The BSA Workshop > Singles

b40 amal carb

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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.

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.

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?

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.

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.


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