have got the West Eurotec engine?
I went through a couple of plugs fast with mine before I ended up with an Enya #6... this one seems to hold.
I had a Proplug #7 (supplied with engine), Rossi R5 (2 of them actually), and an Enya #4 on it, and the were all gone in 2 or 3 flights.
The engine doesn't need high nitro fuel, 5% is more than enough apparently...