BSA Owners' Club Workshop Discussions

The BSA Workshop => Twins => Topic started by: BUNNERS on 26 Apr, 2026, 14:29

Title: A7SS choke/carb issues
Post by: BUNNERS on 26 Apr, 2026, 14:29
Afternoon All
I have an issue with my 1961 A7SS. End of last seasons but the old bike away in workshop, it had run very well all season. took it out of hibernation today and it will only run on full choke. choke off and it stops immediately. My son took it for a quick spin of about 5 miles to get it nice and warm with choke on and ok. turn choke off and it stops. any ideas would be most welcome.
Bunners
Title: Re: A7SS choke/carb issues
Post by: limeyrob on 26 Apr, 2026, 15:08
Clean top hat filter, Blow (gently) up fuel lines to clear tank gauze (if they are still there). If its a monobloc remove and clean the idle jet, remove the hex nut and clean the main jet.
If its a concentric then odds on its the idle circuit which is a pig to get clean.  The bio-ethanol seems to create white power that blocks concentric idle circuits solid.
Try getting a can of carb cleaner with the tube on and blow though the idle circuit.
Title: Re: A7SS choke/carb issues
Post by: BUNNERS on 27 Apr, 2026, 09:12
Good Morning Limeyrob
Thanks for your reply
Late yesterday afternoon I replaced all of the jets with brand new

376/239 1" carb
Main jet   270
Pilot jet  30
needle jet 106

I got these details from the Amal Reference catalogue
But alas the problem still exists
Bunners

Title: Re: A7SS choke/carb issues
Post by: V500 on 27 Apr, 2026, 09:43
You need to clean the passages as well as the jets though.
Air line, High-E string, whatever it takes.
Title: Re: A7SS choke/carb issues
Post by: BUNNERS on 27 Apr, 2026, 10:06
Morning V500

My intention this morning is to put the carb, suitably striped down, into my Ultra sonic cleaner and see if that makes any difference.

Bunners
Title: Re: A7SS choke/carb issues
Post by: V500 on 27 Apr, 2026, 10:45
Worth a go, but no substitute for blowing and rodding, in my experience.
Title: Re: A7SS choke/carb issues
Post by: limeyrob on 27 Apr, 2026, 11:02
The bio-ethanol petrol dries to a rock hard white clag that even carb cleaner struggles with.  At least its a Monobloc which you can rod, the Mk1 Concentrics are near impossible.
Title: Re: A7SS choke/carb issues
Post by: BUNNERS on 06 May, 2026, 10:05
Ultra sonic cleaner did the trick now running as sweet as a nut
Title: Re: A7SS choke/carb issues
Post by: limeyrob on 06 May, 2026, 11:30
I now run my carbs dry after every run.  I'm not sure it will stop the idle circuit blocking but its worth a try.  I don't remember these problems before ethanol in petrol.