Jeff,

About a year ago I had problems when running LOP at 31” MAP & 2500 rpm.  At any setting LOP I would get a “surging” sensation, and at leaner settings (16.5 gph and less) the engine would backfire (forward fire?  Unused gas would burn rapidly in the exhaust) causing a popping or banging sound.   Running at 125° ROP the problem would immediately go away.  Doing a mag check in flight revealed a problem with the right magneto.

When I removed the magneto I found some sort of “growth” (fungus?) inside the right mag.  I ultimately ended up replacing the mag and have never had any backfiring or surging since.  Problem solved.

Also, you should check the pressurization lines going to the mags.  I found that the blue filter device that’s inline between the mags and the intake manifold was loose.  The clamps had loosened.  My first inclination was that I wasn’t developing deck pressure in the mags allowing arcing at high altitude.  This might have been happening but I think the gunk in the right mag was the ultimate culprit.

I don’t think that I’ve ever experienced a compressor stall.  I typically cruise at 31” MAP, 2450 RPM, and 16.5 gal/hr with no problems.  I’ve got an “E” engine.

Good luck,
Hal Woodruff