I own a Mazda MX5 mk2 1.6L.
I had severe problems with engine misfiring, caused by the valve cover seal, to leak. The oil that was leaking was concentrated near the spark plugs, and periodically it grounded them.
First I had the seal changed. The leak stopped, and the repair was a success. However, due to everything being covered by the oil, the misfires continued, which I considered normal.
Both the spark plugs and the plug wires, were in a mess.
Today I changed the spark plugs, to new NGK ZFR5F-11, with the gap set to 1.1mm, and the plug wires.
The engine now run very smoothly with no sign of misfiring. However I noticed a SEVERE loss of power, both at low and high RPM.
Before the plug change, sometime the engine worked without misfiring and was delivering the proper power, so I can compare before and after in a few hours of time difference between the test drives. This leads me to be heavily biased to blaming the spark plug change.
What may I have done wrong?
(1.1mm is what Mazda recommends, ZFR5F-11 are considered a very good option for this car, they are long-reach, as were the previous ones, and firing order is definitely set correctly.)