With the concentric, at all throttle openings there is an area of air flow which isn't in the main air stream,
Which at all throttle openings except WOF is worse on the Monobloc - While the Monobloc will flow more air at WFO the "tea-cup" shape of the slide, and the turbulence it creates actually reduces air velocity once the slide is below WFO. It also has an affect upon how quickly the bike accelerates/responds to changes in throttle opening as it takes less time to reduce the air pressure under a Concentric slide to start the actual process of drawing fuel than a Monobloc. The Concentric, MKII and VM slides are the first step toward the flat slide with ever better throttle response and acceleration.
Also look at the height of the fuel lift between a Monobloc vs a Concentric. The Monobloc is near 2 3/4" while the 4 stroke Concentric is 2 3/4." There is a lot going on hear for a 2 minute answer between phone calls. Read the next issue of Vintage Bike where Kevin Cameron discusses development history of this exact subject.
Being a type of "smoothbore," the Monobloc at WOT will technically have a slightly greater air velocity than a equivalent Concentric. The slight increase in air velocity should provide a stronger signal (more vacuum), and by itself, allow ambient air pressure to lift the required fuel and keep a stoichiometric mixture.
In fact as air velocity increases it doesn't provide a linear vacuum. As air flow increases the vacuum signal increases greatly in a non-linear manner. So much that an air bleed has to be provided to keep the the mixture from going extremely rich. So this isn't where the answer lies...
What goes a long way to help explain the larger main jets required, is the distance the fuel needs to be lifted. With the Monobloc It is about 1/3rd more than the Concentric. We don't have that much more vacuum with the Monobloc. In fact when the slide is closer to 3/4 throttle you probably have less. But we still seem to need that larger main jet. The only way we can get more fuel at this point is decrease the resistance to the fuel flow by enlarging the main jet.
The Monobloc, for a given air velocity would require more flow (larger jet) to compensate for the height the fuel has to be moved with a given vacuum as compared to the Concentric.
Both of these carburetors use the primary and secondary transfer ports through full throttle to supply some fuel, but as the slide lifts it is less, and less, as a percentage of the fuel required. There isn't any other source of fuel. ----------------------- Both TT and GP carbs have tea cup slides. ------------------------ Burlen has billet anodized aluminum slides for the MKI Concentric. They are standard in the new Premium models. ------------------------ You can buy plastic fillers made for Mikuni VM carbs and modify them for the MKII, but I have not seen any for the Concentric. Because of the design you could not use any type of slide filler in the TT, GP or Monobloc. This is fussy stuff and requires a of time on a flow bench with the main jet connected up to a manometer. Smoothing the underneath of the slide greatly increases the vacuum signal at all throttle openings, not just at WOT like the smoothbore designs, and can make acceleration nearly instantaneous. With modern GP bikes it makes the bike unrideable without engine management computers. The rider might turn on the throttle, but the ride-by-wire computer does a whole lot of calculations before it starts to lift the slide.
I would be gratefull if someone could clarify the effect of blocking / increasing the size of the cross drilling of the needle jet, This is on a TT carb.
Might be worth clarifying what you want it to do. Is there a running problem.
On Mk1 Concentrics, blocking the bleed holes is a modification attributed to the Vincent Owners' Club and Dave Degens. It sometimes fixes a problem with throttle response off overrun, apparently caused by momentary richness. The glitch is described as "fluffing."
I have blocked the holes with solder on the twin Mk1s on my Triton. Before I did that, I'd typically be slowing down from 70mph to 45mph for a corner or bridge and when I applied some throttle to carry me round, the bike would actually dip down on the forks as the engine hesitated. It was very annoying. Stangely, blocking one of the two holes on each carb's needle jet made the problem worse. Blocking both holes cured it.
It's only fair to say that such response problems can have any number of other causes, which will not be fixed by blocking holes.
If a TT is a racing carb, will it ever respond well at low speed and small throttle openings?
The needle jet cross drilling has its greatest effect while the straight Portion of the needle is still in the needle jet. It is an air bleed, and as such, making it larger reduces the vacuum signal on the jet (which is just below the holes) and it will draw less fuel. As the needle is raised, and the taper takes over, the bleed has less effect on the mixture.
I have not examined the relationship of the TT needle and needle jet, but on a Concentric the straight portion of the needle doesn't leave the jet until about 1/3rd throttle (depends upon needle clip position).
Thanks John,that was the answer I was looking for.The carb is a Dellorto SSI and appears to be a close copy of the TT,Replacement jets are not availiable so I make/modify what I can get.Some are cross drilled,some are not,some parts are pattern ,I'm on my own with this one.
Joined: Aug 2006 Posts: 188Pelle
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I took the B40 out to play yesterday, after two weeks of inactivity. Immediately the engine acted up, as if the carb magically had re-jetted itself. Taking the carb (Concentric) apart, I found the cross-drilled holes in the needle jet completely blocked by some white, powdery stuff. Removing this restored normal running. I suppose the white deposit is an unwanted effect of modern fuel. However, it surprised me to find this in a carb that's used rather often. Pelle
Just before John comes back with the right answer I did notice that your idle mixture screw was 1 1/8. That seems on the lower side - I wonder if there is enough total mixture able to flow from the pilot jet system
One way of checking if the mixture is too lean (or rich) is to slowly introduce the choke. If the performance improves then the carb is too lean: if it gets worse then it is too rich.
Looking at your description of what happened before you changed from a 106 to a 105 needle jet, could it be the wrong way round? I would have expected the mixture to be too lean at 1/2-3/4 throttle and too rich at 1/4-1/2 throttle, not the way you put it.
If you want to check the idle jet size (assuming the mixture is correct at "kerb idle"),increase the idle rpm by about 500. Then see if the air screw is still in the best position for fastest idle.
The 105 needle-jet should have weakened the mixture from about 1/8 throttle to 3/8 throttle (most effective at 1/4 throttle). At higher throttle opening the clip position would have some effect.
#461656 - 11/02/1210:42 pmRe: Tuning Your AMAL Carburetor
Joined: Aug 2006 Posts: 188Pelle
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The article. Take a close look on the illustration on p. 2...but do not try to assemble your Concentric that way! I´ve seen that illustration elsewhere on the web, probably where it originated. It's a nice attempt to explain the inside of the Concentric, but somewhere it just went wrong. Apart from that, the text may be very helpful.
I've never had any trouble adjusting the brass seat to get 0.080" float height,even cold. I know there can be trouble with the stay-up float,trying to get 0.170" - 0.240" fuel level without the float fouling on the main body.
#461768 - 11/03/1211:16 pmRe: Tuning Your AMAL Carburetor
Hi, I had recently placed an order with Burlen (AMAL)for 2 new concentrics to fit his 67' bonnie (928 - close match to the original 1" 1/8 carb)
Anyway, as a distributer I decided i'd take the hit in the small price difference and order him the premier carbs, plus a few extras.
All new Amals now come with the "stay-up" puncture proof float and aluminum float needle. However for the small amount extra you receive a hard anodized slide ( which is in the new style casting, like the former chromed slides. The standard mazak slides are still using the original type casting) and a removable pilot jet. The pilot jet was what caught my curiosity, I was sure how it would/could function. Anyway its removal made things a little clearer and hopefully if your curious too - the pictures will help.
Now, because my friend isn't currently running chokes, I made the option open to him to use either or, his cables are the ferule type with mid-way adjusters. (mounting ferules do come with the kit - not pictured) for running none chokes, you need a carb top with a screw in choke section, Burlen sell a screw in plug to fit into the top, so no more jamming a ball in the hole or something else which might work loose.
Whilst I had the carb stripped down, I took the liberty of photographing inside the body. The 2 "bleed holes" are which help the carb come off no throttle (tick over) onto the main carburetor, if these are blocked the bike may stall, run lean or fail to pick up, this results in people fitting richer slides and raising needles. This blocks from the poor quality fuel available today and the fact many people don't ride their bikes as regular as they would like to be ridden. Bushmans carb secrets ( http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html ) nor the leaflet which comes with the carbs mentions those 2 bleed holes (and if it wasn't for John Healy, I wouldn't know either)
Hope this is helpful and shows what is available on the market.
After ordering a batch of carbs recentl(one being a generic 2 stroke 930 carb) I had noticed that AMAL had taken some of the improvements from the premier carb and fitted them into the non premier bodies.
As we know, ALL carbs now have the aluminium needle and Stay-up float. The new carbs now have the pilot bush side of the carb drilled out (As the carb I ordered is of 2 stroke origin, it comes as standard screw in pilot jet in the float float bowl region). The drilled section has been and a blanking plug fitted, identified by its cross head and silver colouring (as opposed to the yellow passivate look on the premier pilot jets)
This should help clear some of the cleaning issues, on cleaning out pilot jets (providing AMAL do this same mod on 4 stroke carbs with the pilot bush fitted)
You can mount a Premier style pilot jet in this position, but IMO as a prem pilot jet costs around £12, you are about £12 away from the cost of a Premier carb with a Hard Anodised slide (which would cost around £28 if bought separately)
AMAL are also now producing Premier kits for the Triple range of BSA/Triumph, as you can imagine these are very well built and come fitted with the correct 4 stroke upgrade kit (original carbs came with a 2 stroke setup on Rocket 3's) fitted with correct needle, needle jet, jet holder and semi slash cut spray tube as fitted to later T160's
This is exactly the 'problem' with analogue (non digital) electronic ignition systems. They have the ability to retard the timing way too far at low rpm which makes the idle less stable.
I was always confused by this quote, which in one form or another, has been around for almost as long as we have been fitting electronic ignitions to these bikes. I have routinely been able to set a Bonneville's idle at a stable, and rideable 500 rpm with a analog EI. I find idle problems to be more related to the carburetor than the EI.
The idle stabilization, introduced when Boyer Bransden started using a specialized automotive computer chip to control the advance/retard, was able to compensate for varying idle rpm. It does this by advancing the timing when rpm starts to drop. All this is doing is masking the real problem... an improperly set-up and maintained carburetor.
Set-up, and maintain your carburetor and you can have as steady an idle with your analog EI.
I have a 1967 BSA Lighning with AMAL 930 Concentric Carbs. I am in the process of getting this bike running and willbe rebuilding the carbs soon. I have a questions about jetting. The bike was originally located in Colorada. I live in Phoenix Arizoona. There is a considerable elevation change and was wondering if the lower elevation will affect the performance of the bike. Will I need to repalce the current jets? I would like to get the jets close to where they need to be since the carbs will apart. I don't the existing jet size yet. Thanks.
At 3,000 to 5,000 feet a change of 5% would be in order. Altitudes in excess of 5,000 feet up to 8,000 feet another 4% should be considered. Most dealers in Denver have been fitting a .105" needle jet and a size or two smaller main jet.