Main Motorcycle: Triumph TR7
Join Date: Feb 2012
Location: Norwich, England
Other Motorcycle: T140
Extra Motorcycle: Most of another T140
If you can ship it to England you can expect $3000 on ebay! Check out BSA B25s on ebay uk (there are a lot more BSAs than Triumphs of this design, they are the same bike beneath the cosmetic surface).
On build quality and why you don't see many around, how many CB250 Hondas do you see these days? Hardly any. And they were very reliable. B25s were not! They did suffer big end failure frequently, I'm not sure why: must be some design flaw as the oil capacity was fine (at least as much as a T140). The slightly earlier C15G engine was miles stronger - the only difference was a roller big end and of course the C15 top end with iron barrel and conventional tappet adjusters.
This is just my personal speculation (based on having owned quite a few B25s and a B50 in my youth), but I think there was a serious quality control problem at the Small Heath factory where I believe these engines were made. I've seen several with castings that look like they should have gone straight in the reject bin - blow holes everywhere, clumsy, coarse-edged fins and machined way off, so that the fins on the head overhang those on the barrel by several mm. The B50 engine, in contrast, is extremely strong and very much better made even at a glance. The B50's oil pump is of similar design but made from steel instead of die cast pot metal like the B25s. I don't think it was higher output but I could be wrong.
The 250s can be very nice little bikes, and they can be made to go pretty fast, but without serious modification they won't take thrashing kindly - and what 17 year old with their first 250 didn't engage in constant thrashing!