Rough left Mag

[Tan]

Before my mechanic can come check. Thoughts on a slightly rough left mag. Man drop on both mags is 70. I already got 8 new plugs to rule out bad plugs. EGTs rise all on 4 cylinders both mags.

 

[Doc Holliday]

Rap on them a few times. Seems to fix electric fuel pumps according to some.

 

[RyanB]

Not enough information. Could be a host of things and can still very well be a plug.

 

[Jeff767]

How much time on the mag since the last rebuild?

 

[Magman]

Post - Flight Mag check?

 

[Tan]

Not enough information. Could be a host of things and can still very well be a plug.

How much time on the mag since the last rebuild?

Mags sent out for inspection 200 hours ago. They both drop 70 rpm

 

[Tan]

Post - Flight Mag check?

Haven’t done a inflight check before but egts all rise on each mag. Only slightly rough on left with 70 rpm drop.

 

[Chip Sylverne]

Do you lean for ground ops? Have you done a lean "plug cleaning" runup before the mag check?

 

[EdFred]

What are the cht's doing? Are they all dropping about the same if you leave it run for a minute or is one dropping more?

 

[chemgeek]

Lots of causes, in order of frequency for my airplane:

1. Fouled plug (oil or lead; oil often burns off with a lean runup, lead usually won't) - this is probably 90% of rough runups.
2. Bad plug
3. Bad/corroded lead spring connector
4. Bad coil in mag
5. Carbon tracking in mag
6. Bad capacitor in mag

Mechanic will need to troubleshoot.

 

[Domenick]

Check the plug harness.