Aloha,
Discussion in another thread got me thinking and checking up on some info I thought I'd already researched properly...
When I set out to mount a Concentric to my M21 I was instructed in a thread here (by Ian I seem to recall but may be wrong) to start out by using the same main jet as for the original carb. Now since I know doodley-squat about Amal carbs prior to the concentric it seems that I was a bit hasty in my research.
I have now found from various sources that the "276" has a 160 main jet and the "376" has a 250 main jet.
Now that is quite the difference when put into a Concentric!
Some indication as to which one I should go by would be by me appreciated.
Best regards,
Simon
Assuming you have more or less the same size Concentric as the original carb the correct jet would be the 250 from the Monobloc (376)...
Monoblocs and Concentrics use the same jets....However, as there is not a direct equivalent for an 1 1/16" carb as originally fitted you may need to adjust the main jet size slightly once you have it running.
Concentrics come in 26mm and 28mm sizes, one being slightly smaller and one being slightly bigger than the original...You might find no alteration is needed, after all, it's not like you ride an M21 around flat out all the time and you are only on the main jet at over 3/4 throttle...Ian
email (option): ian@wright52.plus.com
Thanx Ian!
And wheehoo I have the right jet. It runs ok so I thought so, some sorting out and fine tuning will come at a later stage.
The carb is a 28mm with a slight taper on the distance piece (hence other thread about studs).
I think there actually is a 27mm concentric available, intended for the Trident.
Thanx again for quick response, what would my life be without this forum.
/simon
626 on the A75 / T 150.
They are easy to spot because the idle stop is not drilled as the slides were on a common rail with a single adjuster for all 3.
While there is a lot of bumf around triples they only made quite a small number on a BSA type scale.
hus I could not see a cash strapped AMAL making a one off special for the trident.
email (option): bsansw1@tpg.com.au
The original carbs for a T150 Trident are 27mm Amal Concentrics...Probably just a 626 that had the body machined out a bit further.....Ian
email (option): ian@wright52.plus.com
I was also concerned about the different main jets recommended for the same engine when fitting 276 vs 376 and concentric carbs.
I asked Burlen about this and their explanation was that the 276 main jets are considerably shorter than those for the 376/concentric and so the fuel does not have to be lifted so high.
Not sure if I understand the logic how this relates to the jet size but it’s probably the best answer I’ll get (I have asked elsewhere about this and am non the wiser).
Pete
email (option): petercomley@web.de
I got this a bit wrong, I was working from memory which isn't what it was. I've found the e-mails now. Here's what I was told.
My query:
For a given motorcycle engine, the size of main jet recommended varies quite a bit depending upon which carb is used.
For example
BSA B33
289G/1AT main jet 200
376/10 main jet 260
BSA B31
276AW/1BB main jet 150
376/2 main jet 260
Matchless G80S
89B/1AK main jet 180
389/1 main jet 260
I would have thought that the fuel requirement would be the same at full throttle irrespective of the delivery system. I can’t see that much difference in the design of the fuel delivery method between pre-monobloc, Monobloc and concentrics anyway when the main jet is in play. Have I missed something here? What is the reason for the difference in main jet between these carb types?
I would be most grateful if you could enlighten me.
The Responses:
The difference in jet size is caused by the distance that the Monobloc has to draw up the fuel from the Main Jet.
This is further than the distance for the Standard carburetter and the later Concentric and called for larger jets with lower surface tension on the fuel.
The Monobloc lifts the fuel from the Main Jet further than the distance in the earlier Standard carburetter and the later Concentric and so requires larger Main Jets to be able to lift the same amount of fuel for any given air flow.
Following my post above Burlen added:
When changing from a Monobloc carburetter to the Concentric the Amal advice has always been to reduce the Main Jet size by approximately 20%. This is because the distance between the fuel pick up point and the fuel is greater in the Monobloc
When changing from a 276 carburetter to a Concentric we have found that the best starting point is to match the Main Jet size.
I hope this helps somebody and many thanks to Phil Beresford at Burlen for his kind help and the information.:+1:
email (option): petercomley@web.de