Ignition Check

[pilotyoung]

I need some help. My AeroVee Onex had a 500 rpm drop when I turn off the secondary ignition during the ignition check at 1800 rpm. The engine also runs very rough when I turn off the secondary ignition during the check. The really interesting thing is that the EGT's on cylinder 1 and 4 go to zero with the secondary ignition off. This happened the fist time last Friday. On Saturday I did an ignition check at the hangar before the engine was up to full temperature for takeoff and the check was OK. Today I put the cowling back on. Before I put it on I did an ignition check and it was fine. After putting the cowling on, I taxied down to the end of the runway and when the engine was up to temp I did an ignition check. When I turned the secondary ignition off, there was a 500 rpm drop and it ran very rough. Also the EGT on cylinder 1 and 4 dropped to zero.

Since the top magnetron fires cylinder 1 and three and the bottom magnetron fires cylinder 2 and 4, both magnetrons would have to be partially bad for this to happen. And it only happens when the engine is hot.

So it seems there must be a short somewhere. But these magnetrons are almost totally separate. The only connections other than the spark plug wires are the ground wire and the wire that goes to the ignition switch that grounds it when the ignition switch is off.

So I am at a loss. Does anyone have an idea of what could cause this? What could be the common element that makes both of the magnetrons to partially fail when hot?

Thanks.

[Scott Todd]

It could be a bad batch pf plugs. You could swap them or try new ones. I just bought some at NAPA for $2.99 each.

Hate to say it but did you ever verify the cylinder numbers to sensors? If you made a simple mistake a single bad coil could be the culprit.

[pilotyoung]

I recently replaced the EGT sensors so that is a good point. I will check the wiring.

[Onex107]

The secondary is fired by the 12v coils, bottom cylinders. #1 and 2 by one and #3 and 4 by the other. The CHT indicates they are not firing. If your coils are new I'd suspect something else. I had a coil failure on #1 and two and the socket of the coil was corroded and I think the failure was caused by heat at the fire wall. At about 300 hours. I have since added two NACA ducts to add cooling air behind the engine.

[pilotyoung]

Scott,

You were right. I finally checked the EGT sensor wiring to the RDAC today and 3 and 4 were reversed. So cylinders 1 & 3 are the EGT's going to zero when I turn off the secondary ignition and so the top magnatron is probably bad. I am going to order a new magnatron tomorrow.