BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: chaz on 14 May, 2022, 13:28:56

Title: B40 choke or not?
Post by: chaz on 14 May, 2022, 13:28:56
just wondering...
the 376 monoblock lets the bike run and got the tickover stable, but opening the throttle, it misses and pops.
Ive rebuilt the carb, with new cable, lever and slide.
Reading the Amal catalogue it says no air pipe, checking the parts list there is no air pipe listed. so Ive removed cable and all inc the slide. it now picks up ok. BTW, I had tried altering stator plate but made no difference. better with choke wide open. hence removal.

anyone running a choke
Title: Re: B40 choke or not?
Post by: Mike40M on 15 May, 2022, 06:10:54
When I bought the B40WD, it had a 376 Monobloc from a Triumph T110 and a choke slide that could not open fully. Going rich.  Removed the choke and plugged the top. No carb problem after the choke removal.
Title: Re: B40 choke or not?
Post by: Spaceman on 15 May, 2022, 06:36:42
It's hard to see why removing the choke should make any difference to the way the carb works. As Mike40M found, the only conceivable problem is if the choke slide does not retract fully and continues to close off part of the carb inlet causing the bike to run rich. Otherwise, it should play no part in how the carb operates.

The B40 came fitted with a choke or air slide and its purpose was simply to make starting a bit easier under certain conditions. As far as I can see, it is also listed on the AMAL web site for a B40 376 Monobloc.
Title: Re: B40 choke or not?
Post by: chaz on 15 May, 2022, 08:47:13
Ive got an Amal dealers catalogue and it says no air tube. as said  in original post exploded drawing has no mention of air tube so I suspect its the choke which is why Im asking.
with the choke in place , down,it is restricting air flow, when opened in up position the bike runs better.

when the bike arrived it had a rusted throttle cable and top from a 376 but no body or slides so dont know what it was running choke wise but as it had brazed bob weights for flat tracking I guess it had been jetted up. no air filter.
Title: Re: B40 choke or not?
Post by: DAVE BRADY on 15 May, 2022, 10:16:24

I have had two B40s with Amal 376 fitted with choke and neither really needed them.  A good tickle of the carb was enough.  If the choke is removed then ensure that the hole in the carb cap is sealed to prevent leaks.  I am not sure how much difference the air filter will make as it was only a rather open gauze so not much affect on carburation.  The tube connecting the carb to the filter will have some affect as it lengthens the inlet track by a couple of inches and gives a smoother delivery of air to the carb intake as opposed the abruptness of a carb with no tube or bellmouth fitted.
I found that the decompressor was pretty much superfluous for such a low compression engine but a longer kick start lever would have been beneficial.

Title: Re: B40 choke or not?
Post by: chaz on 15 May, 2022, 16:16:21
thanks Dave,
Ive got a hunt on for a decomp cable as cant get one the right length. Ive got a plug over the adjuster so sealed.
Ive not got an air filter or the tube to the air box. the tubes shown are straight but I cant get the tin wear up high enough to get a straight line, presumably as a former flat track bike it ran open carb or had a gauze over.
Title: Re: B40 choke or not?
Post by: AdrianS on 16 May, 2022, 14:58:39
I donít think the decompressor is not needed. It does make the bike a lot easier to start and kick over by kicking the engine just over tdc using the decompressor and then allowing the engine to turn over twice before firing. It also helps protect the kickstart mechanism which is a bit of a weakness on these engines.
I have also found the choke not really necessary but useful on very cold days. I have got an original air filter and connecting pipe and this set up works well.