Engine Operators, I read the CS 061 ... I Give Up ... posts. As a mechanical engineer of many years experience, the primary problems with manufacturing quality (i.e. materials & workmanship) results in the following;
a) engine non-operation or poor operation (low RPM & low Torque/Hp & unsteady RPM oscillation)
b) short operational lifecycle or poor product durability
If an engine is able to turn a defined propeller at the defined engine RPM ... then the engine is NOT failing performance; regardless of its appearance.
Unless the engine seriously fails to operate, poor manufacturing quality primarily affects the engines durability ... this includes porous casting material. "Bubbling" castings do
NOT directly affect performance unless it is extreme ... similar to a metal foam "fluid filter" condition in which required flow pressure cycles cannot be maintained during operation.
The gas flow (and associated pressure cycles) through an engine is far to fast for small amounts of leakage to notably affect performance ... does the engine produce propeller RPM
or NOT ... is this RPM operation steady or NOT ... this is primary engine performance.
Fuel consumption and product lifecycle durability are lower order operational characteristics that do NOT measure primary engine RPM performance ... which is what is required to "FLY." As for "variable throttle" operational performance concerns, only fuel injection and/or sophisticated carburetors can deliver such variable engine RPM performance with stable control, high power output and fuel efficiency (sfc). Sooo ..., just how much weight and cost is one willing to endure? Only operators (i.e. customers) and engine designers can answer these questions directly!