The B44 and B50 are basically built on modified B25 crankcases, which are too small to accommodate a large enough flywheel. That's one reason that they can be difficult to start. Another weak point is the starter gear and ratchet on the end of the mainshaft. I've owned a few of these, and the only one I really enjoyed was a B50 that had extensive head work done to install big valves and a 40 mm Mikuni carb. With the Sifton cam and valve train, it ran very well. It also had a Carillo con rod for reliability. Then I bought an XL600R and sold the BSAs.