B31 carburetor clean

[rhyatt]

Dont forget to check the throttle slide
It's marked at the top face 3and1/2 maybe
See what you have and if correct for your bike.
These slides can wear over time and rattle back and forth.
This slide affects from 1/8th throttle movement to about 1/4 but all the time the needles position in its clip notch will be metering fuel from around 1/4 to 3/4 throttle.
All of these settings blend into each other of course

[rustyboots]

Hi.
so this appears to be 2 1/2, too small according to Mr google,i don't have any spares to play with, so will have to see how it goes.

[limeyrob]

That doesn't look right, hardly ever see a 2 1/2.  I seem to recall its the hight of the cut-away in 1/8" so you could saw 1/4" off the angled part and turn it into a 3 1/2.
Larger numbers are weaker, so is it rich just off idle up to about 1/4 throttle?
Stand it on a flat surface and see what the hight of the cutaway is just in case someone before you has taken file to it.  A lot of tinkering and fettling can take place in 60 years.

[idie]

The cut away is in 1/16 increments if you cut of 1/4 you will wreck the slide.

[limeyrob]

Thanks for correction, so 1/8" would have to come off the angled part if its not been "adjusted" in the past.
I know this is all set out somewhere but I couldn't find it, do you have a ref?

[rhyatt]

Check your bsa book or failing that , ring Burlen in Salisbury.
Verify what slide you need 1st

https://amalcarb.co.uk/carbspec/carburettor/spares/id/4662/

not sure if this is correct for you bike , but main jet 260 and 3 and 1/2 throttle slide and needle clip in 2nd notch

somewhere in my head, the variation between 1 cutaway and the next was 1/64th" - if your is a 2.5 and you need a 3.5 then 1/32nd" increase in cutaway

just found this - unsure if accurate ---
"slides are listed from 2 to 5 in 1/16″ increments. The #2 cutaway would be 1/8″. A #3 cutaway would be 3/16″.

#2      =   1/8″     (3.175mm)
#2.5   =   5/32″    (3.96875mm
#3      =   3/16″    (4.7625mm)
#3.5   =   7/32″    (5.55625mm)
#4      =   1/4″     (6.35mm)
#4.5   =   9/32"    (7.14mm)
#5      =   5/16"    (7.9375mm)

[idie]

All monoblocs have 3 1/2 slide from the smallest size to the largest. A 3 1/2 side is right for a B31.

[rustyboots]

Huge thanks to all for the invaluable information ,links etc. Never thought a carby clean would become so intense.
Looks like its going to cost more than just a gasket set
I will keep you posted as to what the outcome turns out to be.

[Stubaker58]

I've probably got a used 31/2 slide you can have.
PM me

[scifi]

I had a bike that cut out if the throttle was opened too quickly, which I determined to be TOO MUCH throttle cut-away.   So I took 1/16 inch from the bottom flat face of the slide.   It cured the problem, and the pinking which occurred.   

Most Amal slides wear at the back edge, so maybe this metal removed from the flat bottom removed some of the worn part as well.

It is well worth using your files, rather than paying extortionate prices for new parts.

[limeyrob]

I too have taken a file to a carb slide, but its a fine line between a successful adjustment and turning a dining table into a coffee table in an attempt to stop it wobbling.

[V500]

It is well worth using your files, rather than paying extortionate prices for new parts.

Yes and no, after all the slide could be worn enough that it's never going to work satisfactorily ever again. Then you have the prolonged process of filing, rebuilding and replacing the carb, taking the bike for a run, stripping the carb, filing, and rinse and repeat.

[limeyrob]

A perfect description of the joys of old motorcycles  

[rustyboots]

Hi all,
Update on slide,

looks to me that this slide has been got at in a previous life, my best effort to measure is coming out at 5.86 height. I think, using the dims kindly provided by rhyatt,and my rubbish maths i make nearer a 3.5 than a 2.5 as stamped on the slide.

[V500]

As the component parts of carbs are calibrated so accurately from new, if someone's been fannying-around with the slide I would replace with new.
A reliable base-line is needed if the carb is ever going to be made to perform as it should.