RE: Engine name
RCGuy:
I have the catalogue (that's where I got the picture of the two versions of the 61) and parts listings from the German HB factory, There is no listing for a 0.75 size engine, and RJL still lists nothing in an HB engine larger than 0.61 size.
So, if your engine is a 75 and is an HB, it must be a prototype that never made production, or so few were made that it wasn't listed in the products catalogue.
Does the front of the carb hacve the HB logo?
Bill.