The .25BB is, perhaps, the most
conventional looking Fox engine. If it were not for the MKX style
carburetor, most would not recognize it as being a Fox at all.
It has an almost European look to it. The .25BB evolved directly
from the .19BB and is, naturally, of similar design. An important
difference however is the redesigned, lower compression head
While the current head button
is acceptable in stock form, buttons in early versions resulted
in engines that were slightly over-compressed. If you have an
earlier .25BB and find the needle settings finicky, or the engine
makes a "frying egg" sound at full throttle, we recommend
adding a .01" shim between the head button and cylinder,
or install the current series button.
Besides a better head button,
later versions have improved porting and more power as well.
We have found the current .25BB friendly to operate and powerful.
Our only complaint with this engine is that it does tend to vibrate
a bit, especially at higher rpm. A good, solid firewall and engine mount is recommended. Staying with
the larger sizes in the recommended prop. range should also help.