Recently while flying at 7,500 at 21 sq at 11 gallons per hr a vibration came into the control column and was noticeable looking out at the cowl, the edges of the oil filler flap were a bit blurred.
I had flown up to an airport at 3,900 ft above sea level my strip is at 30ft, and had not noticed any vibration in the yolk on the way up.
I have had a bit of a problem with a plug at mag check, the left mag seems to drop 180rpm the right only around 80. I was taught to increase revs to 2000rpm and lean until an rpm drop to clean up any fowled plugs, then enrich to normal operating setting.
As I started to descend to a local strip which is at sea level, and in the circuit the problem disappeared. Again on mag check I seemed to have a fowled plug so went through the routine, on takeoff and the trip to my home strip no vibration.
Could a fowled plug cause this vibration to occur? Or could it be something a bit nastier?
Engine is a Lyc IO540

Thanks for any ideas, John

John,
How old are your plugs? Well 3,900 feet is not necessarily high any increase in altitude also will highlight any deteriorating condition you may have in your ignition harness. At least pull the plugs, inspect them, clean them, rotate top to bottom plugs and see how that performs. Also inspect your ignition harness and see what condition it may be in.

While you have your plugs out, check the resistance with an ohm meter. That will help eliminate or identify a bad plug as a problem.

New spec is 800-1,200 ohms, discard any above 5,000.
www.aircraftmagnetoservice.net/id27.html "is it the mag?"

Dogpilot, Plugs are 75hrs old replaced last annual,will check out harness tomorrow I'm about to do a 25 hr oil change and rotate the plugs.

Rockhopper, thanks I'll do that.

Hotrod, maybe mag hope not $$$ had the right one recond last annual or at least the previous owner did as he paid for the last inspection.

But would this cause the vibration experienced or could a problem with the prop be considered?

New plugs have been known to be faulty right out of the package. I'd put the ohmmeter to them anyway, new or not. Lots of people think the less expensive Tempest plugs are better in this regard & also last longer than Champions. "You get what you pay for" is usually but not always true.
I wouldn't think that the prop would develop a vibration out of the blue, unless a spinner screw is missing or something. But who knows?

Bad primer o rings caused some bad vibes on the 0-470 a while back. Easy fix too!