I think like a lot of engines, it just depends!
On things like tolerances --some guys get them ready to go right out of the box, others have runaways. In the past (maybe even now), the ring gap was insufficient. George Aldrich even claimed that a lot of them had "negative" gaps. Here is a copy of his reply to a post of mine in some long lost forum about a problem a buddy of mine was having with his Tigre 51 (Italian version):
"Everyone of the current ST's I've seen, clear up to the .91, have come with NEGATIVE ring end gap ! The
piston rises to the very top of the cyl. on all of these designs, and if you remove the ring from a new
engine, it will not fit in the cyl. at the top where it get hottest ! The ring ends will actually over-lap !
The end gap should be increased to about .003", and then a more normal run-in will have the engine
purring in about 30 min's. to an hour of slightly rich flying runs.
Above all, at least 20% lube, with 5% min. of castor for fuel."
By the way, I did check mine at the time and the gap was ok.
Also it may be the prop you use, or the fuel, or the ......