BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: Greenfield on 01 October, 2017, 09:40:27

Title: b40 amal carb
Post by: Greenfield on 01 October, 2017, 09:40:27
Checking over my recent b40 (1963) purchase I notice that the carb is an amal 376/231. According to Ratio it should be 376/253. I've managed to start it but its as rough as hell. Can anybody suggest whether this carb will work or do I need to change it. I don't want to start tinkering with settings if its a no goer. Thanks in anticipation.
Title: Re: b40 amal carb
Post by: JulianS on 01 October, 2017, 10:12:20
The 376/231 is a 1 inch bore carb (376/253 1 1/16 bore) originally fitted to a Francis Barnett Falcon 87 200cc 2 stroke. The 2 stroke carbs usually have a different spray tube to the 4 stroke ones - look through the bore you see a brass tube protruding rom below. 4 stroke usually cut straight across the top and 2 stroke usually cut at an angle.

If yours is like that it may explain rough running.
Title: Re: b40 amal carb
Post by: Greenfield on 01 October, 2017, 13:39:25
Having removed the carb, lifting off the cap with throttle slide, there is a brass tube poking out of the jet block into which the needle goes. this brass tube is flat on top. Is the difference between the carb I have and what should be, a matter of internal parts which can be changed, or do I have to replace the entire unit?
Title: Re: b40 amal carb
Post by: JulianS on 01 October, 2017, 13:50:05
If carb is good condition - especially fit of slide in body- with the right jets and slide you will probably be able to adjust it to deal with the smaller bore.
Title: Re: b40 amal carb
Post by: Greenfield on 01 October, 2017, 16:09:45
Many thanks for that. I now have a pile of bits on the bench, and looking at the parts I appear to have a mix of carb parts. Surprise surprise!
 Do you know how to tell the difference between a 1" and 1 1/16" choke carb? ...and yes the throttle slide is worn.
Title: Re: b40 amal carb
Post by: JulianS on 01 October, 2017, 16:45:32
Just measure the bore - same basic 376 body came with different choke sizes.
Title: Re: b40 amal carb
Post by: Greenfield on 02 October, 2017, 09:19:31
Yes 1". There seems to be a separate part, stamped '2' on mine possibly pressed into the body, which would make sense if they're producing a range of choke sizes. Someone may have tried to remove this in the past. I think my best bet is to start from scratch with a new carb, rather than refurbing the current one which will never perform as designed. Thanks again for your help.
Title: Re: b40 amal carb
Post by: Pauldemob on 26 January, 2018, 14:13:10
If you are buying a new carb direct from amal, take care , it took 4 months to get mine. as it turned out they did not have one made up and no orders, so kept putting me off. Terrible customer service. They even charged me postage on a cable adjuster, which they forgot to include in original order. Then it had to go back as bike ran rough, like yours . Turns out they had fitted 2 stroke internals. Mines a 1967 B40, all sorted now and runs lovely.
Best service is from surrey cycles  01342 716 120.
If I had known at the time could have had one next day, lovely people