Originally posted by culero
I owned one of those, that exact year model, but a Special (peanut tank, upswept pipe). It was a lot of fun to ride, but awful in terms of reliability (taught me BSA stands for "Bastard Stopped Again" 
)
culero
I had to rebuild the transmission...Twice. Replaced all the Whitworth philips head hardware with socket head screws. Made my own gaskets for the tranny side covers... That cured the leaks.
I kept that bike for 9 years, selling it in 1980, I think.
My favorite was the RD400 Yamaha. I bought it from Suzuki of Jacksonville (also a Yamaha dealer) in April of 1979. It was a 1977 leftover. The engine was pulled within a week and went through extensive modification. Over-bored, new pistons, radical porting, racing reed valves, polished exhaust ports, gaskets matched to ports, decked heads, larger smoothbore Mikunis, custom tuned expansion chambers, K&N filters and a competition clutch. Power went from the stock 44 hp to 72 hp. The power curve was steep and violent.
The original tires were replaced with high-end Dunlops. The cast aluminum wheels were powder coated to match the red fuel tank and side covers. Front and rear disks were cross-drilled for cooling and better bite in the wet. a set of clubman bars and a quarter fairing, along with a re-contoured saddle and rear-sets finished the bike.
Performance was simply frightening. I recently had the opportunity to ride a friend's 2006 Suzuki GSXR600. That old Yamaha was much faster, but the Suzuki was easier and far less scary to ride quickly as modern suspensions are vastly better (as are the tires). Still, in a straight line, that RD400 would easily run away from the much heavier Suzuki, once you could get the front wheel down on the pavement....
I sold that little monster some time ago.. I actually refused to sell it a teenager, who came with his father to see it. He simply lacked the experience and judgment to own a bike with a power to weight ratio of well under 4 lb per hp. Definitely not a machine for a low-time rider. Especially with a power curve that transformed the throttle into little more than an on-off switch.
I miss the charm of that old beastie, but I don't miss the absolute terror it generated when powering off an apex and having the front wheel lift at 90 mph.. It would leave one's rectum so tight that you would be crapping ribbon for a week.
My regards,
Widewing
