I have a decent collection of 1000 series carbs and parts ranging from 1034 to 1038. None of the slides or tops have any provision for a choke. I have never seen any 1000 series or parts with choke provision.
Never underestimate the human ability to elevate stupid to a whole new level!.
I've never seen a 1000 series Concentric top that had a hole for the choke cable,
Originally Posted by Rich B
I have never seen any 1000 series or parts with choke provision.
Prompted by your comments I looked more closely at the parts list. Only the top with a single hole was supplied for the 1000 so my search was bound to be lengthy. Series 1000s were supplied for a few four strokes at the time which is why I (incorrectly) thought chokes might be available, although scarce.
My carburetor starting ritual with the 930 Concentric on the BB is tickle and full choke, with the choke removed immediately after it starts. However, with the 1-5/32" Monobloc on the Catalina it's no tickle and no choke. So, maybe the Competition will be just as easy to start without a choke as it would be if it could have one.
Just a thought, what type of float needle are you using? The brass/viton type often results in a lower fuel level. AMAL recommend their newer alloy/viton needle to avoid this issue, and I can vouch for the difference.
Just a thought, what type of float needle are you using?
I'm using alloy/Viton. I currently only have three floats in my Concentric box, all the original plastic. I suspect the problem is that all of them probably have warped with the passing of nearly 50 years and, thanks to the leverage, it doesn't take much change in the projection that operates the needle to change the fuel height by quite a bit. As I wrote in a previous post the current configuration of the float is such that even if I pressed the seat down in an attempt to raise the fuel level it wouldn't help because the floats would top out on the carburetor body.
You could add weight to the float, that will raise the fuel level.
AMAL didn't leave much empty space in the float chamber, and even if some Pb did fit, messing around to find the right weight would be a headache. More so when the weight fell off and jammed the needle open. I think it's best for the adjustable stay-up float to arrive.
It's probably worth repeating that the pilot circuit is a completely separate "carburetor" from the main circuit, with both "carburetors" contributing in different proportions as a function of slide opening. The first photograph is a bottom-up view of a partially sectioned 627 AMAL from a Sherpa T that gave its body to science. Air is in green and fuel is in red. The remnants of the threads at the bottom show where the pilot mixture screw goes.
Originally Posted by chaterlea25
Opening the air screw should lean the mixture
As can be seen, opening the air screw does let more air flow through that passage, but the effect of that screw on the mixture isn't just a straightforward diluting of the fuel with the extra air. To find out more, it was back to the flow bench.
The second photograph shows that I drilled out a longer Monobloc screw and turned it down to fit a line to the manometer. I sealed it with grease on the threads. Although the inside of the bowl itself, other than the volume in the jet block, should stay at atmospheric pressure due to air leaks, I set up my manometer for a differential measurement anyway. It turns out that it did stay close to atmospheric pressure up to ~3/4 throttle , but then dropped in pressure as the flow increased to full throttle. Although I expect the amount of depression in the float bowl, and hence the effect on the flow of the pilot jet, would vary between carburetors, I'll show below that the effect of this on the mixture would be negligible.
The third image shows the results of my measurements of the pressure drop inside the pilot circuit with the air inlet at the front of the carburetor blocked and with the pilot jet blocked (top curve) and open (bottom curve). The data was taken at the same 7" H20 as the other curves on this graph paper.
The measured depression is a result of the air flow over the two tiny holes at the engine side of the slide (seen in the first photograph). In the case of the bottom curve this is partially offset by air flowing from the carburetor bowl through the pilot jet. However, the greater depression of the top curve corresponds to actual operation because fuel in the bowl would not allow air to flow through the pilot jet. As can be seen, at 5 CFM (~1/8 throttle) the depression over the pilot jet is 3.06x greater than that over the needle jet.
A #25 pilot jet has diameter 0.0175" for an area of 2.40x10-4 sq.in. The annular area of a 0.1065" needle jet with a 0.0984" needle is 13.0x10-4 sq.in., which is 5.4x greater. What this means is that at ~1/8 throttle, where AMAL's simplified tuning description says the pilot jet is only just losing its influence, already the main circuit is supplying 5.4x/3.06x = ~1.75x more fuel than the pilot circuit.
The fourth image shows the depression in the pilot circuit at higher flows. As can be seen it saturates at ~0.150 psi whereas that of the spray tube of a 2-stroke body (dashed blue/red curve) continues increasing to 0.340 psi at full throttle. Further, by that point the needle is essentially out of the needle jet and flow out the spray tube is determined by the main jet. Assuming a #300 main jet (dia. 0.059"; area =27.3 x10-4 sq.in) the relative flow through the spray tube will be greater by (27.3 x10-4 / 2.40 x10-4) x (0.340 psi / 0.140 psi) = 27.7x. That's a pretty large factor, but it shows the pilot circuit is still responsible for nearly 4% of the fuel supply even at full throttle.
Finally, ending on the point I originally intended to make with this post, what this information shows is that opening the pilot "air screw" simultaneously does two things that alter the mixture. It dilutes the fuel with air from the pilot air passage, but it also lowers the depression so less fuel is drawn into the circuit. That is, turning the screw CCW adds air while also subtracting fuel. Figuring out the relative magnitudes of the two effects is left as an exercise for the reader.
That backs up what I have seen on the dyno in the past, where my 'dyno guy' (a pretty smart operator) altered top end power/air:fuel on a small 2 stroke single with a small tweak of the air screw, not affecting low end function.
No generalisation is wholly true, not even this one. Oliver Wendell Holmes
The front to back asymmetry of the horseshoe shaped float in the Concentric results in greater sensitivity to downdraft angle (as say compared to the floats in a Monobloc).
The Concentric float (with its horseshoe shape) has a large buoyancy bias toward the engine side, so any leaning in that direction will close the float needle at a lower average fuel level than that advised by AMAL.
On my more modestly sloping carbs (pre-unit T120) I settled at a fuel level of 8mm static and level on the bench, with stayup floats. Attempts at any higher just resulted in flooding/dripping carbs when installed (the stayups tend to be nearer the top limit of movement than the plastic floats).
It is possible that you may have to accept a compromise of fuel level, and tune from there.
Attempts at any higher just resulted in flooding/dripping carbs when installed.
The first photograph shows that as installed on the Gold Star the tilt is ~14-15 degrees. The second photograph shows the carburetor rotated by 15 degrees overlaid with a band of width 1.8 mm based on John Healy's figures of .170" to .240" (4.3-6.1 mm). Note that if the fuel level is set at the high end of this range (i.e. the bottom edge of the green band) it will just touch the outer edge of the gasket (the inner edge would be slightly above sea level).
From my previous post it can be seen that the float bowl gasket doesn't just have to stop gasoline from leaking out, it has to stop air from leaking in. At least around the pilot jet where it would affect the pilot circuit. As long as they're undamaged red fiber gaskets do a good job of stopping gasoline but once I get the float adjusted to the proper level I'll use Permatex Permashield gasket dressing to seal it ("this compound resists common engine fluids, including oil, gasoline, ethanol,...").
The third photograph shows that the carburetor is mounted on the test stand awaiting arrival of the adjustable float.
I would recommend satisfying yourself that the "manometer style" technique accurately accords with a bench measurement of fuel level in the centre of the bowl. As with any other measurement, it should be checked against a reliable standard. I did this, and reported my findings several years ago. I was using a similar diameter (1/4") tube and found a 2+mm discrepancy between tube and actual level in the bowl.
Perhaps I failed to explain my reasoning clearly, but the design of the float means there is more fluid displacement when the carb is tilted in a down draft direction, hence earlier closure of the float valve, compared to a horizontal situation.
I would recommend satisfying yourself that the "manometer style" technique accurately accords with a bench measurement of fuel level in the centre of the bowl. Perhaps I failed to explain my reasoning clearly,...
If someone fails the eye test they have to give up their license to drive. If I physicist fails the meniscus-reading manometer test, they have to give up their license to practice physics.
I understand the issue you mentioned. That's why the fuel level has to be measured on the center line, but the carburetor has to be mounted at 15-deg.
2) The minimum depth that the fuel can be at the center if it is to just reach the edge of the bowl when the angle is 15° is 20 mm x sin15° = 5.2 mm.
I measured the angle to be ~14-15° so 15° would be worst case (if I measured correctly). This means if I aim for 5.5 mm, which is within the acceptable range, the fuel never will even touch the gasket. Unless the engine shakes which, of course, none of them do.
Originally Posted by koan58
the "manometer style" technique ... should be checked against a reliable standard.
I'm puzzled by what you mean by this. I can't even imagine what a "reliable standard" for this would look like since the height of the meniscus only depends on the gravitational constant being the same at both ends of the manometer. Since the two ends of the tube are only a horizontal distance of ~2 cm apart even a block of kryptonite under the float bowl wouldn't affect my measurement.
Originally Posted by koan58 [the "manometer style" technique ... should be checked against a reliable standard.]
Originally Posted by Magnetoman [I'm puzzled by what you mean by this. I can't even imagine what a "reliable standard" for this would look like since the height of the meniscus only depends on the gravitational constant being the same at both ends of the manometer. Since the two ends of the tube are only a horizontal distance of ~2 cm apart even a block of kryptonite under the float bowl wouldn't affect my measurement.]
Well you did ask...
It should be remembered that we only use the term "manometer type tube" very loosely for descriptive brevity. It is in no way a manometer, it is merely a level tube.
When I got my stayup floats and alloy needles, I decided to conduct a more thorough examination than I had previously done of what was going on in the float bowl with both the old and new components in different combinations. The "old components" were standard, not very old plastic floats and brass/viton needles. The "new components" were stayup floats and alloy/viton needles. I was also interested to see what difference just changing to the alloy/viton needles made with the old type floats. This was inspired by a post from a contributor (Caulky) who had taken a preliminary look at the functioning of brass/viton needles in Concentrics, and the impact on fuel level, and also that I had never been fully satisfied with the performance of my original setup, in terms of a dependable idle, engine temperature and subtle lack of smoothness in general running when compared with pals who ran Monoblocs on similar engines (pre-unit T120).
I started out thinking to use the convenience of the "manometer type" method, plumbed into the drain plug similar to yours, with the carbs in situ (the only extra convenience being that it was T'd with a clamped pipe for easy draining/refilling of the bowl for repeated measurements). I began with std float and brass needle, with a sight tube being the small gauge tube from a battery breather. I found fuel level indications (on repeated drainings/refillings) varying by several mm, though all were well below AMAL specs. I then tried the largest ID transparent tube I had available (1/4"), and still found several mm variation, and if anything the avarage level seemed slightly lower.
At this stage I took the bowl to the bench vice, carefully levelled, with the 1/4" manometer tube and jury-rigged fuel tank. Using a bridge across the bowl surface I could dip the depth gauge of my vernier into the centre of the bowl to determine the actual fuel level depth, and compare with what I saw in the tube, over multiple refills. What I found was that the directly measured depth varied little (not even a whole mm range), the tube exhibited 2X to 3X this range. Perhaps more significantly, the average tube measurement was ~2+ mm higher fuel level than the direct measurement.
This is what I meant by a "reliable standard" with which to compare the accuracy of your tube. Perhaps I should have said "reality standard", which requires no kryptonite, just a measuring stick to calibrate your tube. In any other measurement you make, you go to proper lengths to establish that your technique accords with a more defined reality standard. I see a couple of reasons beyond a black hole hidden under your workshop why the tube measurement may differ and vary more, compared with a direct measurement.
1) Capillary action. The attraction between the fuel and the tube not only results in the meniscus, it also produces an upward force raising the fuel level in the tube. That there is a concave meniscus in the first place illustrates that the molecular forces are acting in this direction. Assuming all other factors are equal, the smaller the ID of the tube, the more this capillary raising of level, and even 1/4" ID has a significant effect. This is for water in glass, for illustration of the principle only: https://en.wikipedia.org/wiki/Capillary_action#/media/File:2014.06.17_Water_height_capillary.jpg
The other end of the tube is inside the float chamber, with 30+mm diameter at its meniscus. The capillary force here is insignificant in comparison.
2) Wetting effect of fuel on the inside surface of the tube. After repeated tests the tube has a film of fuel molecules adhering to its inside surface, above where the meniscus would usually settle. This then results in the angle of contact between meniscus and tube being closer to vertical, the fuel level in the tube rising in consequence.
"It should be remembered that we only use the term "manometer type tube" very loosely for descriptive brevity. It is in no way a manometer, it is merely a level tube."
Where Im from we call it a sight glass. Very often static level is different from running level, particularly if there is a swirl in the housing. Not really applicable to carbs, but the capillary action thing is a good point.
otherwise very interesting so far. MMs results on the pilot system were not what I expected. just goes to show. Surely the float bowl bleeds air through the tickler, doesnt just rely on leaks ( although the tickler is just a controlled leak).
Last edited by gavin eisler; 04/23/198:53 pm.
71 Devimead A65 750 56 Norbsa 68 Longstroke A65 Cagiva Raptor 650 MZ TS 250 The poster formerly known as Pod
Gavin, I don't think that the tickler is a controlled leak, it is just a passage for balancing bowl to atmosphere, a breather. It doesn't need to do much at all, because fuel taken by the engine is replaced by fuel delivered from the tank. All of the "vacuums" happen above, away from the bowl.
Running level must be slightly lower than static level, otherwise there'd be no opening of the valve for fuel, to replace that being used by the engine. As the fuel only flows through the float valve under gravity, it's flow can only be according to the needle's lift from the seat. Hence for high flow at high rpm/throttle the float must be lower than at idle. This can only happen by the fuel level being lower than at idle.
This says to me that one can only hope to establish fuel level in one standard set of circumstances (which is what AMAL advise) and tune from there. That there are numerous varied applications of one simple carb illustrates how tolerant/versatile it is.
For maybe 20 years I'd put up with the brass needles, unaware that the fuel level was ~8mm lower than it should be.
"For maybe 20 years I'd put up with the brass needles, unaware that the fuel level was ~8mm lower than it should be"
There is only one answer to this statement... No! 0.300" +-????
The aluminum needle followed the introduction of the Stay-Up float. The action of the Stay-Up float doesn't naturally unseat the needle. Thus the search for a lighter needle so the head of fuel unseats the needle instead of the the float. It is to correct a problem that wasn't there with the plastic float.
[quote]As the fuel only flows through the float valve under gravity,quote]
A problem that wasn't a problem with the plastic float as the tangs on the float raised the needle when the bowl needs fuel, especially when float level was done by moving the needle seat. While the action of the Stay-Up float barely lifts the needle as design, bending the tangs to set the float level makes things worse. The float can fall and the tangs never lift the needle. It can be made right! Koan look closely at what is happening.
I expected you to pipe in John at some stage, to defend the use of brass needles in the Concentric. We have been through this in previous discussions. The Concentric was designed to work with a very light float needle (a 0.3g plasic one), because of its unconventional arrangement of the needle being able to close the valve under its own weight (which the brass one is able to do). Alloy is 0.5g, Brass is 1.5g.
The only way the carb float bowl system functions as originally designed is by using either the original plastic or the newer alloy needle, only with the heavy brass needle does the weight of the falling float have to open the valve, rather than properly for the float bouyancy to close the valve. This makes a more than 5mm difference in fuel level drop before the float intervenes to lift the needle, due to the freedom of the float tangs within the needle space. It is laughable that a cistern ballcock type of arrangement depends on the weight of the needle to stop the valve, it is always closed by positive force from the ballcock, and always opened by fluid pressure when the ballcock drops a little, the ballcock doesn't have to pull the valve open.
Respectfully, I question which of us needs to look more closely.
If your float chamber is fitted with a brass needle valve, you may find the valve sealing under its own weight, before the float has risen far enough to press it shut. Symptoms of this problem can be that the carburetter takes a long time to tickle, hesitates on pickup and does not idle reliably. A Viton tipped aluminium needle valve is now available that overcomes this problem. It is now fitted as standard equipment to all new Mark 1 Concentric carburetters.
No mention of "if you have stayup floats", it is clearly talking of the issue of brass needles. Indeed, when I installed my alloy needles in the experiment I described earlier, I found an immediate rise of fuel level of 6-8mm straight away, using just the plastic floats, as compared with the brass needles. I ended up settling at that fuel level with the stayup floats (I settled at 8mm fuel level) because even with the adjustability of the stayup floats there are constraints, one being that they tend to float a bit higher and abut the bottom of the body a bit sooner than the old plastic floats. It was a transformation of the behaviour of the engine. ---------------------------------------------------------------------------------------------------------------- For anybody who isn't bored enough with my tale already, I'll try to kill you off with my original postings from 2014: MkI Concentric float needle/stayup float
Hi folks, this follows earlier posts by Caulky et al about fuel level in MkI Concentric carbs vs various types of float/needles, which inspired me to explore. New 930 MkI Concentric carbs on 8-stud T120. I've used the brass/viton needles since the mid 80's, and I've always felt the bike ran differently from time to time, subtle but feelable. Got upgrade kits, including Stayup float and alloy float needle. Began by making manometer of 4mm ID plastic tube into plastic floatbowl plug, cabletied to side of bowl. Then began measuring fuel levels of carbs on the bike, found this a bit vague, carbs lean toward head, meniscus, parallax etc, so averages of multiple measurements. 2 gal fuel in tank. Original plastic float/brass needle 13.5mm Swapping to alloy needle/original plastic float 8.5mm Swapping to Stayup float/alloy needle 8.5mm So just changing from brass to alloy needles brings fuel level 5mm closer to spec. Decided to study the bowls on the bench, used 20cm head of fuel, Stayups & alloy needles. Had the manometer pipe connected to compare with direct measurement. Bowl levelled. Started by dropping a vernier down between float toe/bowl, found this unreliable, think surface tension between the 2 close surfaces raises the apparent level. So made a bridge of known height to measure down into the centre of the bowl, gotta be the truest measure? Right was 8mm (the nicest pot), left 9+mm Set left to 8mm as well. Went on 40ml run, felt like "on agood day" had to open airscrews 1/4 to 1/3 turn, steady idling. Will need more rides to see if not good days still happen. This idea of the brass float needle closing the fuel off under its own weight(not from pressure from the float) is absolutely confirmed. Changing brass to alloy(1.5g-0.5g) raised fuel 5mm, which corresponds to float toe movement within the free tang space of the needle. When filling the header pipe to the test bowl I would initially see fuel freely passing needle(float fully down pulling needle fully open) float would rise part way (where the tangs just allow needle to seal its seat) and settle until header pipe reaches ~15cm. At this head the alloy needle rattles up and down, letting fuel through to a level which is constant for heads 15cm upwards. My tank running low is about 20cm head. My own calcs said alloy needle would take 9.5cm head, a bit more actually needed to overcome stiction maybe? For brass needles I calc a petrol head of 37cm(over the top of most petrol tanks) is needed to unseat them against their weight, so they will always give toward a 5mm lower fuel level. Why did I choose 8mm fuel level? Bit of a punt really, the original plastic float/alloy needle gave ~7mm and I figured its been working pretty well for decades on up to 5mm lower. AMAL spec 6.35mm max, some good sources have suggested 7.35mm, so I went 8. Phew Dave
then someone replied:
I have brass needles and nylon floats in a pair of Concentrics I just bolted onto my '66 T120R as part of a troubleshooting exercise. I have wondered about setting up those needles in my drill press and drilling them across their axis to reduce their weight. Might give it a try one of these days and compare it to your alloy weight
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
OK, more misinformation has been posted here than I care to address. So, I won't. Either take my word for it that it is possible to read a manometer to better than 0.1 mm. Or don't. It's also possible to misread a manometer if you don't know how to correctly measure a meniscus. As an example of what is possible, if you needed to determine the temperature of liquid helium to 0.001 K near 1.5 K you would need to accurately read a manometer to 0.02 mm, as I have routinely done (using a cathetometer). Again, take my word for it or don't. But, if I don't address other posts don't assume that means I think the information in those posts is necessarily correct.
Originally Posted by gavin eisler
but the capillary action thing is a good point.
Not for a tube has a ~1/4" ID.
Originally Posted by gavin eisler
Surely the float bowl bleeds air through the tickler, doesnt just rely on leaks ( although the tickler is just a controlled leak).
Air does bleed in through the tickler, but in the carburetor I tested that leak/bleed wasn't fast enough to keep up with the pressure drop once the air flow exceeded ~3/4 throttle. The tickler isn't a precision bleed so the magnitude of the effect will vary between carburetor bodies.
Whether or not it helps speed up the process remains to be seen, but while waiting for the adjustable float to arrive I made a jig I sawed from a few pieces of scrap Al that gives me access to the float while the bowl is full. My hope is this will let me tweak the actuating arm for the needle in "real time" rather than having to remove the bowl from the carburetor body each time I need to make an adjustment. The AMAL site says these floats have "stainless steel tangs which can be bent to alter the fuel level," but whether or not I can accurately do this in situ remains to be determined.
The first photograph shows that the jig holds the bowl flush against the bottom surface, with cutouts to hold the pivot pin in place and give access to the area around the needle. The second photograph shows that after I welded a "stand" to it I then faced the bottom of the stand to make its surface accurately parallel with that of the surface that supports the bowl. Measured on a surface plate the two surfaces are parallel to within the 0.1-deg. resolution of the digital protractor. The final photograph shows the jig mounted on an adjustable table set at 14.5-deg. I bolted it into the slot on the left so the scale on the table also can be seen but in use it would be in the slot on the right to give room for the tubing from the bottom. This also relies on the mounting surface of the adjustable table being level (in this case, the mill's table), which it is.
You could add weight to the float, that will raise the fuel level. Just exactly how , I dunno, never done it, whatever glue is fuel resistant and a short bit of cable tie maybe?
Or you could take weight off the needle. Or change the seat height as we have been doing with Concentrics for ages to get them properly balanced on A65L's Apparently their is both a breass & alloy needle available and it makes quite a difference to the fuel height depending upon which one you use
[quote] While the action of the Stay-Up float barely lifts the needle as design, bending the tangs to set the float level makes things worse.
I'm hoping John's comment refers to ham-fisted attacks on the tangs with Vise-Grips, not delicate changes made with surgical precision.
Unfortunately, tracking now shows my 'stay-up' float won't be delivered until tomorrow. To further be ready for its arrival, the first photograph shows I marked the float bowl at the 4.3 mm and 6.1 mm upper and lower limits for the level in the center of the bowl.
Because of the 15° tilt the float will have to rise higher than if it weren't tilted, the second photograph shows how far into the main body the float can move before it tops out. It can go into the body by 0.080", plus another 0.034" for the thickness of the gasket, for a total of 0.114". If it projects further above the float bowl than this when I set the level I'll have to devise a solution.