Hi,
I recently purchased a F2 engine - since purchasing I have had problems with not being able to achieve 1J of FPS, this is while using 120psi, 99dP too.
What I have tried to doing to resolve - change inner barrel (almost doubled the length, is now 250mm), got a different hopup rubber (unrelated), fixed my regulator (separate issue), corrected alignment of the engine, tried using closed bolt mode, tried a different hopup unit/inner barrel combo from a different gun I own (it had an even longer inner barrel but made things worse).
This is incredibly frustrating as I have owned this for about 2-3 weeks by now and haven't been able to field it once. Hope someone can help!