Several years ago, I purchased an OS 140 RX with the OS tuned pipe &
header.
Due to other priorities, It took me a while to finish my pattern
airplane (USA-Star) and have it ready for flight.
I performed the break-in with the engine installed in the airlplane
using an APC 16x14 prop, using cool power fuel with 15% nitro and 17%
synthetic oil.
In the first flight the engine quit after 2 minutes.... It seemed to me
normal for a recently bronken-in engine, and after a closer inspection,
I saw that the soft mount was too soft, so I decided to remove it. ...
and mounted the engine directly on the mounting beams.
On the second flight, the engine quit again after two minutes...
Right after landing, I restarted the engine and observed that the RPMs
we unstable - the engine would accelerate for a few seconds and then
slow down a few seconds, and then accelerate again, etc, etc...
I figured maybe the engine was overheating, so I took off the cowling
and started the engine with no cowling.
The RMPs keept oscillating as described and the engine shaked violently
at idle.
I took off the backplate and cylinder head and found everything as good
as new.
I checked the fuel tank, fuel lines, tuned pipe, etc and found
everything in order.
I tried again, this time using cool power 15%nitro, 17% synthetic, and
I added 5% castor oil.
The results were the same....

I have a question on wich I would like your comments: Assuming that the
engine overheated during the break-in and the first flight, what damage
could that have done to engine? As I mentioned the engine looks as good
as new, both on the inside and the outside, has no free play, has
excellent compression, and has no friction. - I am totally disoriented
as to what might be the problem.