1000-Series 2/4-stroke Concentrics

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1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sat Mar 30, 2019 2:27 am

I have a question about 1000-Series Amal Concentrics that I hope someone here can answer. Making a long story short:

I'm reconfiguring a 1036 to use on my DBD Gold Star. I realize that it's not a unit BSA,[*] but please bear with me for a moment. Few large British singles remained in production by the time the 1000 series was introduced so most of these carburetors were configured for 2-strokes until disappearing in favor of the Mark 2. The only 4-strokes I've found that used 1000s were the 1970 Velocette Thruxton (1036) and 1974 CCMs (1036 and 1038). Anyway, probably as a result of this there is very little technical information to be found about the differences between 2-stroke and 4-stroke Concentrics other than they use different needles, needle jets, and jet holder, and that the 2-stroke spray tube is cut with a slash.

To set up my question, one relevant fact is that Amal's generic recommendation for their Mark 2 is to use a 3.5 air jet for 4-strokes and a smaller 2.5 for 2-strokes. Although it doesn't ever seem to be discussed, the second relevant fact is the compensating air passage to the jets in 4-stroke Mark 1 Concentrics is ~0.189"-dia. for all three series (600, 900 and 1000). However, in a 627 from a Bultaco Sherpa-T that I happen to have the air passage necks down to ~0.098"-dia. just before reaching the jets. I find the same ~0.098" constriction in the much larger 2-stroke 1036 and 1038 that I also have.

OK, finally, here's my question: should I drill out that passage in my 1036 to give it a constant 0.189"-dia. all the way to the jet cavity? Rather than speculation, what I'm hoping for is that someone has actually done this and has data on this specific question to support what they did. Or, absent that, perhaps someone reading this owns an original Mark 1 Concentric that came with a CCM and knows (or is willing to measure) if its air passage has a constant diameter, or if it is necked down to form the equivalent of the Mark 2's smaller 2-stroke compensating air jet.

Thanks in advance for your help with this.

[*] I own a 1963 BSA C15S so I'm not a complete interloper to this forum.

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Re: 1000-Series 2/4-stroke Concentrics

Post by madsundaysurvivor » Sun Mar 31, 2019 9:12 am

Hello magneto,
I dont know if I can help you
In spain the manufacturer with license of amal carbs was Arreche carbs and I think I will be possible to find in the store room :roll: some drawings with sizes.
I dont speak well english and have a doubt, Do you refer to conduct which intersects with the conduct after the air jet?
With the changes you mention, I have used carburetors of 2t bultacos in 4t engines. apart from the gasoline steps the main one was to change the spary cut tube for one cylindrical shape, discard idle jet and press a Bush in the conduct, like did amal in your four strokes units.
If I remember correctly, because this have been many years ago, with the mk2 there was not this problema, only change the pilot, jets and needle,

Is not the issue, but only for information eventually I eventually end up using dellortos and then mikuni
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sun Mar 31, 2019 3:58 pm

madsundaysurvivor wrote:
Sun Mar 31, 2019 9:12 am
Arreche carbs and I think I will be possible to find in the store room :roll: some drawings with sizes.... Do you refer to conduct which intersects with the conduct after the air jet?
It would be great if you could find information from any drawings they might still have. Yes, the conduit I'm asking about is the one along the centerline of the carburetor below the main 36 mm bore. There are three passages: one not on the centerline is blocked, one on the other side of the centerline leads to the pilot circuit, and the one I'm asking about it runs down the center and leads to the cavity for the jets.

The issue with the angle of the spray tubes is one that also interests me. Amal made those tubes in four configurations: angle-cut for 2-strokes, straight cut for most 4-strokes, and two special cuts for Trident/Rocket3s and for Nortons. I have a Norton 932 and tomorrow will receive a 2-stroke and a straight-cut 4-stroke 932. I've already made a manifold for my flow bench and will measure the effect of the various spray tubes on the relative pressure above the needle jet. I've never seen information on this published anywhere so the results should be quite interesting.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Sun Mar 31, 2019 4:46 pm

This is getting interesting. I knew there were differences in the 2 stroke and 4 stroke Amal. But I had never heard of a difference for Norton or the 3 cylinders. Are these differences in any of the Amal Tuning pamplets that Amal used to publish?
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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Sun Mar 31, 2019 4:52 pm

I just found this old post about the 2s vrs 4s Amal differences. I am not sure if it apply to the 1000 series. Do some web searching for old parts suppler catalogs of the era. They should have the information.

http://www.britbike.com/forums/ubbthrea ... mber=50499

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sun Mar 31, 2019 5:11 pm

Jeff K wrote:
Sun Mar 31, 2019 4:52 pm
I just found this old post about the 2s vrs 4s Amal differences. ... Do some web searching for old parts suppler catalogs of the era. They should have the information.
Thanks for your post. However, before writing my post to B50.org I searched everywhere I could think of, including every catalog I could find. Unfortunately, there is very little information available, and what is to be found is contradictory. For example, you can find authoritative-sounding posts saying the slash cut on 2-stroke spray tubes makes the mixture richer, and others that say it makes it leaner. Hopefully, the measurements I'll make on my flow bench this next week will provide a definitive answer.

As for the air compensating passage in 2-strokes vs. 4-strokes, I have found exactly nothing about it. By inference from the fact Amal recommends a smaller air jet for 2-strokes in their Mark 2, the restriction in the passage in the two 2-stroke Mark 1s I've measured wouldn't be present in 4-stroke versions. But, I'd like actual information on, say, one supplied to CCM to be sure.

Amal supplied a 1036 for 1970 Velocette Thruxtons. However, only 140 of those were produced before the factory shut and, it seems, the reason for the 1036 was a theft of the GPs they had used previously. Amal's records show the same body was used for those 4-strokes as for a Maico, but I can imagine that the factory was in no position to request anything special at the last minute so they used what they could get. Perhaps the situation was different by the time CCM ordered 1036 and 1038 a few years later.

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Re: 1000-Series 2/4-stroke Concentrics

Post by minetymenace » Sun Mar 31, 2019 5:25 pm

<135 to the forum Magnetoman.

Would you update your profile and let everyone know roughly where you are please?

Before searching the net, try searching this forum, there is lots of historic stuff here. I'd try itr without drilling it first...but I'm no expert. This is page has a host of Amal stuff...
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sun Mar 31, 2019 5:44 pm

minetymenace wrote:
Sun Mar 31, 2019 5:25 pm
Would you update your profile and let everyone know roughly where you are please?
Done.
minetymenace wrote:
Sun Mar 31, 2019 5:25 pm
Before searching the net, try searching this forum,
Trust me, I searched B50.org (and everywhere else I could think of) before joining B50 and posting my question. I have a "Carburetors" tab in a bespoke Gold Star shop manual I created for myself. That tab is 3/4" thick (~190 pages) containing everything techncial I've found over the past ~20 years about GP/TT, Monobloc, and Concentric carburetors. I could be wrong, but I suspect the information I've asked for is not contained in anything in print or on the web, but only in someone's notebooks of measurements and experiments they've conducted themselves.
minetymenace wrote:
Sun Mar 31, 2019 5:25 pm
I'd try itr without drilling it first...
Yes, that's reasonable advice. But, I'm hoping someone has gone through all of this before and has the results in a notebook.

I have a wideband air/fuel meter and will be sorting out the jetting of the 1036 using it. It's just that it would shorten the process if someone had a definitive answer to the question I posed in my first post.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Sun Mar 31, 2019 6:10 pm

So it is 1036 data you need. I am off to my mother's birthday party (86). When I get back I will look through some of the old dealer catalogs that I have, The older Domi Racer catalogs used to have a lot of data in them and I have almost a full set of them.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sun Mar 31, 2019 6:24 pm

Jeff K wrote:
Sun Mar 31, 2019 6:10 pm
So it is 1036 data you need.
Thanks for that offer, but it's very unlikely anyone will find the information I'm looking for in catalogs, including the ones from Domiracer (I have a full set of them, from which long ago I extracted everything relevant about carburetors). Basically, catalogs contain information of the sort "for this model motorcycle buy X, for that model buy Y," and Amal literature is good for telling you how to work with the particular carburetor that is specified for a particular bike. What I'm looking for is technical information on the how the main 1000-series casting is machined for 2-stroke vs. 4-stroke applications.

Given how much I've already looked, for present purposes it's a pretty good assumption that the information I need will not be found in anything in print, and is very unlikely to be found in anything posted on the web. If it exists, it will be in someone's own notes, or possibly in the form of an original 1036 or 1038 supplied for a CCM. I have to hope that someone who has the information frequents B50.org, sees this thread, and responds.

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Re: 1000-Series 2/4-stroke Concentrics

Post by JB » Mon Apr 01, 2019 12:22 am

I wish I could help with a direct answer to your question but I can't, I've only ever owned one 1000 series carb from a CCM 580 which was sold many years ago.
I never knew there was a difference between 2t & 4t Amal carb body machining on the central air feed for the jet block, it's not something I've come across over the years. As you say the other differences are well documented. The spray tubes differences appear to be straight forward until you come across the anomalies that appeared later on, which only seem to confuse things!
I'm aware of the non standard spray tube types fitted to the T160, MK3 Commando and also the later T100R Daytona's, what they all seem to have in common is that they were an attempt to resolve fueling issues as a result of tighter noise regulations. The Commando's solution in particular was the most drastic that did solve a serious loss of mid-range acceleration that worked very well, I have some info on the technical reasons for this somewhere and can find it if you want?
I've done a small amount of experimenting with different spray tubes using a wide band sensor the only conclusion I came to was that small changes seem to make either a big difference and throw out ALL settings across the board or hardly any difference at all! (flat, various angles and cut away) Needle types(all 4-stroke types) and needle jets (105-106+) make a big difference whereas main jets, needle position and slide cut away are all minimal.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Mon Apr 01, 2019 12:43 am

You are correct, very little in any catalogs even in the 1970 to 1973 range when it was being made. By the late 1970 all they show are common(900) replacement parts. Also nothing in any carburetor books. Roy Bacon's Carburettor book from his Restoring Motorcycles series has only two brief paragraphs on page 46. They did note that the 1000 came with a 0.125" seat versus the 600/900 having a 0.10". And there being a optional 0.156" for use with Alcohol.
Looks to be a very rare carburetor with a very short production 1970 to 1973 when the MK2 came out.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Mon Apr 01, 2019 12:51 am

This link (Amal) has photos of the four different spray tubes used.

http://amalcarb.co.uk/rebuilding-mark-1 ... arburetter

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Mon Apr 01, 2019 1:48 am

JB wrote:
Mon Apr 01, 2019 12:22 am
I've done a small amount of experimenting with different spray tubes...
Jeff K wrote:
Mon Apr 01, 2019 12:43 am
very little in any catalogs even in the 1970 to 1973 range when it was being made.
Actually, there's very little (i.e. none whatsoever, as far as I can tell) information about difference between the smaller 2 and 4-stroke 600 and 900-Series bodies, either. Possibly the only reason spray tubes are mentioned by anyone is they are a very obvious visual difference. And, even then, discussion of them seems to go no deeper than "4-strokes use this shape, 2-strokes use that."

For what it's worth, I just spent the day (and part of yesterday) preparing my flow bench for a series of measurements. I had to fabricate an adapter flange and a rigid way of reproducibly setting the slide height. I have a 932 Concentric with Norton spray tube and on its way to me is a box with 2-stroke and straight-cut 4-stroke 932 bodies. I'll measure the pressure in the jet cavity of all of these under identical conditions of flow and slide-heights to experimentally determine the effect of spray tube shape. I hope I'll be able to unambiguously infer from the measurements the effect of the "air jet" (i.e. constriction) in the air compensating passage of the 2-stroke 932, which will tell me whether or not to drill it out of my 1036. It would have been easier if someone here had the answer but...

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Re: 1000-Series 2/4-stroke Concentrics

Post by BSA_WM20 » Mon Apr 01, 2019 2:01 am

Funny how one compartmentalizes ones brain.
Same post on a different forum brings different things to mind
Stan Millard ( Hp by Stan ) would be the go to person.
Fairly sure he did a bit on amal 1000's some time back.
He has not really posted much since his hospital visit last year but might respond to a P M

I assume that you have already found this thread
viewtopic.php?f=3&t=855&hilit=amal+1000
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Mon Apr 01, 2019 2:24 am

BSA_WM20 wrote:
Mon Apr 01, 2019 2:01 am
Same post on a different forum brings different things to mind ... I assume that you have already found this thread...
Trevor, I joined B50.org at Shane's urging. He thought someone here might have the answer I seek. Yes, I had found that thread some time ago but it doesn't address my question.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Mon Apr 01, 2019 3:43 am

I am sure that you have seen this, but if not, here it is. It has some jetting data on the 1000 series.

https://triumph3ta.nl/Downloads/Amal%20 ... Manual.pdf

and this

https://partsbooks.britishonly.com/part ... mbined.pdf

1036 on the bottom
https://v-tec.velocette.info/carbs.html

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Re: 1000-Series 2/4-stroke Concentrics

Post by madsundaysurvivor » Mon Apr 01, 2019 8:49 am

Now I havent a 1000 serie, but checking parts in the disaster drawer I have seen another differeces that I didn't remember:

1-the needle jet in 4t have a hole in the nut in 2t no
2-the main jet holders are differents, in 4t is more long than in 2t, and consequently the main jet is more far of the carb body, about 3 mm.
I think that this is for the fuel weight and the aspiration of the engine, (more heavy in 2t)

I comfirm your measure of the hole in amal 900 series 4t.
I will try to find the spanish amal brouchure, but think that no will find this diameter measure.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Tue Apr 02, 2019 12:10 am

A note in the Superflow flow bench Operator's manual mentions that carburetor tests sometimes need to be run at a reduced test pressure, as opposed to the 30" H20 that might be used on a head. The manual gave 1" H2O as an example so I started out there but then doubled the test pressure. This gave a manometer reading of 0.061 psi in the jet cavity of a 932 body with a Norton spray tube and with its 3½ slide approximately halfway open.

The Superflow's feedback controller holds the pressure and flow rate constant such that averaged over a short time reduces the error to below 1%. Further, Superflow claims measurement repeatability of +/0-.5%, which is important for switching between carburetor bodies. Anyway, my test today showed that with my present setup I can achieve better than 1% precision measuring the pressure in the jet cavity.

OK, my setup can measure effects at the 1% level, but will this be sufficient to look for relevant performance differences between the various spray tubes and air correction passages? Raising or lowering an AMAL needle by one notch changes the mixture by ~12%, which certainly has an effect in the region where the taper is active. Mikuni gives a finer control of ~6% on their needles. Changing an AMAL needle jet by one size (e.g. from .106 to .107) has a similar ~10% effect on the mixture, and needle jet wear of 0.0005" changes the mixture strength by enough to be noticeable, which is ~5%. Anyway, what these examples show is the ability to measure 1% differences between the three otherwise-identical carburetors will be sufficient to disclose what effects the different spray tubes have at various throttle settings. One other thing is I have a miniature Pitot tube for probing air velocity. I won't know until I try whether it will reveal any useful information about the actual air flow in close proximity to the spray tubes.

The 932 bodies with a "standard" flat-top 4-stroke and a slash-cut 2-stroke spray tube (and restricted air correction passage) are supposed to arrive in todays mail to go with the 932 "Norton" body I already have so I hope to start measurements tomorrow. Doing such measurements carefully takes time, and we have out-of-town guests arriving the day after tomorrow, so it might take a week before all the results are in.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Tue Apr 02, 2019 3:08 am

I am impressed and I will be following your results!
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Re: 1000-Series 2/4-stroke Concentrics

Post by BSA_WM20 » Tue Apr 02, 2019 4:08 am

Magnetoman wrote:
Mon Apr 01, 2019 2:24 am
BSA_WM20 wrote:
Mon Apr 01, 2019 2:01 am
Same post on a different forum brings different things to mind ... I assume that you have already found this thread...
Trevor, I joined B50.org at Shane's urging. He thought someone here might have the answer I seek. Yes, I had found that thread some time ago but it doesn't address my question.
Send Stan a PM.
He did a lot of work on his flow bench & was regularly myth busting about concentrics.
Minety might have a phone number or private email address in the records us mere members are not privy to.
Should have thought about Stan a while back.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Tue Apr 02, 2019 4:26 am

BSA_WM20 wrote:
Tue Apr 02, 2019 4:08 am
Send Stan a PM.
He did a lot of work on his flow bench & was regularly myth busting about concentrics.
At this point I'm far enough into this briar patch that I want to find answers for myself unswayed by the findings of others (assuming those findings exist[*]). Once I've made my measurements I'll see if I can contact him in order to compare notes.

[*]Where did he post his work on Concentrics? I searched this forum, and everywhere else google took me, but didn't find anything relevant to my question, which is why I started this thread.

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Re: 1000-Series 2/4-stroke Concentrics

Post by minetymenace » Tue Apr 02, 2019 9:01 am

Sometimes things just happen and the real reason is lost in time, there may not even be a real reason, I have seen many accidental changes occur that people or systems can't be bothered to reverse, and if a change was unintentional and/or ineffectual, it may not get written up. This even happens in the highly regulated ISO9001 industry I work in, so I'm sure there will be a few obscure inexplicable anomalies in the Amal production history. The different models for Norton and Triumph is probably more to do with brand identity than any scientific reason.

Stan is a fountain of knowledge and well worth contacting, but quotes from him would indicate that he thinks that carburettors are made better by other manufacturers rather than by fettling Amals.
HPbyStan wrote:I'm always amused by the "how do we make our Anal Concentric carbs actually carburate" questions. This place does have mechanics and tuners BUT damn few magicians me thinks
HPbyStan wrote:I can't believe anyone would fool with an Anal carb for a year.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Tue Apr 02, 2019 4:14 pm

minetymenace wrote:
Tue Apr 02, 2019 9:01 am
there may not even be a real reason, ... The different models for Norton and Triumph is probably more to do with brand identity than any scientific reason.
Starting with the Air Quality Act of 1967 automotive manufacturers were faced with tightened emissions requirements in the U.S., with additional requirements introduced with the Clean Air Act of 1970. Continued access to the U.S. market provided a very real reason for carburetor manufacturers to step up their game at that time.

Tailpipe emissions were something that were measured so even if we think workers at Amal blindly messed around until they found something that worked, and didn't understand why a slashed spray tube allowed one bike to meet standards while a notched worked in another, those shapes were a matter of functional necessity, not fashion. As for brand identity, I doubt even a single customer ever removed the air filter on the showroom floor to look inside a carburetor of a bike they were considering buying, let alone used the shape of the spray tube to influence their purchase decision.

There is no doubt the shapes of the spray tubes and the presence of the restriction in the air compensating tract of 2-stroke carburetors are for functional reasons. The experiments I'm about to conduct with my flow bench should provide quantitative data resulting in a scientific explanation for these features. Of course, after I post the results I fully expect some people to say "That's obvious, I knew it all along"...

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Tue Apr 02, 2019 4:40 pm

Looking back at the Norton 850 MKIII that I had purchased new and then looking at a Commando 750. Just looking at the exhaust on the 1976 850 MKIII - Huge black capped mufflers, crossover pipe and then look at that huge enclosed air box. All this was done not only for "emissions " but to quiet the bikes down. Then look at a 750 . Basic Pea Shooter mufflers with low back pressure and a large paper air filter. Messing with Exhaust and intake had to require the resulting adjustments to the Carburetor.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Tue Apr 02, 2019 4:59 pm

Jeff K wrote:
Tue Apr 02, 2019 4:40 pm
that huge enclosed air box. All this was done not only for "emissions " but to quiet the bikes down.
The U.S. Department of Transportation is the other important U.S. agency needed to understand why and when things happened. The E.T. ignition system disappeared from bikes in 1967, not because of any technical reason, but because after that date motorcycles were required to have a brake light that still functioned when the engine was off. The crossover of the shifter from the right to the left in 1975 was again due to a DOT regulation.

In some cases there is unintended interaction between DOT and EPA regulations. For example, DOT mandated a lower noise standard, which only could be met with a new airbox and silencers, which affected carburetion, which affected emissions. In the end both the DOT and EPA standards had to be met so both noise and emissions had to be solved together because they were interrelated.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Tue Apr 02, 2019 11:05 pm

Another little tidbit. Early H*nda 150 twins had turn signal on them stock every where in the world that they were sold. That is except the USA. We had a old law back then that the turn signal had to be a certain distance apart, Look at a full dress Harley's rear signals with the long stalks. They were way to big for the little H*nda so H*nda just do not install them on USA bikes. They had to use a special handle bar switch and blanking plates on the USA models. So the DOT effectively out lawed a safety feature being used world wide. :shock:
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Re: 1000-Series 2/4-stroke Concentrics

Post by minetymenace » Wed Apr 03, 2019 9:04 am

Magnetoman wrote:The experiments I'm about to conduct with my flow bench should provide quantitative data resulting in a scientific explanation for these features.
..............or not. I would suggest that a hypothesis is proposed before the tests are performed so that you are not tempted to predict the results.

While the US was the largest market it was by no means the only market for this British manufacturer, I agree however that at the time, the legislation in the US drove the majority (but not necessarily all) of the changes around that time.

The interaction between all the elements will inevitably lead to different manufacturers arriving at different solutions to the same problem. As previously been stated, emissions and noise would be the focus of the design process, but lead content, ignition & valve timing (and lift), chamber shape, required torque curve, and the restraints of the existing chassis design all play their part. With so many variables, trying to establish the reasons for a particular carb design, using just a flow bench will be a challenge.

I don't mean to discourage you form performing tests and producing data, but caution against the assumption that your results will provide the answers you seek. Please publish all your data as I anticipate it being very interesting.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Wed Apr 03, 2019 3:23 pm

minetymenace wrote:
Wed Apr 03, 2019 9:04 am
I would suggest that a hypothesis is proposed before the tests are performed so that you are not tempted to predict the results... caution against the assumption that your results will provide the answers you seek.
My hypothesis is that the tests will provide the answers I seek.

Seriously, this is straightforward, and my preliminary measurements already show I have the resolution to discern differences well outside error bars. Now it's down to the time-consuming slog to actually make the comparative measurements.
minetymenace wrote:
Wed Apr 03, 2019 9:04 am
trying to establish the reasons for a particular carb design, using just a flow bench will be a challenge.
I think you misunderstand. This isn't an attempt to read the minds of designers, but rather to determine the differences in behavior of the final designs.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Wed Apr 03, 2019 3:34 pm

Unleaded gas and low leaded gas showed up around 1970. But it was in 1975 that the mandated catalytic converters were here and with them was the wholesale switch over to unleaded , electronic ignition and some horrible early fuel injection units. Most gas stations stopped selling Ethyl gas about then and converted those pumps to unleaded. I can remember the problems with valve seats showing up a couple of years later. I do not know why, but it was very rare to see a Norton with worn seats, but very common on the BSA and Triumph. Harder seats? I also remember the drop in octane causing problems with spark knock. Again Norton seem less prone to this. I was told it was largely done to the flat top piston design versus the domed/hemi and the effects on the flame front.
All of this effected carburetor design and function. Fist it was the noise restrictions and then in the mid /later 70's it was the emission started for bikes also.
How many here are old enough to remember leaded regular and ethyl. Some stations even used to sell pre mix 2 cycle gas at a pump( mostly for boating) and white gas too. Also not that many stations sold Diesel fuel at the time. I can remember as a kid in the 60's working at a gas station. We pumped the gas, cleaned the windows, checked the oil and the wipers. We would get .10 cents for every quart of oil and .25 for every wiper sold. And we put it in and installed them free.
One thing that I do like about the unleaded is how clean a high milage engine is compared to the old days when rebuilding it. The old blow by with leaded gas used to turn the oil black in no time.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Easterwood » Thu Apr 04, 2019 3:04 am

Courtesy of the Gold Star Forum of Britbike.com. As suggested for DBD34.

Recommended settings for a 1038 Concentric are:
Mainjet: 380
Needle jet :622/122/108
Needle :622/124
Pilot jet :30
Throttle slide :3

Hope this helps.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Thu Apr 04, 2019 5:06 am

Easterwood wrote:
Thu Apr 04, 2019 3:04 am
Hope this helps.
Thanks very much for taking the time to post this, but suggested settings aren't what I'm looking for. I'm interested in differences between 2-stroke and 4-stroke bodies. Also, note that the settings you posted don't list which type of body they're for, e.g. which type of spray tube it has.

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Re: 1000-Series 2/4-stroke Concentrics

Post by ghislain » Thu Apr 04, 2019 9:02 am

On the french forum about british bikes , a friend tells he managed to open up the central passage up up to 4.8 mm and says it runs now nicely on his Velocette Thruxton ...
sur une VMT: achat d'un 1034 2 temps (neuf) montage du kit conversion 4T, remplacement spray tube, et beaucoup d'essais, bof...
je me rends compte que le fameux trou central est trop petit, comparé au 1036 démonté dont l'alésage est de 4.8 mm.
Rien à perdre, donc porté cet alésage à 4.8 mm et maintenant cela fonctionne très bien, hormis au démarrage ou il faut
en partie obstruer l'admission d'air quelques secondes (il n'y a quand même pas de choke)
En fait, j'ai l'impression que beaucoup de carbus série 1000 sont à l'origine des 2 temps prévus pour Bultaco et transformés à la mode de chacun.
Mon avis est que si une VMT démarre trop facilement, la carburation à bas régime est trop riche et va encrasser la bougie, ce qui était le cas avec l'ancien 1036.

https://www.motos-anglaises.com/phpBB3/ ... SA#p350984
Translation by the moderator wrote:On a VMT: I purchased a 1034 2-stroke (nine) 4T kit conversion kit, replacement spray tube, and a lot of tests...
I realised that the famous central hole is too small, compared to the 1036 disassembled whose bore is 4.8 mm.
With nothing to lose, I bored this to 4.8 mm and now it works very well, except at startup unless the air intake is partly obstructed for a few seconds (there is still no choke)
In fact, I have the impression that a lot of 1000 Series carbs were originally planned for Bultacos and then transformed to suit other needs.
My opinion is that if a VMT starts too easily, carburation at low speed is too rich and will foul the plug, which was the case with the old 1036.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Thu Apr 04, 2019 5:39 pm

ghislain wrote:
Thu Apr 04, 2019 9:02 am
On the french forum about british bikes , a friend tells he managed to open up the central passage up up to 4.8 mm ...
Translation by the moderator wrote:On a VMT: I purchased a 1034 2-stroke (nine) 4T kit conversion kit, ...
Merci beaucoup to ghislain for posting this valuable information, and to the moderator for the translation. I've been to France many times, most recently to lovely Besançon in November, but my skill at technical French is not equal to my love of the country.

Although a small amount of not always consistent information can be found about spray tube shape, the air passage seems to be the component that dare not speak its name (to misquote Lord Alfred Douglas). The information in this French post is consistent with what I've obtained with my preliminary flow bench measurements so it is great to read that someone else has come to this conclusion. Out-of-town guests arrived yesterday so I'll only be able to steal a few minutes on the flow bench here or there until they depart on Sunday, but I hope to have a full set of repeatable measurements within a week.

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Re: 1000-Series 2/4-stroke Concentrics

Post by minetymenace » Thu Apr 04, 2019 8:18 pm

I do hope you share your results, it is quality info like this that keeps this forum alive. :thumb
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Fri Apr 05, 2019 5:30 am

minetymenace wrote:
Thu Apr 04, 2019 8:18 pm
I do hope you share your results,...
I'm not into old motorcycles for the vast amounts of money, and haven't signed any Non Disclosure Agreements, so I do plan to share the results.
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Re: 1000-Series 2/4-stroke Concentrics

Post by tridentperu » Fri Apr 05, 2019 6:12 pm

This is an extremely interesting post even to an Amal luddite like me! So far I have learnt about the percentage differences a needle notch / needle jet change can make and more importantly, how a tiny amount of wear on the needle in the tapered bit can make a fair difference.

Re the special angled cutaway on the spray tube, this was only for the triples I believe and it affected the high rev carburation. The std 4 stroke flat topped spray tube makes the mixture too rich. This cut away was specific to the triple as it is so high revving - in comparison to a twin that is!

Also triple carbs were, originally, 27mm bore not 26 as the numbering would suggest.

There is a lot of historic discussion about this on the triplesonline forum, much of it by a well known expert on the marque

Will be v interested in the flow results whent hey are posted

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Fri Apr 05, 2019 8:02 pm

"Re the special angled cutaway on the spray tube, this was only for the triples I believe and it affected the high rev carburation. The std 4 stroke flat topped spray tube makes the mixture too rich. This cut away was specific to the triple as it is so high revving - in comparison to a twin that is!"

Since the 750 was basically three 250 singles, did the 250 singles also have the slanted cut out section? I have got boxes of the old Amals. Now I am going to have to dig them out and to check for differences in spray tubes. It could be interesting because I can take the numbers that are stamped on the carbs and check it against a chart and see what bike they came off.
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Re: 1000-Series 2/4-stroke Concentrics

Post by BSA_WM20 » Fri Apr 05, 2019 11:57 pm

MM will most likely post his final results here because he is a man of his word and he said he would.
However if you want to follow the work as it progresses search the same titled thread in the Gold Star section of the Britbike site which is where this thread originated from.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sat Apr 06, 2019 1:53 am

Jeff K wrote:
Fri Apr 05, 2019 8:02 pm
The std 4 stroke flat topped spray tube makes the mixture too rich.
After I started this thread it took two days to get the first response so I assumed it had fallen on deaf ears. However, although off to a slow start, it slowly gathered interest to the point where only three other 'Tech' threads (out of over 200) have received more replies in over a year.

I've managed to find more time at the flow bench than I thought I would. OK, hands raised, how many of you, like Jeff K, think a slant cut spray tube makes the mixture leaner (i.e. decreases the relative pressure above the spray tube at a given air flow and thus draws less fuel into the air stream than a straight-cut tube)? How many of you think it makes the mixture richer? Of course, one group of you has to be wrong, but the other group is only half-correct. It's actually more complex. I'll leave it with that annoying teaser for now because I'm in danger of being overwhelmed with data so there will be a pause while I connect a computer to the flow bench.

I've had a computer connected to it in the past, but it was a different computer with older flow bench software. The flow bench has an RS-232 connection whereas any computer made in the past decade has USB ports. Because of this the connection requires an active adapter and not all adapters work with all instruments at one end nor all computers at the other. With luck, it will be simple. However, it could be a rabbit hole waiting to draw me in.

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