Serious question, I swapped a Cessna 172's spark plugs with a different brand's recommended gap

I was doing a 100-hour on a 172 last week and used Champion plugs gapped to .018 instead of the usual Tempest at .016. The engine ran rough on the post-maintenance run-up, with a clear drop on the right mag check. I learned the hard way that even a tiny spec change can mess with the ignition timing on these old Continentals. Has anyone else had a mag issue pop up just from switching plug brands?

2 comments

2 Comments

the_felix15d ago

Two thousandths can wreck a mag check.

jessicab3315d ago

Disagree with the idea that two thousandths alone caused it, honestly. @the_felix is right that it matters, but a rough run-up points to a deeper mag timing issue the new gap just exposed. Seen this before where the real problem was worn breaker points or a weak impulse coupling. The plug swap just showed the weakness that was already there.