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When a Concentric float chamber is fitted with a nylon or aluminium float needle the operation of the float and needle is controlled by the level of fuel as it rises in the float chamber. The fuel rises to the correct level and the tangs of the float press the needle valve shut.

When the brass viton tipped needle valve is used, the float bowl behaves differently. The weight of the brass needle valve causes the carburetter to become sensitive to the pressure of fuel entering the float bowl. Lighter pressures of fuel cause the needle valve to self seal under it's own weight before the float rises sufficiently to close it and the fuel level is correspondingly lower. The function of the float becomes to open the valve as the fuel level drops - it plays no part in shutting the valve. This also makes the system sensitive to the weight of the float. Lighter, more buoyant floats sit higher in the fuel and “drop off” the needle valve sooner, leading to an even lower fuel level. That is why older heavier floats appear to work better in conjunction with brass valves.

The most obvious symptom of this is the slow tickling carburetter which leads to the corresponding practice of lengthening the tickler. Carburetters fitted with the aluminium valve can be left to fill the float chamber once the fuel is turned on and will flood in a couple of seconds when tickled.

To generate conditions where a brass needle allows fuel flow until closed by the float requires such a force of fuel entering the float chamber that, in a float bowl secured separately for observation, fuel floods over the side of the float chamber due to the velocity at which it is passing through the valve. As the flow of fuel is reduced the weight of the needle causes the needle to drop and seal the valve sooner and sooner with correspondingly lower and lower fuel levels.

Now obviously brass needles have been working in Concentrics for the last thirty years and did away with the wear and leakiness associated with the nylon needles. They were introduced at the time of a very short lived consideration of adopting the 0.125" needle seating as standard. A heavier needle will work better with a wider seat as higher pressure is required for sealing. The 0.125" seating idea was dropped, the needles remained.

The aluminium needles are intended to restore the original operation of the float chamber. By sealing when closed by the float they give consistency in the operation of the float chamber, and between carburetters on twins or triples. When the petrol tap is turned on the fuel rises to the set level, doing away with tickling problems and improving starting. Consistency between float chambers is presumably why they often make a noticeable contribution to smoother low end running.


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Originally Posted by John Healy


I will send down a couple aluminum ones and you can try them...

edit: they are in the mail.


You are too kind. Thanks for indulging me! beerchug

Steve


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Thanks to all who have contributed so far.....keep it coming!

Pete and Phil, I understand your assertions....Magnetoman, I only wish I could understand yours! It's not your fault. I think I must have been sleeping when they discussed Fluid Dynamics in physics! smile

I think John H. may be on the right path. The Concentric can be counted on to do what is asked of it
with the brass float needle. Having said that, I'm willing to explore and see if there's a difference! :bigt

Steve


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Whoa, hold up there a bit. There is an awful lot to digest.

Quote
The most obvious symptom of this is the slow tickling carburetter which leads to the corresponding practice of lengthening the tickler.


I think we are getting ahead of ourselves. When the float level is set as with the drawing: top parallel with, and above the top of the float bowl, there was never a problem with slow tickling. I have never seen a "slow tickling" carburetor where the total travel of the tickling pin (which is changeable) is set to displace the float and thus upset needle from its seat. This is a problem only when the pin is pushed too far into the button, as when the 622/172 tickler conversion kit is installed, and the total travel is less than that required to displace the float and thus the needle.

When one undertakes lowering the fuel level, as described in Triumph Bulletin, the standard setting for the tickler pin does not allow for enough travel for the pin to reach the top of the float. You get conditions where the pin doesn't touch the float at all or just touches the float not lowering it enough to displace, or lift, the needle. This has nothing to do with the weight of the needle, and is simply cured by drawing the pin out of the button enough for it to reach the top of the float when the button is depressed. This is often made worse by the installation of the update kit where the pin is pushed too far into the button as described above.

I find a lot of scepticism when I suggest that lengthening the travel of the tickler pin will cure slow tickling after resetting the float level to the .080" below the top of the bowl level. Many cannot get the mechanics of the parts involved.

Given that the engine supplies manifold vacuum in such a quantity and quality as required, there is no reason at all that the bike fitted with an AMAL Concentric carburetor set-up properly using a brass needle will not start on the first kick when cold and idle at 500ish rpm once the engine reaches operating temperature, and take throttle without stalling. I have taken 2 old AMAL carburetors that were pronounced dead by a couple of AMAL experts, straightened the twisted bodies, replaced all of the brass, made sure the pilot jet was SIZED (not cleaned), etc. and put them on a motor I built for btour... It idles at 500 rpm.

Now one must also consider flywheel weight, and how it was accelerated (the rotational kinetic energy stored in the flywheel makes a big difference in how easy it is to start an engine), and carburetor size, changes to valve diameter, cam duration, inlet closing figures, type of ignition, modifications made to the inlet tract, and ignition timing etc. when making comparisons on how different Triumph engines idle and I suspect they will have a greater effect than the difference between an aluminum or brass needle.

People are always ready to look for something other than the stupid thing they did. A bike that is lean will idle poorly if the spark plugs are not set to the recommended gap, but I see a lot of people espousing the virtues of running .035" plug gap when they should be using .024-.025" gap and complaining that the bike doesn't idle well. Then there is the elephant in the room - these engines vibrate - the needle isn't sitting in a static float bowl and changes in barometric pressure change the head pressure and thus the pressure on the needle.

Must go, 14 hour day is enough for a 72 year old....

Where am I wrong?


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BTW - John, package arrived, thanks.

I'll install them in a couple of weeks and will report back!

Steve


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Long overdue follow-up to close out this post. I FINALLY put the alloy float needles in my carbs on Saturday. Went for a 100+ mile ride on Sunday and, unless it's just wishful thinking on my part, noticed an improvement in low-throttle running and coming off the pilot jet and slide.

Before I switched it out, I would get a slight burble/blub/8-stroke sound at ~1/8 throttle. Perhaps this is doe to the fuel "flooding" the float bowl with the brass float needles. That coondition is gone now with the alloy needles in place, so perhaps the alloy version is contributing to a more consistent fuel delivery.

Of course one could make the argument that brand of gas (altho I consistently use the same), relative humidity, temperature, etc, could play a part. At any rate, thanks for all the feedback!

Cheers,

Steve


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One of the other advantages of the newer aluminum float needles is our sticky gasoline/ethanol blend does not seem to gum them up as bad as it does brass. At least that has been my experience.

I prefer the aluminum ones.

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Originally Posted by elefantrider
I prefer the aluminum ones.

I think there are more ethanol blend issues with the pilot jet and fuel transfer port passages than there are with the float needle(s).

I have found that using STA-BIL and draining the float bowls when the bike is going to be laid up for more than a couple of weeks helps.

Steve


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The difference in weight between the float needles can make a huge difference to the fuel level, with the alloy one much nearer the design weight of the original plastic needle. Funnily enough, on a Concentrics carb, the heavy needle should be weaker, with a lower fuel level, unlike on a Monobloc, but WTH.

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The reason the Concentric carb is so sensitive to float needle weight is that the Concentric design is very unusual.

Almost all other carbs have the needle weight tending to open the valve (including all pre-Concentric Amals).

The Concentric has the needle weight acting to close the valve. So the weight of the needle has a profound effect on how the system operates.

Original nylon needles are 0.3g, alloy 0.5g, brass 1.5g.

If you do the maths, it takes less than 4” of fuel head to lift the alloy or plastic needle from its (0.1” ID) seat.
All normal fuel tanks will do this, even when nearly empty.
This is the way the Concentric was originally designed.

The heavier brass needle requires at least 16” fuel head to lift it from its seat.
Few normal fuel tanks will enable such a head, even when full.
So in this case, the needle must be lifted from the seat by the weight of the float descending.

This results in ~5mm lower fuel level with the brass needle, compared to either of the lighter needles, if no ther changes are made.

The following drawing may make it clearer (apologies for the crudity).

[Linked Image from i.postimg.cc]


The upper half shows the mechanism as originally designed. The angle of the float of course follows fuel level. The light needle follows this float movement as intended.
Almost all other carbs work this way, as well as the cistern in your toilet or water tank, ie float pressure to close.

With the heavy brass needle, the fuel cannot open it. It needs to be opened by the weight of the float dropping (so fuel level dropping) such that the tangs of the float actually pull the needle of the seat.


The crucial thing to appreciate here is that there is quite a bit of clearance between the float tangs and the upper/lower surfaces of the needle (indicated by arrows).

This vertical movement of the tangs equates to ~5mm difference in fuel level.

The Concentric can be forced to work with brass needles (as JohnH says), but the needle seat needs to be knocked down to compensate for the above consequence.

If a light needle and adjustable stayup float is used, no knocking of seats is necessary.

If the brass needle is used without any adjustment of the seat, a dramatically lower fuel level will be “enjoyed” which I did for many years before seeing the light.

Nothing of what I describe has anything to do with vibration or barometric pressure, which are largely the same in both scenarios (though I do experience less vibes/roughness with the alloy needle) and the idling is 100% reliable. The engine also runs much cooler.

Hope that is helpful.

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That’s a far better analysis than my words, but you agree that the fuel level, and thus the mixture, should be weaker with the brass needle, the opposite of what is reported!

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"but you agree that the fuel level, and thus the mixture, should be weaker with the brass needle, the opposite of what is reported!"

If you're simply swapping the needles, without knocking the seat about, incontrovertibly so!

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Originally Posted by koan58
"but you agree that the fuel level, and thus the mixture, should be weaker with the brass needle, the opposite of what is reported!"

If you're simply swapping the needles, without knocking the seat about, incontrovertibly so!
I agree with the above. When setting up a carb with a brass needle I never set the float height below the flange, either flush or slightly above.
It's been a few years since I changed my Trident carbs back to nylon needles, it became a different and much better bike to ride.
I collect all the nylon needles I come over, not just because I'm cheap but they work as the design intended and they don't really wear out either. I clean them with Autosol (or similar) on a rag and polish the float seats with Autosol on a Q-Tip. I've had more flooding issues with the brass needles.
The aluminium needles are fine.

SR

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Just be a little bit careful when converting to alloy needles.
When I did this on my Trident the the float on the timing side and the middle carbie [sometimes] started sticking on and fuel flowed everywhere.
I re installed my nylon ones and the problem went away. I then did some close inspection of the alloy needles with a magnifier and the three corners had small machine marks on them. I then sanded and polished the three corners of each needle - as well as modifying the top radius of each seat and retried. Needle operation was improved but the timing side would still stick open occasionally. So I did some more sanding/polishing on this needle and fixed it....now all works as it should.
I installed the alloy version to help improve fuel shut off. Where you leave the bike for 10=20 minutes but no longer have to turn your fuel off...exp. when on the side stand and all fuel tends to flow to the left.

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Originally Posted by tridentt150v
Just be a little bit careful when converting to alloy needles.
When I did this on my Trident the the float on the timing side and the middle carbie [sometimes] started sticking on and fuel flowed everywhere.
I re installed my nylon ones and the problem went away. I then did some close inspection of the alloy needles with a magnifier and the three corners had small machine marks

In my own experience, there’s a risk of something similar with almost every Concentric part that AMAL supplies.


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#425265


Originally Posted by Pete R
[The original nylon needle weighed 0.3 gram.
This should give you a fuel level 0.205" +/- 0.035" below the top of the bowl,when the float toe is set 0.080" below the top of the bowl.
The viton tipped aluminium needle weighs 0.5 gram.This will give you the same fuel level,as long as you have a fuel head 5.5" above the bottom of the float bowl.That means it will work OK if there is some fuel in the tank.It takes 5.5" of fuel head to overcome the weight of the needle.]

MM
[I'm in an airport waiting for a weather-delayed flight, so I haven't taken the time to reverse engineer your calculation, but it appears you've calculated the pressure of a column of gasoline (pounds/sq. inch, or grams/sq. cm) and equated that to the mass difference. If so, that's an incorrect calculation. And, it's not how the float needle works anyway.

The needle is pushed against the seat by the float acting through a ~4:1 lever arm, and flow ceases when the pressure on the seat equals or exceeds the pressure head of the fuel in the tank above. It's true that the float has to lift a bit more weight if the needle is brass instead of plastic or Al, but that's a minor effect. The level of the gasoline just has to rise enough around the large float to displace the additional 1 gram (or whatever) of weight, at which point the pressure on the seat will be the same as if the needle weighed less.]

Though MM is a master physicist, in this description he is massively erroneous in his understanding of the functioning of the Concentric carburettor, with the different weight needles available.

It isn’t about pressure that much, it is about the force, and in which direction it is exerted by the float tangs (ie up or down depending on your chosen needle).

Sometimes being clever is a dis-advantage.

A little later post:

[Indeed, just as you say (and as I said, albeit in a more lengthy way), a rise in fuel level in a Concentric pushes the needle down against the seat and stops the flow. In this endeavor it is helped in a teensy way by gravity acting on the weight of the needle, which pulls a brass needle down a bit harder than it does a nylon needle. As a result, if a nylon needle were replaced by a brass needle of *precisely* the same dimensions, and if nothing else were changed, the float level would be lowered by ~0.04" or so after the substitution.]

A yet later rather pompous post:

[When provided with evidence that brass needles actually do work (and, indeed, have worked in countless motorcycles for 30 years), there are two choices: 1) continue to argue they can't work, or 2) use it as a learning opportunity, by examining one's underlieing assumptions to determine the reason for the incorrect belief.]

This is unnecessarily condescending, as MM hasn’t realised the fundamental fact that a heavy brass needle can close the valve on its own. It doesn’t need a float to push it down
A brass needle, contrarily, needs the weight of a descending float to PULL it open.

The gap of the tangs between the upper and lower limits of the needle make ALL the difference.

I have mentioned the necessary fuel heads previously.

If you just put a brass needle in the carb, without a float, it would never fill with fuel.

Put an alloy or plastic needle in, it will flood freely.

I’ve actually measured the difference it makes, it is approximately 5mm in fuel level.

Think again MM.

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I remember that exchange well. It is my experience, as well as my opinion, that what Pete wrote was correct. The nylon needle will let fuel in with the lightest touch, indicating that the float needn't pull the needle off its seat. The head of fuel seems to be enough to lift the needle. Consequently, the float needs to push the needle shut. A brass needle on the other hand needs the float to pull it off of the seat, but will shut off the fuel supply as soon as it lands om the seat. This is why I need to set the float level very high with the brass needle, and lower with the nylon or aluminium needle. In my view, the lighter needles offers a more precise fuel level to be maintained.

SR

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Agreed Koan👍.

I would have loved to have met Pete R. We lost so much with his passing. A true gentleman as well as knowledgeable. A great example!


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I didn't join BB until later in 2012, so the exchange passed me by, unfortunately.
It wasn't until the thread was revived that I looked back to find it.

Had I seen it at the time, it would have advanced my own exploration of the matter by a couple of years.
As it turned out, my interest was provoked by a post by a fellow named Caulky on Rat in 2014.
Though Caulky lived on the same small island as myself (caulkhead is an old term for Isle of Wight folk - related to the building of wooden ships) I never actually knew who he was, and he seems to have disappeared off the face of the forum several years ago, though he was a moderator there).
Perhaps someone on Rat could enquire of him?

It inspired me to obtain the alloy/viton needles and stayup floats, and do as methodical an examination of the system as I could with my limited means in my workshop. With a bowl clamped in the vice and a header tank of fuel, I could see the effect of different fuel heads on the various combinations of components.
It was probably the longest periods in my adult life that I wasn't smoking!

What I found in practice was very similar to my calculations. I did actually find that the alloy needle required 2-3cm more head than the calculated 9.5cm to lift it from the seat, but this could easily be experimental error (my judgement of head was quite crude, try it, it's not easy without laboratory facilities!).

My posting of my findings produced a fair bit of interest on Rat, this was my reply to Snakeoil:

[Glad you found it interesting Rob!
This post was only the headlines, along the way were many other subtle observations/realisations:-
Original nylon needles are 0.3g, alloy 0.5g, brass 1.5g
but weight isn't the whole story, approx densities of materials:-
nylon 1.7, alloy 2.7, brass 8.5
The needle has a certain bouyancy in its fuel-filled housing (density of petrol~0.74) which reduces the effective weight that has to be overcome by the fuel head to open.
The impact of this is tiny on brass needles (8.5 minus 0.74 effective density) but is significant on nylon & alloy needles (1.7-0.74 and 2.7-0.74 respectively).
Drilling out brass needles to 0.5g (if possible to remove 2/3 of the brass?) will greatly reduce head required, but not as much as replacing with alloy needles, because that 0.5g of brass will feel almost no benefit from its bouyancy in petrol. I wouldn't waste the time when the alloys are cheap.
My calculated minimum fuel heads required (bouyancy allowed for):-
nylon 4.7cm, alloy 9.5cm, brass 37cm.
Another benefit of the alloy needle is its upper float tang mushroom is 1.5mm above the lower tang flange(as compared to brass needle 1.7mm). Tangs are 0.7mm thick, so the float dropping as fuel level falls lifts the alloy needle sooner/further.
I find tickling now a 2-3 sec affair, compared to say 10 sec with brass. This is because the tickler had to actually lift the needle from the seat, so only opening valve a little, whereas the alloy voluntarily lifts the full distance allowed by the float tangs, opening the valve much more.
I found using the manometer tube problematic, not very repeatable and hard to judge level, would probably be improved by using a bigger ID and glass tube.
However I found direct measurement on the bench at centre of bowl to be convincingly reliable. Dave ]

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