airportkid
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The right O-320 of a GA-7 Cougar has developed a sudden high mag drop in both mags, as much as 160RPM. Observation 1: leaning the mixture decreases the drop to about 120. Observation 2: leaning the mixture on the left engine has a negligible affect on its normal drop of about 50-60RPM each mag.
The left mag acted normal during one brief runup session, but now produces similar RPM loss as the right as when the problem first surfaced. The right mag seems to be more seriously affected than the left, so our diagnostic efforts thus far have addressed the right mag. Diagnostic 1: swapped plugs top to bottom to switch the plugs driven by each mag. No effect. Diagnostic 2: swapped the right mag from the left engine. No effect. Mag timing to engine is correct.
A hot compression check yielded 74, 70, 72 & 72. Plugs are clean and dry top & bottom, no oil fouling of any kind. The high mag drop is accompanied by a very mild roughness. Running on both the engine develops smooth power equal to the left engine (no yaw trim req'd during flight).
We're out of ideas - any imaginative thoughts here?
Thanks!
The left mag acted normal during one brief runup session, but now produces similar RPM loss as the right as when the problem first surfaced. The right mag seems to be more seriously affected than the left, so our diagnostic efforts thus far have addressed the right mag. Diagnostic 1: swapped plugs top to bottom to switch the plugs driven by each mag. No effect. Diagnostic 2: swapped the right mag from the left engine. No effect. Mag timing to engine is correct.
A hot compression check yielded 74, 70, 72 & 72. Plugs are clean and dry top & bottom, no oil fouling of any kind. The high mag drop is accompanied by a very mild roughness. Running on both the engine develops smooth power equal to the left engine (no yaw trim req'd during flight).
We're out of ideas - any imaginative thoughts here?
Thanks!