Hi!
I ran one of these several years ago using 10% nitro 20% castor oil. I used a 11x6 prop (11x4 is to small and so is 10x6).
Nipple at the rear is for ventilation of the crankcase.
Timingmarks should be found inside if you remove the front cover.
Power is a little less than the first OS FS .40 from the early eighties.

Regards!
Jan K
Sweden