My ped was running well, relatively speaking, until I did the following (it's a 505/1 A Sachs engine):
Adjusted my decompression/starter lever (the lock nut had fallen off).
Failed at fixing the top end of my decompression/starter cable (it was fraying shaved too much, now I have to pull on the cable with my hands to start the ped).
Wiped down the spark plug with a dry rag (it's black, can't install a new one until my rethreading kit arrives. It's been rethreaded once, but the insert is stuck to the plug).
To keep the engine from immediately dying I made adjustments to my throttle adjuster and idle screw. While it can idle okay, the engine will now die when I try riding faster than a few miles an hour.
None of these things were big deals, I'm just confused why there's been such a drastic change to the performance.