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Wrestling with the T10TT carb...


I've been unsuccessfully attempting to eliminate the probably not quite 2 drips per second petrol leak from my carburettor.  The float bowl is positioned level.  The factory metal float has no leaks.

As i understand it, there is no float level setting; only the single notch in the float needle the clip positions itself around.  Is my understanding correct?

I have determined petrol leaks (and no place else) through these holes, in particular the one hole just below and behind the pilot needle.  Logic suggests the cause of the leak is that the float valve, i.e., the float needle is not making a perfect fit in the seat and therefore petrol leaks past this float valve.

It would seem my next step is to bench test the float chamber to rule out petrol is not passing through the float valve when the float chamber is filled with petrol. 

Is it worth a try to move the float bowl one direction or another to see if a change in the float position might make the leak stop?  If after trying/troubleshooting what I have mentioned, is it reasonable to consider using a modern plastic float which might have more buoyancy that the factory float?

Will appreciate hearing your thoughts.

What is the function of the 3 holes near the base of the carburettor?

Thanks in advance and kind regards.


I had a similar dilemma  with a Ducati, struggle on with a TT carb  or fit the  concentric that came in a box  with the bike.   Half the visual appeal of the bike was the  orriginal  exotic  carb and  erratic but orriginal  chronometric Veglia  competition rev counter.  I'm glad I stuck with it , although the type of usage  plays a big part in the decision. I steer well clear of traffic and mostly use the bike in track events .

Hi Steve 

specs as below

310 mains, n/jet 106, pilot 30(bush removed)

slide 3

And yes i have probably a 1/4 “ fibre spacer between carb and head 

the premier version of the concentric  wasn’t around when I did this but recently fitted one to my ‘68 Atlas that was converted to single carb use and has performed perfectly so could be the way to go

all the best



Here's info I gathered from others on this forum. I set up the Monobloc just so and found that it starts and runs great. Still running it in so haven't run it wide open. There is a 1/16" insulator in there that you can see. I found that the upper edge of the air cleaner fouled the right rear tank bolt so I put a short stud into the tank there to locate the rubber pad. I prefer the looks and function of the Monobloc.  

As for leaks...this photo was taken before the leaks had really had a chance to develop. Major leaks via the telltale and the vertical shaft tube. Cambox not too bad due to use of Ian Bennett's roller rockers and restricted oil delivery.

Jetting for International Model 30:

389 Amal Monobloc  

1 5/32"

Main jet 340

Idle jet 30

Needle jet 106

Needle D middle notch

Throttle slide 3


930 Concentric

310 mains

needle jet 106

pilot 30(bush removed)

slide 3

...for that information on the rwo carb types.  You mentioned you needed to bore the Atlas manifold larger, was that from 1-5/32" to 1-7/32" ?

Another first in my 69 years...  i am oddly reassured hearing your engine leaks are "major."  fwiw, i was able to nearly stop the Tell Tale leak fitting a 3/32" thick leather washer atop the packing.  there is a very slight weep whereas prior to addition of the leather washer i had this (:

Thanks David for the picture, especially considering you removed the lines to get a better pic, i sure don't want you to mess further with a perfectly working, non-leaking set up!  Attached are pics of my float attached to carb in forward, middle and rear positions.  i tried to take pics as horizontal as possible.

Thanks again and kind regards.

...  for my clarification, what is "cranked transfer tube?" what are you referring to, "mine is straight?"   i will post pictures in response to your clarification.  And/or if you can post pics of the "transfer tube..."

Please note my reply to David, with pics of float attached to carb in forward, middle and rearward positions.

Also, please note i have pictures of front and rear views of float off carb.  One can see the numeral "15" at bowl's bottom.  interestingly not the "0 / 7 / 15" seen on Paul Norman's website.

Thank you and kind regards.


... give up when i can't find the answer for the leaking...  It would be one thing if i knew the cause of the leak and it could not be remediated for an understandable reason. 

To simply give up "because it leaks," going with a different carb just leaves the 15' float and/or TT carb in a box with the unanswered question of "Why?" gnawing at me...

I am more than happy to post any pictures anyone requests.



Then don't give up !!.   I ran my  250  for 4 years  including  four 10 day  IOM riding  holidays and  many parade  rides  before  I  finally nailed the  difficult  running issues.  Now it performs like crazy  but  oddly  I find myself riding   something  else that needs sorting !,   how perverse  can we be ?.

A small circle is engaved on the air jet plug ( brass plug in the right hand side at the bottom of the air slide )

The bottom of this circle is the correct fuel level

I hope this is of some help John

i hope to goodness you did not take your carb off your bike!!!  it doesn't look like you did, it appears you have placed paper as a background to better show only the float bowl's position in relation to carb.  Thank you for all you are doing to help me!

I'll follow back in the next day or two.  I'm pretty busy this summer through July 24, busier than i want to be at my age, so don't get to devote as much time to my bikes and joy rides as i would like.  Winding down to end work responsibilities with one employer, so then i'll only have a 3 day per week commitment with the other employer for the remainder of my work life...

David, thank you again!



You fine fellows are amazing in your generosity and willingness to help with this colonial cousin slog through the petrol in my attempts to "stop the leak."

Below follows in its entirety the reply i received from Paul Norman this morning.  I am posting it with the intent it may be helps to anyone else.

"Ahh, the joys of TT carbs! Great when working, but often prone to issues.

I have to say, your issue is unusual, in that if you are using a fixed float chamber (of which more in a moment), there is less adjustment and less to get wrong . . . i.e. the float attaches to the main carb by the fixed casting, and therefore it is theoretically impossible to get the height setting wrong – unlike remote chambers.

A couple of points worth checking first:

  • Top of float needle:  does it have free and full movement in the screw on locator at the top of the float chamber (might be worth sending me a photo, but assume you are using normal TT float chamber with screw in top . . . as opposed to later Manx type larger chamber, held on with hex screws . ..  which have different float stop, which are more prone to leaking when worn).  I ask this, as it sounds like when you are testing, you do not have the top on (so you can see), but does it leak in the same test, once top lid is fully screwed down?
  • Again, in your test, when you are saying it is not leaking (which is good), have you done this with the screw in top affixed  . . . if so, try it with the screw in top taken out .  . . and assumedly it will still not leak .  . . but then try and touch the float needle at the top and move it fractionally sideways in all directions . . . do you get a leak in any direction?
  • I know it sounds obvious . . . but tickler .. . is it touching top of float?  If just touching it can hold float down

With both of these, what you are trying to ascertain is – is the seating between float needle and float chamber needle taper marginal . . . in past experience this has always been the most likely cause of issue . . . sometimes just worn needle or female face, sometimes because needle slightly bent (so taper is not totally parallel to shaft, and when top put on it leaks), or just assembled from different spares.

Worth just checking all of above first.  If seat has got worn over the years .. . then a float new needle is worth trying.

You mention you have a float with 15 written underneath . . . if you have it on a Norton downdraught head – then that is correct, they should have a 15 degree downdraught (7 I think is Velo normally, and 0 is any head that has horizontal inlet tract).

With the float in relation to emulsion tube .. . I have always worked on theory that if you have a matched fixed floatchamber to a TT body . . . and all in reasonable nick and you can see float is right angle (i.e. vertical when bike is off its stand) .. . then you have to assume float to emulsion tube is correct angle . .. and it is something else causing it to leak.

However, if it helps – Remote Float Chambers.  I normally favour these on race bikes (before I got too fat to get in my leathers!),  and the way I found best to adjust the height of a remote float chamber (which do normally have a line cast into the side of float chamber, but I use that just as a rough guide) . . . was to adjust it so (that on a racing Norton), when the bike is absolutely vertical, the carb does not leak.  But as soon as the bike is pushed away from me off vertical, towards the timing side (I am standing on the drive side of the bike) – and the bike is approximately 10 degrees off vertical, the carb starts to dribble.  I had always found this gave best running and full flow (normally using methanol on singleknocker, or avgas high octane on DOHC … where I know both need good fuel flow).

If you have a fixed float . .. the degree angle might vary slightly . . . but same principle should apply . . . ideally, should not leak when vertical ..  . and just push bike away from vertical, at some point it should start dribbling from carb body.  If it is dribbling at vertical – pulling towards you (i.e. 10 degrees or whatever from vertical to drive side) – should at some point stop the leak, if it is caused by float height (which would surprise me on a fixed chamber) . . . but if it doesn’t stop leaking (as I suspect) . . .then it is not float height in relation to body . .. it is something else causing leak!

Hope this logic makes sense, and hope this might help, let me know if not

Best wishes Steve,


 ps – I am not going to recommend it . . . but if seat in float chamber is marginal, I know in past that some people have taken a tapered pin and keeping it absolutely vertical have tapped it against seat .  .. to try and restore it (preferably not the needle you are using though . . . as will just indent needle.  I will leave that one with you!"

valid point well taken... 

i suppose i could move the needle in closer to the chuck.  When i hold the distal/top end of the needle between my left index finger and thumb and twirl the tapered end with my right index finger and thumb i can see run out...  i do have a granite surface plate i can roll the needle on...


Otherwise mark it with a Sharpie and roll it on a plate.  Note the stopping position on the circle, and repeat with the same starting point as marked.  If it stops in the same place its "Bent"...

 Eggs and sucking come to mind.  Build it with the cap on to confirm it leaks or not.

I took a note out of my own book and created a small spacer to lower the seat of the float valve and statically its cured, but the proof is in the machine.  But I'm running and starting and ticking over on a 930 Amal now...    and like it....   310,106 3, 25 pilot.


 Good  hunting




...if i go with a Monobloc, is this the one i need?


...i go with a Concentric, is this the one i need?


I would buy from a dealer and not an anonymous ebay site. You need a seller who is prepared to jet it correctly as per the info others have posted previously. Burlen Fuel Systems market 389/3732. That is 1 5/32" , which is the size fitted originally. I don't know why they list what looks like 29mm , since that doesn't match an " inch" size.

The 30mm concentric is more or less the same size. Arguably it is not quite as good as a monobloc but that's theoretical and of no consequence. But monoblocs are much more expensive. So why is the one you list actually cheaper? If it is, and if it is the right size and has the correct slide and jets, then you might as well go for it. Buyer beware...there's a good reason for something to be cheap. Whether the reason is sinister is the question.

I'd avoid ebay myself.

In the motor cycle factories they used subscript numbers to denote the internal jetting and slide, so they could collect the right carb for the bike from the stores. The subscript is irrelevant today. 



...put it to the surface plate, the needle is true.

More from Paul Norman as follows below:  (at this point, i am going to step back a day or two before i do any more.  on the one hand i don't want to throw money at the float with a new needle and seat and on the other hand i'd like to have a properly working float)

Hi Steve,

Well nothing obvious from photos, but a couple of observations:

  • Float is correct type pre-war Inter 15 degree type . . . but I think are slightly more difficult to fix if leaking – from memory the needle seat is quite deeply recessed (later alloy TT floatchambers had a screw in brass piece with bigger flow if I remember correctly.  I am still wondering if it is the seat on this that could be marginal
  • I notice your floatchamber has lots of knocks on its side . . . classic signs of the float chamber having been hit with a large hammer or spanner, because it was stuck and leaking!  I had a speedway Type 27 with twin float chambers on a JAP speedway bike the same ..  . and almost impossible to find Type 27 carbs that had not been hit with a spanner in the pits . . . that was the standard way to try and stop them from leaking (particularly with methanol, which turns to jelly).  But again, points to the fact that the float needle seat could be damaged or worn.
  • Float needle . . . hard to tell in photo, but does not look new.  Really difficult to make an old one stop leaking if it has any wear marks.  That type has a taper with a smaller surface area.  The later alloy floatchambers had a bigger flow . . . and they just made a float needle where they extended the taper down further, giving a wider diameter. 
  • If there are any marks or a question mark over needle . . . might be worth trying a different one.  We actually do make both types for the TT (total bitch to make) – this type is here:
  • Float . .. not sure, but looks like solder on it.  Again, I know it is obvious, but just check to make sure no pinprick leak in it . . . tiniest bit of fluid in it will affect it.

Obviously, with these older carbs . .. it is difficult, because to recut float seat does affect height slightly, but I would still maintain, that if all fitted originally, and in good condition, should not really leak when bike upright




I know that if we put a price on our leisure time we would not play with old bikes but  even so, there are limits! Why not buy a new carb - whichever one you like the look of, and play with the TT next winter when the weather is rubbish. You are missing using it. I did 45 thoroughly enjoyable miles on mine last night and came home with a smile from ear to ear. I couldn't see the carb at's under the seat!


...Thanks David, that's exactly the direction i am going...  Have just started the application for  title process today; pretty easy to do.  Since the Inter was built from parts, it came with a bill of sale only. Will be at least a month before i have title, registration and plate.  I'm too lazy to remove the pretty little fasteners holding plates on one of my other bikes to put on the Inter and at the rate it's taking to either get the TT to not leak and/or procure a replacement carb, there's no real rush.

I've ordered a plastic float and 0.222" needle from Paul Norman.  Removing the float chamber top with fuel tap open  filling the chamber suggests the original brass float might not have enough buoyancy to make a seal between the needle taper and the needle seat...

My search for a 389 Monobloc appears they are not available; I'm not seeing them on the Burlen site...?  Who has them?  30 mm Concentric Premiers are available.  Doing the math, 1-5/32" is 29.37 mm, so the 30 mm Concentric is 0.63 mm larger than 1-5/32"

if i can purchase the 29 mm Monobloc off ebay for $98, I'm buying it; i'll just need a smaller main jet, i have a variety of sizes in my spares.  If it is actually 29 mm and not 1-5/32" it will only be 0.37 mm smaller.  if it doesn't work out, then what the hell.

From what i see, Atlas intake manifolds aren't exactly plentiful, one is on ebay in England; I haven't tried the usual American vendors.  Yet.  Will appreciate any suggestions you fellows have to source one.

Whether or not i get that Monobloc, I may go ahead and purchase a 930 Premier; it's only money...  i want to take the Inter for a ride or two before the snow flies...


Burlen do not sell direct. In the  UK I deal with Surrey Cycles.  It's a one man and his wife outfit and he is very helpful. RGM and Hitchcocks may also help. Prices may look different but some quote with VAT (20% sales tax) which you do not need to pay.


... looks fine to me and much cheaper than from a dealer (although of course a bidding war may follow).

If not I've always found Hitchcocks an excellent source for Amal bits & bobs.


I have always sourced by Carb items on line with them.  They were down during the covi times but back at now I believe.

Steve; your bracket looks nicely made but you could do with a more vertical design as that is the direction of the forces that will cause you issues.  





...with float chamber on the carb and cover removed, i suspicion the factory brass float may not provide enough buoyancy to lift the needle fully into the seat...

I have ordered a plastic float from Paul Norman.

I passed on the ebay Monobloc.  Have ordered a 930 RH Premier.

More will follow after goodies arrive.


Why would you ever want to run a TT carb on a street machine?

Am I correct in that the TT carb does not have an idle circuit?



My carb is stamped 10TT34 so that makes it Amal on page 3 of your list.  It is an early one and several changes followed as far as I've been able to work out.  The slide return spring in the early 30's was a parallel spring about 1/4" diameter.  They then changed to the conical type a couple of years later.  (I  recently fitted a conical spring to make the return more certain).

The inlet tube on mine opens out sooner upwind of the slide, and is supposed to carry a shorter, wider flared inlet trumpet.

I assumed the "34" was the year code.  But from your list that doesn't seem to be the case?

Some AMAC had no needle, didn't they?


David Cooper, You may be right but as I understand it AMAL mixing chambers had an internal thread for the Bellmouth and AMAC had an external thread. I understand AMAC did use the last number as the production year. I have enclosed another ''AMAL'' list for your perusal, you will note it covers all the Amalgamated Makers. I have been chasing this subject up for over 12 months and am getting more and more confused. If only we could download the ''Old Blokes'' brains before they depart this mortal coil. Thank heavens for the Interweb, we would be even more confused without it.

Yours in motorcycling and research

Bill Moline



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