The .19BB was introduced in the early 1980's and represented a significant departure from the previous .19 bushing design. Besides being schnuerle ported, it incorporated a separate cylinder casting that could be rotated to locate the exhaust in any direction.
In the late 80's a .25 version was also introduced and, for a few years, both were available. The .19BB was eventually discontinued but the .25BB remains in the Fox lineup.
The 19BB was powerful, but also a bit over-compressed, and tended to run
hot. It was common for varnish to quickly form on the piston.
A .01" shim between the head button and cylinder should
help. There were also some mid-range carburetor problems caused
by the design of the low speed needle. We recommend installing
the low speed needle from the later .25BB engine, which seems
to have a more suitable taper design. The .19BB is no longer
produced, but "some" parts are still available.