I use an NGK BP6HS on my 3800s. You can use as hot as a B4HS. Many people use a B5HS. The Flash Power is not worth looking for. Here are some other good ones for Solex:
Bosch W 175 T1 (aka W8AC)
Denso W16FS, W16FS-U
gap is 0.5 mm or 0.020 "
Make sure the spark plug cable has not been melted,damaged or become brittle from touching the cylinder.