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Slick mag timing—impulse couple

447BB

Member
A little confused by the documentation I’ve been able to find for Slick 4370 and 4371 on a Lycoming O320. Engine has been modified to produce 160 hp with 8.5:1 compression. Left mag has impulse coupling. Data plate says lag is 20 deg on that magneto. Question: At what point on flywheel rotation should points open on this magneto to be properly set?
 
Impulse trigger

Plugs out!


If just doing a quick sanity check, the impulse should snap right at or just past TDC (when moved slowly through forward rotation).


Right after the impulse triggers reverse engine rotation until ~25 degrees BTDC, connect your timing light and make sure BOTH magnetos open up at the same time as you rotate past 20 BTDC
 
Slick mag timing

Lycoming procedure is to rotate coupler in it's normal direction until snap and seeing a spark from the #1 plug lead, then (2)reverse the rotation approx 25 deg until pin slides into vent hole, (3)mount magneto to engine (previously turned to 25 BTDC on the cylinder 1 compression stroke), (4)remove pin (5)keeping engine at 25 BTDC, adjust (rotate slightly) the magneto case until the indicator (buzz box) indicates agreement.
I think your “sanity check” is essentially the same but getting to the coupler snap by rotating the engine. After the snap, both sides (L & R) set for the same points opening time. Thanks for the help.
 
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