I thought I'd kick off the new year by doing something incredibly stupid. So I bought yet another project to add to my list of work in progress. I've bought a 1970 Starfire, or at least half of one. I've got about 85% of a rolling chassis, but no engine whatsoever as yet. Which leaves me with the proverbial blank canvas. Probably a blank canvas that's going to be expensive to colour in, but it does give me the opportunity to get an engine that suits me exactly. From a collectors point of view, the building of a bog standard Starfire engine with 1970 dated cases is the route to maximising resale value, but as I intend to use the bike, not sell it, it is not necessarily the route I intend to follow. I've already read through umpteen web pages about alledged cures for the dreaded B25 clutch. I'm starting to wonder if I might do better buying and rebuilding a B44 lump instead. On the other hand I've always fancied the hard revving high compression B25 engine with its raspy exhaust bark. But being realistic they don't have a good reputation for holding together long term. I'm undecided as yet. This is going to be a long term project, it won't get finished for a couple of years at least. I'd be interested in other people's opinions on what sort of engine I should be investing in.
Seriously though, The crank is reasonably robust but the bolts holding the weights on can come loose. The con rod is the real weak point, ideally replace with a Carrillo or similar. The clutch is the usual wobbly BSA/Triumph item, not the best but not a complete disaster The best thing you can do on the primary side is ensure the sprockets are EXACTLY in line. aim for less than 0.5mm
Seriously though, The con rod is the real weak point, ideally replace with a Carrillo or similar.
Yes that is my perception too. But as I so far have no actual hands on experience with the unit singles, everything that I "know" is just hearsay. The B25 conrod is the same part as used in the Triumph T100R Daytona. At the Daytonas 84 BHP/litre apparently this will hold together. At the Starfires 98 BHP/litre the rod becomes the victim of stress fatigue and fails. Usually at maximum RPM. It's hard to believe that a once great company like BSA would sell such a seriously flawed machine at all. When you consider that this was an entry level sports motorcycle it makes matters far worse.
A Carrillo rod maybe if I win the lottery. I think that I read that a B40 crank with it's forged steel rod could be fitted with a little work. Would this solve the problem or just introduce some new ones ?
An interesting read. It sounds like they know what they are doing with those crankcase and crankshaft mods. But my understanding of this engine is that the connecting rod will eventually break anyway if you use all of the available performance. Presumably the combination of a 10:1 compression ratios and 8000 plus RPM is just too much for the forged duralumin rod in the long term, no matter how carefully you look after the lubrication.
Of course there will be plenty of owners who will never throw a rod, because they wouldn't dream of riding the bike like the kids who bought them in the '60s. When I bought my first real(ish) motorcycle, a Kawasaki S1 in 1977, there was just one British motorcycle still being ridden in my local area. A light blue Starfire that seemed to lurk a couple of miles away from my parents house. The word on the street was that this beast was very unreliable but brutally fast when it did run. People who had raced it on their Japanese 250s had lost decisively. I never did get chance to find out if this was true, one day it was leaving a car park just as I turned into it, by the time I had got to the exit, the beast was nowhere to be seen.
In 2016, at the vintage 1/2 mile flattrack races at Mid-Ohio, I watched a B25 beat out three Bultaco Astros handily to take a win in its class. Sorry I don't remember the owner's name but I know he's from Canada. The B25 sounded like a 500 on the track. I asked my engine man in Syracuse (who knew of the bike and owner), what do you do to a B25 to make it run that way? And he said, "Everything".
I'm starting to think that a B44 might be the way to go. It might be no more expensive to build than the 250 and a damn sight more durable.
You have to bear in mind that as a 17 year old, myself and most of my motorcycling friends were still riding 250cc bikes on L plates. It was generally accepted that a good Japanese two stroke 250 could hit 100 mph, but the electronically timed road tests of the day told a different story. About a true 90 mph was about all that they could manage without modification, over optimistic speedometers and youthful exuberance was responsible for the phantom extra 10 mph or more. Our concept of "fast" had to be taken relative to the bikes we owned.
Quoted horsepower figures for the Japanese 250 sports bikes varied around 28 -32 Bhp, but when a contemporary magazine decided to dynometer test them, rear wheel figures were actually in the low 20s. Possibly less than the unsophisticated BSA. The conclusion was that "British horses had big hairy legs "
It was the marginal oil supply plus the use of white metal big end shells that does it in for the rod plus the addition of an oil hole in the rod IMHO. Get hold of an undrilled T100 rod, a set of Al/An shells and the sort out all the potential oil pressure leak points and you are good to go.
I have a B44 and a B25, the B44 is easier to use with power at low revs so no need to stir the box but the B25 is not much slower if you keep the engine on the boil by changing gear a lot.
I also own a Starfire and decided in the fall to completely rebuild the motor. While my on rod seemed to be in fine shape, I’m not taking the risk and am getting a new rod for it. I am also sorting out the oil filter issue, as well as fitting an oil pressure gauge to it, just as some extra insurance. They’re fun bikes, but like a fussy super car, need a lot of attention! I can help if you need anything. Do you have the workshop manual? That’s a good first place to start!
My '70 250 has Ed V's 280 kit and is currently getting his oiling mods and a Carillo rod and a cam. Considering that I am built like a sheet of plywood and am pushing 300 lbs, maybe its performance will approach that of a stock b25!
'71 bikes seem to be fetching good money now. The was a time when they were just considered to be Umberslade freaks and a good source of engine spares for the earier models. When something is unpopular like that most get scrapped, then all of a sudden the remaining bikes become fashionable and start making big money. I've got to admit that I have a sort of perverse attraction to the idea of a highly tuned 250 rather than the big lazy 441. I'm just wondering how much of a months salary it would take to pay for a Carrillo rod ?
Last edited by ferretjuggler; 01/10/185:22 am. Reason: My poor spelling
When i had my star fire tuned i had the ports opened and i think a 30mm Concentric carb fitted, a 10/5 piston, BSA Spitfire rev and speedo, swept back pipe, standard 68 silencer and rear sets, so it was mildly tune and believe it or not, the conrod held together unlike my friend's B25 SS which snapped a rod. My star fire was gearing was slightly raised and at 9,000 in the gears would do 48, 68, and 90 in third. Part exchanged it for the first ever Trifield made by a dealership called Gander and Gray, Romford,42 BHP weighing 315 pounds 110 top speed