My Vespa burns 1 spark plug every month or so

Hi everyone, my Vespa PX (model without indicators from 1978) burns about one spark plug per month. Here (https://photos.app.goo.gl/DzfbCATe4LjJQuWx5) the photos of the burnt ones that can already give some info to the most experts and some other info. I live in London and it will already be a miracle to find a mechanic capable of putting his hand on a classic, at least I would like to try and direct him a little. I would therefore appreciate your contribution! 

The diagnosis:

  • I've always had this problem (for about 10 years), it hasn't gotten worse or better. 

  • Occasionally the engine makes some rumbling during the journey, it seems it struggles to accelerate, sometimes the rumbling ends with a sort of burst

  • During a recent rainy day the problem of "rumbling" was very pronounced, the Vespa went off at a traffic light and I was sure it would abandon me at any moment (I arrived home luckily, the next day without rain, the Vespa went like normal!)

  • Usually when it stops I am at low speed. I immediately understand that the moment of spark plug change has arrived :( 

  • I keep the "burnt" spark plugs because in reality they apparently are reusable, after a few weeks (when the same problem occurs) I clean them with some gasoline, put them back and usually they work again! Obviously I know that they will last less than a new one.

  • Whenever I forget to turn off the petrol tap I seem to notice that the Vespa takes a couple of more attempts to switch on (maybe the carburetor pin is ruined?) - maybe this problem it's not related to the candle problem