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Carl:
Your high altitude backfire could possibly come from your sparkplug leads. My engine came from the factory with several metal clamps supporting the wires. These metal clamps did not have any insulation like an Adel clamp therefore as the insulation of the wire ages due to heat, vibration, etc, the clamp provides a good path to ground. The problem only gets worse as altitude increases.
A neighbor here at KTCY had approximately 40 hours on an RV-6 which began to have problems with RPM loss during the magneto check. The insulation on the wires was discolored where the wire was supported by a clamp. The neighbor returned the set of wires to the engine mfgr who replaced them. Problem went away. After that, I replaced all my metal sparkplug supports with loose fitting
Adel Clamps.
Good Luck, Bill Hogarty
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