In my case it was carry out the normal checks and divert to the nearest a/d. I was en route Popham to Henstridge and talking to London Inf so I told them what was happening and that I was diverting to Compton Abbas. They asked if I wished to declare an emergency, which I declined. CA was in sight so I switched to them explained the problem and they cleared me straight in for an uneventful landing.
The problem was a drop in RPM and an increase in vibration. Drained the gascolator to check for water and/or debris. Unable to reproduce the problem on the ground despite extensive checks so eventually took off without incident for return to Popham.
About 5 mins after departure it happened again. I established that a slight power reduction caused it to go away and opening it wide caused it to reappear after a minute or two. With Bowerchalke, Old Sarum, Middle Wallop and Chilbolton all along my track plus plenty of fields it wasn't a crisis.
After about 3 days and lots of running the engineers ran it to ground as a valve guide breaking up. Most of the time the valve seated fine and compressions were normal but clearly at high power settings it was vibrating out of alignment, the valve wasn't seating properly and she was only running on 3 cylinders.
Your question doesn't give enough information for a sensible answer. Running rough? The Beagle Pup I used to fly did that. The answer was simply to turn it the right way up again, whereupon the problem went away.
