Doesn't sound like its running well at all. Maybe not on all cyls. Maybe just an exhaust leak, The diaphram doesn't affect start/idle, only higher rpms. After it runs for a few minutes, splash a few drops of water on each header. If one or more doesn't evaporate really quickly, its a non-firing cylinder. The flame I saw out of your pipe makes me think that might be your problem too.

Check your plugs and make sure you have spark to all 4 cyls. Easy way to do this is when its idlng, pull off a spark plug cap. If the rpms drop, its a firing cylinder. If its no effect, then you know it wasn't firing.