Today's brain teaser:
My 1942 Aeronca L3B finally flew after the FAA decided they tortured me enough and gave me my registration (17months, but that's another story).
My friend do flew it around the pattern and all seemed good. Second outing today and we got rough running on the left mag. On the first flight the was some very slight roughness on the left but we put it down to the lower plugs getting a little fouled. Seemed to disappear at high rpm.
Today it was bad enough that we investigated further. It seems we were getting no spark on No 4.
The mags are old style Bendix. But virtually new, with all new parts, straight from Fresno Air Parts. Timing triple checked and perfect.
Taking out every plug and grounding it, we got sparks on 1,2,3, but not 4, when turning the prop over. Swapped plugs and swapped wires in every possible combination: All wires and all plugs fire when connected to 1,2,3 but not on 4.
Plugs (Tempest) are new and all have proper resistance. It's a mystery....
I can take any known good plug and known good wire, in any combination, and all will fire 1,2,3. But not 4.
Here's another point: the same contact on the rotor that fires 3 should fire 4 as it turns around, but it doesn't. So nothing wrong with the rotor, it fires fine on 3 but then when it gets to 4 - nothing.
My only remaining theory is that somehow the No4 plug wire isn't reaching far enough into the mag to reach the rotor. Although I measured it and it looks the same as 1,2 and 3.
Discuss.....
My 1942 Aeronca L3B finally flew after the FAA decided they tortured me enough and gave me my registration (17months, but that's another story).
My friend do flew it around the pattern and all seemed good. Second outing today and we got rough running on the left mag. On the first flight the was some very slight roughness on the left but we put it down to the lower plugs getting a little fouled. Seemed to disappear at high rpm.
Today it was bad enough that we investigated further. It seems we were getting no spark on No 4.
The mags are old style Bendix. But virtually new, with all new parts, straight from Fresno Air Parts. Timing triple checked and perfect.
Taking out every plug and grounding it, we got sparks on 1,2,3, but not 4, when turning the prop over. Swapped plugs and swapped wires in every possible combination: All wires and all plugs fire when connected to 1,2,3 but not on 4.
Plugs (Tempest) are new and all have proper resistance. It's a mystery....
I can take any known good plug and known good wire, in any combination, and all will fire 1,2,3. But not 4.
Here's another point: the same contact on the rotor that fires 3 should fire 4 as it turns around, but it doesn't. So nothing wrong with the rotor, it fires fine on 3 but then when it gets to 4 - nothing.
My only remaining theory is that somehow the No4 plug wire isn't reaching far enough into the mag to reach the rotor. Although I measured it and it looks the same as 1,2 and 3.
Discuss.....
