403 Sparking plugs

[GrahamH]

The plugs on my 403 are prone to "coaking" up after a few months after service. Fuel mixture has been checked and ok. I always put the choke in as soon as possible and make sure the engine is warmed with oil hot before parking. Plugs suffer from carbon deposits that cause misfiring and loss of power.
The plugs are standard NKG.
I believe that it is possible to obtain 10mm plugs that run at higher temperatures that may resolve the problem. Current plugs are I think grade 5 or 6.
I also believe that choice of petrol my help. I do not nornally seek out particular brand but perhaps I should.