Spark Plugs

[Rich]

A couple of days ago I noticed that the engine was somewhat rough at lower power settings. After landing I did a runup and both mags exhibited some roughness. Many recent flights we had spent considerable fractions of the flights at very low power settings (below any documented power settings) doing instrument approaches. I figured that the plugs had started to carbon up and decided to pull and clean the (Tempest fine-wire) plugs.

Sure enough, a fair amount of carbon on all, but #1 lower was particularly grungy. This plug is powered by the Surefly and together with #2 upper, fires every revolution. As a matter of course I also checked the resistance of each plug. All were 850-950 Ω, well below concern except #1 lower, which reads as open circuit. I can't get a resistance reading on it. All plugs were fine when insalled, but this one is now out of spec. Fortunately I still have an old unused Unison Fine-wire that tests as 1250 Ω, which is still within spec, so I put that in its place.

Moral: It's worth retesting the resistance any time your plugs are removed.

[mhoran]

Interesting. I got my plane back from annual at the beginning of June. It ran great on the way back but after a couple flights I found ground run-up to be pretty rough. In flight mag checks are fine, so I've just lived with it for now. Engine is going in for overhaul in August and I'm having fine wire plugs installed. Despite 8 years of ownership we didn't have a spark plug socket, so I just got one and I'll check the bottom plug. But since everything is getting swapped in a few months I probably won't replace anything. Just found it odd that the ground run-up shows issues but things are fine in flight (even if I switch to the rough mag in flight.)