Engine loses power after 30 seconds
For the last month I've been wrestling with a problem with my 912ULS. After running at full power (5770 RPM and 29.4" manifold pressure) for 30 seconds, it suddenly loses power, going to around 5300 rpm, and manifold pressure increases to 29.8" at exactly the same time. Reducing and increasing throttle does not help unless I reduce throttle all the way to idle for a second or so, at which point I can increase it to full and regain full power, at least temporarily. In addition, oil temperature is much higher than it should be.
Compression is good, there are no leaks (the mechanic pressure-tested the cylinders), we replaced the fuel filter, electric fuel pump, and carburetors to see if it would help, checked and calibrated the throttle cables, but the problem still occurs.
Oh, also the magneto check results in a ~300 rpm drop at 4000 rpm but it used to be only a 150 rpm drop at most. (Each carb drops about the same amount though.) What can possibly cause this?
