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#375359 - 05/26/11 2:37 am Amal pre-monobloc question  
Joined: Oct 2010
Posts: 42
Nark Offline
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Nark  Offline
BritBike Forum member

Joined: Oct 2010
Posts: 42
Boston
I've been comparing parts in an 89 and a 276, and don't understand some differences.

The 89, which is 1 1/8" bore, has a longer needle jet and needle than the 276, which is only 1" bore.

Is the needle & jet longer because the carb is bigger so the slide has to raise farther, or do I have some weird parts?

Also, the needle jet isn't marked. Were they all the same size back in the 30's? The application is Matchless MX4, and the manuals I have don't mention what the needle jet (or main jet) should be.

I checked the needle jet from my '55 Triumph, and it is marked, but the net in a 276/012 (not sure what it's from) isn't marked either.

Also, how do I check the float height? It's not adjustable, but I've got a dual float bowl, and the float valve seats hit the float needles at a different spot than the single floats I've got. I don't know if that's a sign that something's wrong or that the float valve is just a different diameter and at a different height.

Petrol is constantly dribbling out of the emulsion air holes on the side of the carb body, so I'm not sure if the float height is off, or maybe I'm supposed to have a long mixing chamber nut, or what.

Any clues will be appreciated.

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#375396 - 05/26/11 11:57 am Re: Amal pre-monobloc question [Re: Nark]  
Joined: Sep 2008
Posts: 131
Phil Beresford Offline
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Phil Beresford  Offline
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The needle jet and needle in the 89 carburetter are longer because the carburetter body is bigger than the 276 body. The hexagon section of the 89 needle jet measures 1 3/16" in length. The same measurement for the 276 is 15/16". The needle should be stamped 29 at the top, a 276 needle will be stamped 6.

The majority of pre-Monobloc carburetter needle jets measured .1065" and were not stamped as this was considered the standard size. Any which were a different size to standard were stamped with their size. .1065" is no longer available and is commonly replaced with a .106"

The carburetter for an X4 was a 76/012 which was a 1" carburetter with a 140 Main Jet, 3 Cutaway and a Standard needle jet. When the engine was supplied for a Morgan the carburetter remained the same but was supplied with a smaller 130 Main Jet.

Longer mixing chamber nuts are used where the carburetter is mounted at a down-draught to keep the fuel level correct in relation to the carburetter.

There is no float height given for a pre-Monobloc carburetter, because they are not adjustable. If fuel is leaking from the overflow slot then either the needle valves are faulty, the fuel level is too high, or the jet block washer is defective.

Float valve seats can be dirty or have been damaged by lapping until they recede. In a twin float bowl, the size of the valve may have been opened up for performance, the wider seat causing the needle to rise higher. Bent needles and dirt in the recess in the float chamber lid where the top of the needle runs are also causes of leaks.

If everything is clean and the needles are straight then try testing the float chambers by sealing the outlet from the float chambers, removing the float chamber lids and running fuel in. If the valves seal then a longer Mixing Chamber Union Nut or an extra fibre washer above the float chamber mounting are your best options for stopping the leak. If the valves seal but a leak develops when you replace the lids shorten the top of the needles.

#375723 - 05/27/11 5:26 pm Re: Amal pre-monobloc question [Re: Phil Beresford]  
Joined: Oct 2010
Posts: 42
Nark Offline
BritBike Forum member
Nark  Offline
BritBike Forum member

Joined: Oct 2010
Posts: 42
Boston
Originally Posted By: Phil Beresford
The needle jet and needle in the 89 carburetter are longer because the carburetter body is bigger than the 276 body. The hexagon section of the 89 needle jet measures 1 3/16" in length. The same measurement for the 276 is 15/16". The needle should be stamped 29 at the top, a 276 needle will be stamped 6.

The majority of pre-Monobloc carburetter needle jets measured .1065" and were not stamped as this was considered the standard size. Any which were a different size to standard were stamped with their size. .1065" is no longer available and is commonly replaced with a .106"


Thanks! That makes perfect sense.

Originally Posted By: Phil Beresford

The carburetter for an X4 was a 76/012 which was a 1" carburetter with a 140 Main Jet, 3 Cutaway and a Standard needle jet. When the engine was supplied for a Morgan the carburetter remained the same but was supplied with a smaller 130 Main Jet.


The X4 is a different engine - I think it's an air cooled sidevalve, the MX4 is a water cooled OHV.

The carb I'm using is an 89/011. I have a Morgan application chart around somewhere, it's the correct bore, but I wasn't sure about the needle jet. The carb was supposedly built by an expert, but the needle jet looks worn and the float valves are different. Do you know if there were different float needles that would fit these float bowls?

Originally Posted By: Phil Beresford

Float valve seats can be dirty or have been damaged by lapping until they recede. In a twin float bowl, the size of the valve may have been opened up for performance, the wider seat causing the needle to rise higher. Bent needles and dirt in the recess in the float chamber lid where the top of the needle runs are also causes of leaks.


The twin float bowl is new, but the float valve seats hit the taper on the needle valves noticeably higher up than they do on other remote floats I've got. This doesn't necessarily mean the seats are machined incorrectly, but it does make me wonder.

I'll try making up a jig with a clear tube sticking up so I can read the float height and compare it to other floats. I think I'm just going to be left guessing and making random changes unless I know for sure what's wrong.

Thanks for the info!

#375765 - 05/27/11 8:24 pm Re: Amal pre-monobloc question [Re: Nark]  
Joined: Sep 2008
Posts: 131
Phil Beresford Offline
BritBike Forum member
Phil Beresford  Offline
BritBike Forum member

Joined: Sep 2008
Posts: 131
United kingdom
The 89/011 on a Matchless engine uses a 200 Main Jet, Std Needle Jet and a 4 cutaway.


Moderated by  Alan_nc 


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