Hi all.
just doing a carby clean for the first time since ownership and need to ask a couple of questions.
My terminology / parts ID may be incorrect, Be Gentle with me.Hoping the pics help.
Background!
Bike is a 1960 ish B31 bitsa, fitted with an Amal 376/58 carb,
main jet looks like 106,
pilot jet is 25.
not found any blockages so far.
None of the butchery is my doing by the way
Q1.Does the main jet holder require any sealer? came off dry but was ridiculously tight.
Q2. should there be a gasket under the filter within the inlet banjo?
TIA
Little nylon strainer under banjo - no fibre washer as the plastic makes the seal.
Jet holder - yes there is a fibre washer. Easy enough to get, may come in a kit with the tiny one on the pilot jet cap. These often split then go missing.
Brass nut on the main jet, no this seals on the taper, and yes these can get really stuck.
Your carb may or may not have an o ring on the face where it makes to the engine. Check the face for flatness, over tightening the two nuts bends the carb quite easily. Flat it with some fine emery on a plate of glass. It looks OK in your first photo.
many thanks for the info, i do have a gasket set from feked, still looking for a place for the paper one and a red fibre one.probably haven't stripped it back enough yet.
I think 106 jet is the needle jet. The main jet should be a 260 for the Monobloc.
UPDATE ON INFO
I have posted wrong info for you guys to work on
Needle jet is 106
main jet is 180
pilot jet is 25
That main jet looks a bit small. I have attached a pic the 1955 carb settings for the B series models. The jetting remained the same over the years. You can see that for a B31 with a 376 Monobloc the main jet should be a 260. If an air filter is fitted it might be a size or two smaller - 250 or 240.
Hi Derek, thanks for the info, i'm getting more confused by the minute, google tells me this.
The Amal 376/58 is a 376 series Monobloc carburettor configured specifically for AJS 30 Twin and Matchless G11 Twin motorcycles, produced between 1955 and 1957.
only took it off to give it a clean and check it out, as bike would not rev after an 8 week lay up.
gasket kit from feked for the 376 went on fine, but have no idea whatsoever regarding jets etc. the needle is marked with a C, again google tells me that is a standard fitting.
with the bike being a bitsa and no history its a big learning curve.
do you think i should throw in a larger main jet anyway to see how it goes?
No air filter fitted, only an alloy trumpet which i cannot get off
Quote from: rustyboots on 17 Jan, 2026, 18:24 Hi Derek, thanks for the info, i'm getting more confused by the minute, google tells me this.
The Amal 376/58 is a 376 series Monobloc carburettor configured specifically for AJS 30 Twin and Matchless G11 Twin motorcycles, produced between 1955 and 1957.
only took it off to give it a clean and check it out, as bike would not rev after an 8 week lay up.
gasket kit from feked for the 376 went on fine, but have no idea whatsoever regarding jets etc. the needle is marked with a C, again google tells me that is a standard fitting.
with the bike being a bitsa and no history its a big learning curve.
do you think i should throw in a larger main jet anyway to see how it goes?
No air filter fitted, only an alloy trumpet which i cannot get off
Of course it depends how you ride it but a slightly too big main jet is safer for your engine than too small, you can adjust size later once run in a bit and you can do a plug chop (full tilt for a mile or so under load).
Quote from: rustyboots on 17 Jan, 2026, 18:24 the needle is marked with a C, again google tells me that is a standard fitting.
The C marked needle is usually the float needle for a copper float on a 276, but your bike has a Monobloc installed?
Quote from: neil1964 on 17 Jan, 2026, 18:31 Quote from: rustyboots on 17 Jan, 2026, 18:24 Hi Derek, thanks for the info, i'm getting more confused by the minute, google tells me this.
The Amal 376/58 is a 376 series Monobloc carburettor configured specifically for AJS 30 Twin and Matchless G11 Twin motorcycles, produced between 1955 and 1957.
only took it off to give it a clean and check it out, as bike would not rev after an 8 week lay up.
gasket kit from feked for the 376 went on fine, but have no idea whatsoever regarding jets etc. the needle is marked with a C, again google tells me that is a standard fitting.
with the bike being a bitsa and no history its a big learning curve.
do you think i should throw in a larger main jet anyway to see how it goes?
No air filter fitted, only an alloy trumpet which i cannot get off
Of course it depends how you ride it but a slightly too big main jet is safer for your engine than too small, you can adjust size later once run in a bit and you can do a plug chop (full tilt for a mile or so under load).
Hi Neil, riding is occasional and gentle tbh,
I dont understand what this means though.
you can do a plug chop (full tilt for a mile or so under load).
A plug chop is a recognised way to check the mixture on the main jet. You get the engine warmed up and find a decent quiet straight road. You then ride at full throttle for a mile or so (more if possible) then cut the engine quickly, do not let it idle. Pull in the clutch and valve lifter and coast to a stop. Then take the plug out and look at it. It should be light straw.
Of course its too hot to hold, its almost impossible cut the engine cleanly but its still a reasonable test. I do still occasionally do plug chops because its a good way to separate the main jet from the needle.
Way back , 60 odd years ago , when AMAL and the British motorcycle industry were at their zenith, plug chops were part of the factory's development , alongside Amal.
They did these chops at the main jet end and then worked their way downwards , checking mixture strength against performance , needle position , then downwards to throttle slide then downwards to pilot.
In the knowledge that at full performance , the carburettor was correct
learning everyday
thanks guys
Quote from: rustyboots on 17 Jan, 2026, 18:24 Hi Derek, thanks for the info, i'm getting more confused by the minute, google tells me this.
The Amal 376/58 is a 376 series Monobloc carburettor configured specifically for AJS 30 Twin and Matchless G11 Twin motorcycles, produced between 1955 and 1957.
only took it off to give it a clean and check it out, as bike would not rev after an 8 week lay up.
gasket kit from feked for the 376 went on fine, but have no idea whatsoever regarding jets etc. the needle is marked with a C, again google tells me that is a standard fitting.
with the bike being a bitsa and no history its a big learning curve.
do you think i should throw in a larger main jet anyway to see how it goes?
No air filter fitted, only an alloy trumpet which i cannot get off
Carbs get swapped around. At some point in the bike's life a previous owner has fitted a carb from some other bike but it sounds like the jets haven't been changed to match the B31. Monoblocs are all pretty much the same except the jets get changed to suit different bikes. The /58 indicates what bikes it was jetted for when it left the factory.
Quote from: Derek996 on 18 Jan, 2026, 10:42 Quote from: rustyboots on 17 Jan, 2026, 18:24 Hi Derek, thanks for the info, i'm getting more confused by the minute, google tells me this.
The Amal 376/58 is a 376 series Monobloc carburettor configured specifically for AJS 30 Twin and Matchless G11 Twin motorcycles, produced between 1955 and 1957.
only took it off to give it a clean and check it out, as bike would not rev after an 8 week lay up.
gasket kit from feked for the 376 went on fine, but have no idea whatsoever regarding jets etc. the needle is marked with a C, again google tells me that is a standard fitting.
with the bike being a bitsa and no history its a big learning curve.
do you think i should throw in a larger main jet anyway to see how it goes?
No air filter fitted, only an alloy trumpet which i cannot get off
Carbs get swapped around. At some point in the bike's life a previous owner has fitted a carb from some other bike but it sounds like the jets haven't been changed to match the B31. Monoblocs are all pretty much the same except the jets get changed to suit different bikes. The /58 indicates what bikes it was jetted for when it left the factory.
cheers Derek, will try a larger jet or two
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
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.
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.
The cut away is in 1/16 increments if you cut of 1/4 you will wreck the slide.
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?
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)
All monoblocs have 3 1/2 slide from the smallest size to the largest. A 3 1/2 side is right for a B31.
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.
I've probably got a used 31/2 slide you can have.
PM me
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.
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. :-[
Quote from: scifi on 19 Jan, 2026, 22:09 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.
A perfect description of the joys of old motorcycles ;D ;D
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.
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.
Do also check the condition of the needle and its needle jet , over time, the needle rattles around in the jet and wears.
As mentioned before , start with a baseline and remove unknowns
I've seen a few needle jets which have worn visibly oval. The bikes (all singles) ran OK-ish but were pigs to start, so a new needle and associated jet is always fitted if the existing carb is being reused.
.Update.
Hi all.
Today i have fitted a new 260 main jet, a new 3.5 slide and a new 30 pilot jet, needle set at position 2.
Previously for starting it would need about 3/4 advance, today it just coughed, spluttered and kicked back quite hard at the same setting.
It did however start with the lever barely pulled back from the top (Fully retarded)? position.
If the work done today is affecting the retard/advance, can someone who knows explain what if anything is going on and do i need to do something else.
Unable to road test at the moment, and cant run for long in the workshop as my remote fuel tank is scuppered .
TIA
If the timing is mechanically the same as before , (ie , you've not changed anything) then the different carburation brought into play with these new bits has had an effect for sure.
The burn rate and mixture are different , once available , a road test will prove things
I would say that's a good sign. With a manual A/R the mag "strong" point is set at fully advanced so as you retard you get weaker spark. More noticeable if the mag is already weak. Before the carb clean mixture was wrong, now it looks like its right and you are getting combustion with more retard. Once you find the sweet spot for the new carb settings it should be easier to start.
Hi,
I would have thought that fully retarded is the starting position so a bit of that sounds OK espcially as it started.
The kicking back at 3/4 advance can be expected.
How much throttle did you use?
Open too much can create a weak mixture which combined with the advanced position will also potentially cause kickback.
It will be interesting to see how it is when you can run it for a bit longer.
Dave.
Cheers guys,
Those comments are extremely encouraging. Thought I was in for a nightmare.
Its many years since I sold my B31, but I seem to remember it started at around 1/2 way on the lever travel, about in line with the bars. This was for a slack wire advance.