I think you are right CK1. Crappy fuel was also my doubt but I couldn't really figure out how to improve it. 35:1 oil ratio seems fine to me and the premium gasoline down here is supposed to be 98 octane or so. Would one of those octane boosters (STP for example) benefit the combustion diagram in a two stroke engine as well?
Then I started to focus on the ignition timing, which I didn't touch yet because I am not sure how to tune it. Should I measure 28 - 30 degrees prior the TDC from the sensor axis position or from the point at which the plug sparks? Moreover, due to the tolerance gap between piston pin, connecting rod, etc. at the very TDC point the piston neither travels upward nor downward for a couple of degrees. So, which is the true TDC point to be considered in this instance? Well, not much of a deal, I reckon that but it is always better to know how things work before taking an action.
Thanks for your help.