1951 B31 Plunger BSA Carburetter

[Foghorn]

Hello all,
My B31 is now restored and running.  But the carby is rather worn in the slide tunnel, which may not be the only fault with it.  The symptoms are that, though the bike starts first kick and runs nicely, after some 2 miles or so the fully re-built engine loses power, farts a little and then just sighs to a halt.  Judicious tweaking of the throttle may keep it going but without substance and it dies.  It is an AMAL 275 separate float chamber model.  I have not been able to find a new replacement for it.  I wondered if someone can tell me if there is a later AMAL model carby which could be pressed into service in lieu.  An AMAL monoblock would be satisfactory, but I do not want to spend money on what will really be a trial without some re-assurance that it will essentially be the same carburooster.
Regards, Foghorn.

[JFerg]

Sounds like a typical carb fault that will be traced to the magneto.  There's an open circuit in the HT winding that has generated a carbon track.  When cold, the resistance is low, but once it warms up, the resistance increases with heat until the spark weakens and the engine dies.  By the time you have pushed it home, it has cooled off again.....

[R]

It does sound entirely like an ailing magneto....
You are having fun with this bike....

[Rex]

+1 for the mag, but an Amal Mono should be a direct replacement, as should a later Concentric. Hard to see how the present carb could be at fault with those symptoms though.

[chaterlea25]

Hi,
A new 276 is available
http://amalcarb.co.uk/carbspec/carburettor/spares/id/4658/

Did you have the mag rebuilt?

John

[JFerg]

I bought a new carb from Burlen for my Panther.  Cannot praise them enough.  Excellent service, excellent communication, and the carb was perfectly set when it arrived.  Bolted it on; idle perfect, mixture perfect.  Really impressed.
JFerg

[Foghorn]

Yeah!  The magneto was totally rebuilt.  One assumes the job was done well and at this time I have no reason (well, not until I opened today's forum replies) to think otherwise, but I will pursue the thought.  The magneto and the bike has only 15 miles on it since rebuild and most of those were put on by the bloke who did the roadworthy certificate.
Yes R, I had 3 years of fun- now the fun starts.  I will see if I can attach a pikky.
Burlen, JFerg, for a mono or concentric as suggested by Rex.  Thank you all.
Initially at the first ride, the bike seemed to work without stopping- [it actually started FIRST kick].  There was just a faltering, an inconsistency of operation which made me look at the carby.  It was after the replacement of the throttle slide and the required honing of the tunnel mouth to fit same (see my reply post to another member) that the symptoms altered for the worse.  It seems that the worn slide worked better than the new one.  Now with the old slide back it place perhaps it may be a little over-honed, but a new carby is sounding better and netter.
Foghorn.

[R]

Have you tried it with a new spark plug in it ?
Its just possible the plug may be overheating, with these carby ? problems.

If a new plug gives an instant improvement, if only for a while,
we are still undecided about a weak maggie or a weak mixture.
Or maybe both...

Smart looking bike.
4 wheeler behind looks pretty nice too.

[rgsjohn]

Something else to check. Tappet clearences a bit on the tight side perhaps? Once engine has warmed up the gap could be closing up.

[Foghorn]

I like the suggestion about the tappet clearances- haven't checked them.  Spark plug too.
The car behind is a 1939 Flying Standard 12.  Must finish it off too!  The rear gearbox seal seems to be non-existent.
How does one test a magneto apart from option one?  [Option one- hold the spark plug lead while your 'friend' kicks the engine over].
Foghorn.

[Rex]

That only works if you can differentiate one yell of "kinell" from another as good or bad, they'll both hurt!
Run the bike until the engine starts to play up, then whip the plug out and have a look. When that's done, stick another plug in for the ride home.

[R]

And when the new plug doesn't make any difference and won't get you home,
you KNOW you have a magneto problem !

The local magneto guru here has a small oven, that he tests maggies in for heat-related problems.
If it won't spark for an hour when its good and hot, its junk....
The test for aircraft magnetos is a lot tougher...