ORIGINAL: MJD
In four flights I've eaten two plugs and one prop, that works out to about $26.00, or $6.50 per flight plus fuel.
At this level of performance, that is actually pretty good. The danger with using old plugs is having the element break off and end up getting wedged between the piston and the liner.
OTOH, that can happen on the 1st flight if you are too lean, overly compressed or too much vibration.
I was hoping to hear that these engines could easily take a little plane like this to 200 mph without any issues, but as usual my wishfull thinking is met with the reality that there is no free lunch in physics.