Mysterious Ailment

[airportkid]

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!

[Walt]

Running overly rich will cause a high mag drop on both mags.

[az_gila]

Originally Posted by airportkid

....A hot compression check yielded 74, 70, 72 & 72. ...

If those numbers were using a "Bonanza" compression checker with a 0.060 orifice instead of the Lycoming recommended 0.040 orifice, then you could have low compressions.

Some mechanics just use the one they have without checking the orifice size...

[rocketbob]

I would have swapped the plugs between engines first.

[mahlon_r]

Make sure the carb heat door isn't leaking hot air into the airbox. and also make sure the primer isn't leaking extra fuel into the intake system.
Good Luck,
Mahlon

[TS Flightlines]

Glad to see Mahlon on the forum!!!!!!
Hi Mahlon!
Tom

[airportkid]

Originally Posted by mahlon_r

... Make sure the carb heat door isn't leaking hot air into the airbox. and also make sure the primer isn't leaking extra fuel into the intake system ...

Now why didn't we think of that! We'll check the carb heat door straightaway and post the result. Too much of my recent life spent fixing injected engines - I forgot all the gadgets on the carbureted ones. Thanks!

[walkman]

Swap the left and right engines, retest.



Sorry, couldn't resist

[airportkid]

Originally Posted by airportkid

... We'll check the carb heat door straightaway and post the result ...

Well, it's not that. We've now completely transposed the right mag ignition system engine to engine, plugs, harness and mag, with no effect - the right mag on the right engine loses 160-170RPM and is subtly rough, even though every component of it has been taken from the left engine. Meanwhile, of course, the removed system installed on the left engine operates perfectly.

Could carbon buildup on the piston or elsewhere in the head degrade the force of the mixture charge when fired from one source? The plug appearance is normal in all cylinders, showing no signs of detonation.

Still haven't tried inverting half the harness to fire all the top or bottom plugs from one mag, but would that tell us anything?

[C-FAH Q]

Next time you run the engine, laser temp the cylinders to make sure they are all around the same temp( no cool cylinders). Check for intake leaks at same time.