1000-Series 2/4-stroke Concentrics

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sun Jun 09, 2019 5:21 pm

My goal was to see if I could jet a 1036 Concentric to work on a Gold Star without changing the spray tube or drilling the compensating air passage. That is, I wanted to determine if only the simplest swapping of a few screw-in components could make it functional. Yesterday the work I did to reach that goal paid off. While the specific jetting I found only applies to Gold Stars, the conclusions should be of interest to anyone who wants to put a 1000-Series Concentric on their B50.

First, some background. The "stoichiometric" A/F ratio for gasoline is 14.7:1. It varies from this depending on the ethanol content. Another way of displaying this is λ but calculating it also depends on the stoichiometric ratio of the particular fuel being used. Maximum power from most 4-stroke engines will occur for A/F ratios in the range 12-13, although for maximum fuel economy a ratio closer to 14.7:1 would be needed. If it gets much richer than 10:1 the mixture can fail to ignite repeatably, i.e. the engine will start missing, and if it is much leaner than 14.7:1 the engine can overheat and be prone to damaging pre-ignition.

I made A/F measurements under two sets of conditions, "static" and "transient." I made the former by holding the throttle in a fixed position for ~5 sec. before changing the throttle to a different position. For less than ~1/3 throttle these were made on relatively level ground as well as when going uphill and downhill. Higher throttle settings only were made going uphill to keep the speed within reason given the road and traffic conditions.

"Transient" measurements were made by snapping the throttle open. I use a 1.55 V battery in my throttle position sensor so the first graph shows a 15-sec. section of the data where I was in 2nd gear at ~1/3 throttle (the red curve), then snapped the throttle closed and back open, shifting successively into 3rd and 4th where I finally gave it full throttle. The magenta A/F curve shows brief lean pulses occurred roughly 1/2-sec. after each of these throttle movements, but none are excessive and I felt no hesitation from the engine. In fact, I couldn't be happier with how the engine felt throughout this entire run.

As can be seen from the first graph, even in regions where I kept the throttle constant the A/F curve fluctuates by ~+/-0.5 around a mean value. Added to this is the mean value at a given throttle setting at different times during the run (i.e. possibly under different conditions of, say, uphill one time and downhill another) could vary by nearly that much as well. As a result, I've drawn a band around a central line in the second graph to more realistically represent my measurements.

A few comments about the second graph: The curve starts at an AFR of 12.2 at idle. I set the idle after the engine was fully warmed up by adjusting the mixture screw to give the maximum rpm and this was the AFR that resulted. As can be seen, below ~1/8 throttle the mixture is richer than it needs to be. Comparing this region with the measurements I made with a #3.5 cutaway I judge that a #4.25 might be slightly better since it would raise the AFR in the valley at ~0.05 throttle. However, I don't anticipate spending much time riding this Gold Star with the throttle barely off idle so the fuel saved by making this change doesn't seem worth the effort.

For throttle settings between 0.2 and 0.5 the mixture is slightly leaner than the "optimum" 12-13 for maximum power. Raising the needle another notch isn't an option because that would make it much too rich in that region. However, there are two reasons I don't think this region is an issue. First, the 12-13 "rule" is just a guideline, and only time spent on a dyno would determine if more h.p. could be extracted with slightly richer mixtures. Second, most time on a machine is spent cruising at mid-throttle so having a slightly leaner, but still rich, mixture will save a bit of fuel without costing any actual performance.

Before disconnecting the Innovate AFR meter setup, re-installing the unmodified pipe, and declaring this episode 'done' I'll make at least one more jetting run to fill in a few additional data points between half and full throttle. However, I had a close call with a sheriff's car yesterday. I had just come down a hill and made a U-turn onto a side street and was waiting for traffic heading up the hill to get far enough along that I wouldn't catch up with it. Had I taken off 5 sec. earlier I would have been going well over 60 mph (with a 35 mph speed limit) when the sheriff's car came around the curve at the top of the hill.

What these results show is a 2-stroke Concentric can be made to work well on a 4-stroke bike by making only changes of components that screw into place, i.e. no drilling or pressing. It's worth repeating that these are the best results possible with an unmodified air compensation passage and 2-stroke spray tube, not necessarily the absolutely best results possible (still, my bike runs great with this jetting).
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AFR_8June19_01.jpg
AFR_9June19_03.jpg

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Re: 1000-Series 2/4-stroke Concentrics

Post by gunnag » Sun Jun 09, 2019 8:22 pm

Fantastic work MM and thanks for sharing this info which may be of some use on my B44 which currently uses a JRC 28 (Kehin PWK replica). I've often wondered about getting more performance from my B44 including gas flowing, bigger valves & carb etc. and why the unit single BSA's couldn't quite get the same performance as the earlier Gold Stars, maybe I need a 1000 series concentric? Anyway keep up the good work :)

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Mon Jun 10, 2019 6:35 pm

gunnag wrote:
Sun Jun 09, 2019 8:22 pm
why the unit single BSA's couldn't quite get the same performance as the earlier Gold Stars,
I'm sure the following has been discussed ad nauseum somewhere on this site before, but all h.p. is made in the head, and the upper limit on h.p. is determined by the total quantity of air that enters on the intake stroke. My SuperFlow manual gives the limit on the h.p. of a naturally-aspirated gasoline racing engine as CFM/1.67. Power can be less than this due to air that escapes because of valve overlap, or because of combustion inefficiency due to the design of the head, but it can't be greater.

A B50MX came with a 32 mm carburetor, whereas a Clubman Gold Star came with a 38 mm (1.5"). Aside from any differences in the intake ports, the air flow through those carburetors alone makes a very large difference in the h.p. potential of the engines.

Catalina Gold Stars had smaller inlet tracts than Clubmans and used 1-3/16" (30.1 mm) carburetors. Despite the smaller carburetor they produced ~37 h.p. at 6500 rpm, which I'm pretty sure is at least a few h.p. more than the B50MX. Assuming everything possible had been done with the B50's cams, this points to something fundamental with the design of the B50MX head as being the limitation.

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Re: 1000-Series 2/4-stroke Concentrics

Post by JB » Tue Jun 11, 2019 6:36 pm

Well done on succeeding with your quest of getting that carb to work differently to how it was originally supplied.
With my B25T trail bike I approached this from the opposite end from you in that I wanted the fueling to be as good as possible at the bottom end all the way through, simply because clean carburation is essential for a tractable trail bike.

I have no doubt that you know this; that your 1065 needle jet needs to be smaller to cure the bottom end richness and that it's effect will stretch as far up as the mid throttle position.

There are many reasons why the B50 won't match a Goldie engine that have been covered here, including the over short con-rod, it's a shame BSA never made the Goldie into a unit construction motor as they did, to some extent, with the A10 to A65 twins.

Cheers
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Tue Jun 11, 2019 7:24 pm

JB wrote:
Tue Jun 11, 2019 6:36 pm
I have no doubt that you know this; that your 1065 needle jet needs to be smaller to cure the bottom end richness and that it's effect will stretch as far up as the mid throttle position.
I didn't go into all the gory detail in what I posted earlier, but I started with a #3 slide, machined it into a #3.5, and then machined it again into its current #4 cutaway. At each stage I made jetting runs with my A/F meter and, based on those results, after making the slide a #4 I then raised the needle one slot which enrichened the mixture down to ~1/16 throttle (i.e. lower than is commonly believed).

All these measurements show that the cutaway has a significant effect from just off idle (to ~40% throttle) and the needle clip position from ~1/16 throttle, which means that getting the mixture "perfect" would require a fine dance between cutaway, needle jet ID, and profile of the taper. That is, not just the start of the taper with respect to the position of the clip, but possibly having two tapers as is the case with some Mikuni needles. Also, for what follows, it's worth noting the obvious that instead of a needle jet with slightly smaller ID the same effect could be achieved with a needle having slightly larger OD.

Eighteen months ago I used my Catalina Gold Star on a ~1200 mile ride. Prior to starting that ride I had measured the needle jet to be 0.1065"-0.1066", but after returning home I measured it to be 0.1069"-0.1070", i.e. in that time the steel needle had bashed the brass jet larger by 0.0004"-0.0005". In only 1200 miles the needle jet was completely worn out. What this means is it would be futile to try to fine tune the jetting of my Gold Star by carefully sizing brass needle jets since any change of, say, 0.0002" would be obliterated by the needle within 500 miles. This means fabricating bespoke steel needles would be the only "reasonable" way forward if I decided to go down that rabbit hole.

Showing that I have nothing in principle against rabbit holes, and with bespoke needles as my motivation, a few years ago I bought a "super-precision" live center with 0.0001" TIR for my lathe. However, thus far I've forced myself to be happy with standard needles.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sun Jun 30, 2019 5:13 pm

After reviewing, and revising where needed, all of my results, the first graph shows the best jetting I found for "2-stroke"[*1] and "4-stroke"[*2] AMAL 1036 bodies after ~15 jetting runs covering ~200 miles and resulting in ~160 pages of printouts. I'll soon be creating a multi-part thread in the Gold Star section of Britbike Forum http://www.britbike.com/forums/ubbthrea ... star-forum that will provide much more detail than given here.

[*1]Most 1000-Series Concentrics were supplied in 2-stroke configuration which, aside from easily interchanged internal components, have a slant-cut spray tube and a "hidden" restriction in the air jet passage. What I mean here by a "2-stroke" body is only the needle, needle jet, needle jet holder and pilot jet (and slide) were changed to 4-stroke items, but nothing was done to the body (i.e. the spray tube and restriction were left in place).

[*2] The same easily-changed components as for the "2-stroke" were changed but, in addition, the spray tube was replaced with a flat-top one and the restriction drilled out of the air jet passage. These modifications left the body in the same configuration as 4-stroke 600 and 900-Series Concentrics.

I made all of my tests runs over the past few weeks with temperatures between 95 oF and 101 oF. Keep in mind that at a more human ~75 oF the equivalent main jets would need to be one size larger to give the same results. Or, keeping the same jets, the curves would be ~0.5 AFR leaner.

From full-throttle flow bench measurements I made a few months ago I would have predicted that given an accurate '200' main jet in the 2-stroke body, it would have taken an accurate '255' main jet in the 4-stroke body to give the same AFR at full throttle. Given issues with the marked sizes of AMAL main jets[*3] the results on the graph have to count as in excellent agreement with the flow bench measurements. If the two jets were actually '193' and '248' (i.e. less than one size off from the marked numbers) the agreement would be perfect.

[*3] A decade ago I measured a dozen new and used jets of the same size on my flow bench and found 75% of them to be within +/-1 size of being correct. Unfortunately, the other 25% flowed too much or too little by as much as 3½ sizes. At one point when determining the jetting for the 1036 Concentric I installed a jet marked two sizes leaner than the one that had been in it but the air/fuel gauge showed the nominally-smaller jet actually flowed a half-size richer. Keep this in mind if your jetting becomes too rich or too lean when changing to a jet whose marking makes it seem it "should" be correct.

As a general comment, during all of my jetting runs the bike "felt" like it ran well for all AFR between ~10:1 and ~15:1 so relying on "feel" when jetting a bike for best performance is problematic.

Referring to the first graph, it's not clear to me that the "flatter" overall behavior of the 4-stroke curve gives it any sort of performance advantage. However, although both versions of the 1036 body work well on a Gold Star, the 4-stroke version does have the advantage of being leaner (but still rich enough) at nearly all throttle settings so it would be less damaging to the wallet. As a very rough estimate, if most riding is done between 1/4 and 3/4 throttle the fuel savings with the 4-stroke version will be ~5%.

The second graph shows that starting from near the top speed of 1st it took 15 sec. at full throttle[*4] heading up a hill before the AFR stabilized at its final value. I was going over 75 mph at the end of that run which means during that time I covered ~1/4 mile. A shorter run reaching a lower speed would have given an erroneously high AFR reading. Assuming someone actually could read a plug to accurately determine if the mixture was too rich, too lean, or just right, the plug chop still would have to be done only after the bike had been under full-throttle for a considerable time.

[*4]The red line on the graph is the output of the throttle position sensor, which is 1.56 V at full throttle.
Attachments
AFR_4stroke_2stroke.jpg
FullThrottle_240Main.jpg

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Mon Oct 28, 2019 4:39 pm

As an update on the Amal 1036 conversion, last week someone I'm working with on a project that will open in a year came to town so we could finish one aspect of it. We took time off from our work to ride to the top of the neighboring 8000 ft. mountain for lunch, riding my Gold Star Catalina and Competition, the latter with the "4-strokerized" 1036 Concentric discussed in this thread. I had tuned the carburetors of both using my Innovate-based AFR instrumentation package. As an aside, the Gold Stars are 95% the same, but the last 5% make them quite different.

On the way there I rode the Competition through 9 miles of stop-and-go city traffic and then another ~30 miles up a twisty road at a reasonably sporty pace to the top of the mountain, while my colleague followed on the Catalina. After lunch, we swapped bikes and rode back down, stopping at a filling station after having covered 72 miles.

The elevation change from the valley was ~6000 ft., which is enough to make the main jets several sizes too rich at the top, i.e. use more fuel than if all the riding had been done in just the valley. Remarkably, despite that, the Competition got 69 mpg (83 miles/UK gallon -- since you're leaving the EU I won't bother converting to liters since you'll never have to use that Continental unit again after a few more days...) and the Catalina got 79 mpg (95 mpUKg). For comparison, an "eco-friendly" Toyota Prius is rated at 50 mpg for combined city/highway driving.

For ~60-year old motorcycles with not-new Amal carburetors to achieve 40-60% better mileage than a modern, eco-friendly sedan is remarkable. This shows that when you use an AFR meter to properly adjust carburetors for performance, you get eco-friendly as a bonus. Your motorcycle, your wallet, and the environment will thank you so using an AFR meter is a win-win-win situation.

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Re: 1000-Series 2/4-stroke Concentrics

Post by stew79 » Mon Oct 28, 2019 9:38 pm

hi magnetoman, have you ever looked at any mikuni carbs to see the difference between 2 and 4 stroke types ? (there is some relevance here as they started of copying amal carbs) big singles can benefit from the non conventional use of the 2 stroke system (especially for racing) old amal on the ccm was re machined to take mikuni parts (very successful) before being replaced with a bigger mikuni
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Mon Oct 28, 2019 11:08 pm

stew79 wrote:
Mon Oct 28, 2019 9:38 pm
have you ever looked at any mikuni carbs to see the difference between 2 and 4 stroke types ? (there is some relevance here as they started of copying amal carbs)
Yes, I have looked at Mikuni in some detail, but discussing them would open up a whole new can of worms. For your enjoyment, one attached photograph shows both sides of an early Mikuni and the other is one side of a Japanese-language instruction sheet for one of their early carburetors made under license. The model number '392' has a special significance. The digits 3-9-2 in Japanese are pronounced Mi-ku-nee (actually, it's more complicated since there isn't a single set of numbers that apply to all things, but rather different names for the numbers depending on what is being described, e.g. the number for 3 sheep would be different than the number for 3 motorcycles, so the company took some poetic license with the '392').
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Re: 1000-Series 2/4-stroke Concentrics

Post by stew79 » Wed Oct 30, 2019 12:14 pm

hi magnetoman, great old pictures. amal dont use the terms "bleed" or "primary" to distinguish to the different ways of getting petrol into an engine, so that must be a mikuni invention ? (although used by keihin as well) as simpler is often better have you used lectron carbs on anything ? or seen there recent addition for 4 strokes ? they can out perform much more complicated jap carbs.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Thu Oct 31, 2019 5:08 pm

stew79 wrote:
Wed Oct 30, 2019 12:14 pm
have you used lectron carbs on anything ?
I have modern bikes when I want modern performance so when I ride old bikes it's for the old bike riding experience. For example, riding across the U.S. on a modern bike takes nothing more than a credit card for food, fuel and motels, but last year I rode a 90-year old rigid-frame, girder fork, Amal-carburetor, magneto-sparked motorcycle across the U.S. in the Cannonball Rally. Completing that event took a bit more resourcefulness than it would have on a modern electric-starter, disc-brake, fuel-injected machine. As a result, the riding experience was quite different as well.[*]

Anyway, for no good reason, my interest is in getting the best performance possible from magnetos, Lucas electrics, Amal carburetors, etc., not replacing them with anything newer and better. I have a flow bench, high frequency oscilloscopes, Air/Fuel instrumentation package with data logger, Magnaflux setup, etc. so I have no qualms about applying modern technology to this effort. But, it's with the intent to optimize the original base components, not to replace them.

[*]On the way back from the fuel mileage test I described a couple of days ago the oil line to the rocker box of one of the Gold Stars broke, requiring me to cobble together a temporary fix in order to get home. The satisfaction of devising a roadside fix, and the anxiety of hoping it would hold, is a typical part of the old bike riding experience which riders of modern machines are deprived of ..
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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Thu Oct 31, 2019 11:33 pm

How about something the same age range or older like a Wal Philips "fuel Injector"
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Re: 1000-Series 2/4-stroke Concentrics

Post by minetymenace » Fri Nov 01, 2019 10:15 am

Magnetoman wrote:The satisfaction of devising a roadside fix, and the anxiety of hoping it would hold, is a typical part of the old bike riding experience which riders of modern machines are deprived of ..
Hear, hear!
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Re: 1000-Series 2/4-stroke Concentrics

Post by eebtr7 » Fri Nov 01, 2019 6:33 pm

Have you been reading your,'Zen and the Art of Motorcycle Maintenance' again?

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Fri Nov 01, 2019 8:45 pm

Jeff K wrote:
Thu Oct 31, 2019 11:33 pm
How about something the same age range or older like a Wal Philips "fuel Injector"
I am quite interested in period aftermarket components like that, but mostly out of curiosity, not because they work better than what they were intended to replace. Because, typically they work worse. If for no reason other than its complexity, a favorite of mine is that someone went to a lot of trouble to engineer a complete desmodromic valve actuating kit for Velocettes in the late 1950s. It appeared in small ads in the back of 'Cycle' magazine in the U.S. for a few issues and then disappeared.

It's not that I try for concours accuracy on my bikes, because I certainly don't. The problem is that enjoy riding them, which is inconsistent with maintaining "concours-ness." For example, I haven't had a chance to deal with the broken oil line mentioned in my previous post, but when I do I'll use it as an excuse to replace the stock manifold with a Webco piece that has been waiting patiently at the bottom of a box for ~25 years. But, overall, I prefer to stay as close as reasonable to the original engineering for the reasons explained in my previous post (nb. 'reasonable' is in the eye of the beholder).
eebtr7 wrote:
Fri Nov 01, 2019 6:33 pm
Have you been reading your,'Zen and the Art of Motorcycle Maintenance' again?
Given sufficient motivation and or desperation, most serious problems with our old bikes can be fixed on the road, whereas with a modern bike that's just not possible. When I started the Ariel after a refueling stop halfway through the Cannonball I noticed it had lost most of its compression, although it was still running, forcing me to replace a valve guide in the motel parking lot that (long) night. It was ready to ride again in the morning. Try doing something equivalent to that with a modern bike. Whether or not such skills are useful to have in the 21st century can be debated, but such skills are being lost. On the other hand, Pirsig made the same prediction 45 years ago so maybe I'm wrong about this.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Fri Nov 01, 2019 11:38 pm

Being old enough to have purchased some of these bikes new. I always have to laugh when "expects" argue over "concurs" accuracy. Many of these bikes were modified by the dealers before then customer had a chance to purchase them. Many times the factory used up what parts they had and some times had to dip into a special stash to complete the bike. I always figured that if it was period aftermarket then that was rarer then stock and that would be a better representation of what the bike actually looked like.
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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Sun Nov 03, 2019 1:26 am

Jeff K wrote:
Fri Nov 01, 2019 11:38 pm
Many of these bikes were modified by the dealers before then customer had a chance to purchase them. Many times the factory used up what parts they had and some times had to dip into a special stash to complete the bike.
If you replace "many" in both of these sentences with "occasionally," I agree with you. Otherwise, I have to say you're repeating an urban legend. Bulletins from Hap Alzina make clear that bikes shipped to dealers were configured as described in the literature, and if a customer wanted any changes the costs were to be borne entirely by them, and that any parts taken off the bike as part of this were to be retained by the dealer because they wouldn't be given credit by Alzina for them. If a customer left the dealership with a bike that had been modified by the dealer it was because the customer had paid for those modifications, not because the dealer had done them gratis without anyone knowing.

Sales catalogs were prepared well in advance so there are differences between what is shown and what was in the showroom. Also, taking the Gold Star as an example, the parts manual issued in 1958 seemingly covers all DBDs. However, like for other bikes, I have a very large stack of Bulletins detailing the precise specifications of given bikes in subsequent years. What this means is, if someone wants to restore their bike to the condition it had when it left the factory, the information exists (although, it can be hard to find) to let them do that with a pretty high confidence level in the final result.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Jeff K » Sun Nov 03, 2019 3:42 am

It might have been different here in Michigan. Norton 500T came with a BTH magneto and yet a dealer told me decades ago the some hated the BTH and wanted the lucas, so the bike left with what the customer wanted. Mostly it was the dealer who "added" items and charged more for them. I have seen more then a few bikes close to a model year with parts on them that were not of that model year. My 1974 TR5T , one of the very last made has several differences then my 1973 TR5T. I have seen dealers swap gas tanks from new models to last year model's models to help sell them. I picked up a NOS Norton MKIII red white and blue John Player tank long time ago from a large Norton dealer, he had swapped it with the more common black one to sell the bike faster. When I purchased my 1977 Norton MKIII new years ago, another dealer tried to tell that the Red White and Blue tank and side covers were from the USA Bi-centennial. LOL
One of the 1st things the dealers replaced was the hard grips on some models and they replaced them with the "softer NAVA Gran Turismo style. A dealer told me that most sales are made when the customers sits on the bike for the first time and what is the first thing he holds. A old and long gone dealer friend of mine Sam from Saginaw cycle told me that the 1st thing they removed was the complete choke assembly, too many bikes came back because it loosened up and closed down. The twin pin stripping on the Norton side cover, I had never seen that on a Norton around here after the early 1970 model and I had a couple that I purchased new. Common on the East coast, not here. The local Norton dealer routinely pulled the headlight bulb and replaced it with Quartz Halogen one and the jacked up the price $20.
Triumph TR25W up here was the the go to 4 stroke Enduro bike and was up against and badly mismatched by the slew of 2 strokes. I saw a few on the floor with Pretty Fenders, Tail light, number plates, Short seat, Plastic gas tank, Tires and sprocket. Dealer set them up sold them at a premium and told me the best part was he now had NOS parts for his shelf. I can remember when the Triumph and BSA dealer had a very hard time getting spare parts start of the 70's and the chance to remove new parts, add aftermarket and to sell at a premium was a win win for them.
You seem to like bikes from a different time then most of mine, your Gold Stars are a fantastic and premium bike. I only know of a couple around here. I relate more to the later 60's to the mid 70's and I think things were much different at the dealers and factories at these times.
I would agree the it was mostly the dealer who did changes. But they influenced the factories who in later years were struggling. Also many difference in East Coast - West Coast. Look it the parts catalog at the Triumph T100C wide ratio gearbox, different gears offered for the East and West.
Keep up the good work on the Amal's!
Jeff

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Re: 1000-Series 2/4-stroke Concentrics

Post by BSA_WM20 » Sun Nov 03, 2019 11:28 am

Sales catalogs were prepared well in advance so there are differences between what is shown and what was in the showroom. Also, taking the Gold Star as an example, the parts manual issued in 1958 seemingly covers all DBDs. However, like for other bikes, I have a very large stack of Bulletins detailing the precise specifications of given bikes in subsequent years. What this means is, if someone wants to restore their bike to the condition it had when it left the factory, the information exists (although, it can be hard to find) to let them do that with a pretty high confidence level in the final result.
Unless of course it is a concours event held by the AHMA when the bike has to look like the image in the catalogue , correct or not.
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Re: 1000-Series 2/4-stroke Concentrics

Post by hhh » Sun Nov 03, 2019 6:31 pm

Magnetoman wrote:
Tue Jun 11, 2019 7:24 pm
JB wrote:
Tue Jun 11, 2019 6:36 pm
I have no doubt that you know this; that your 1065 needle jet needs to be smaller to cure the bottom end richness and that it's effect will stretch as far up as the mid throttle position.
I didn't go into all the gory detail in what I posted earlier, but I started with a #3 slide, machined it into a #3.5, and then machined it again into its current #4 cutaway. At each stage I made jetting runs with my A/F meter and, based on those results, after making the slide a #4 I then raised the needle one slot which enrichened the mixture down to ~1/16 throttle (i.e. lower than is commonly believed).

... What this means is it would be futile to try to fine tune the jetting of my Gold Star by carefully sizing brass needle jets since any change of, say, 0.0002" would be obliterated by the needle within 500 miles. This means fabricating bespoke steel needles would be the only "reasonable" way forward if I decided to go down that rabbit hole.

Showing that I have nothing in principle against rabbit holes, and with bespoke needles as my motivation, a few years ago I bought a "super-precision" live center with 0.0001" TIR for my lathe. However, thus far I've forced myself to be happy with standard needles.
Great stuff Magnetoman. Could you please clarify something in the above quoted post: I think you meant "needle jets" the last three times you wrote "needles."

On another note, something I don't think has been mentioned in this thread yet: additional richness can be introduced at any throttle position desired by carefully filing the needle flat. The downsides include: difficulty of matching twin carbs on a twin, reproduction on a replacement, taking too much off, making the range too small, too large, or in the wrong place etc. Also, I would recommend that anyone messing with Amals get a set of plug gauges to gauge needle jets. I've seen a lot of variation from what's stamped on them. Unfortunately, once the jet is worn, plug gauging no longer works because they wear oval. And actual flow can be compromised by lead-in chamfer or lack thereof before the orifice; the same goes for main jets.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Mon Nov 04, 2019 4:06 pm

hhh wrote:
Sun Nov 03, 2019 6:31 pm
Could you please clarify something in the above quoted post: I think you meant "needle jets" the last three times you wrote "needles."
I carefully re-read what I had written and I did mean what I wrote, i.e. fabricating needles, not needle jets.
hhh wrote:
Sun Nov 03, 2019 6:31 pm
additional richness can be introduced at any throttle position desired by carefully filing the needle flat.
That's a good comment. Thanks for mentioning it. As hhh wrote, the fuel only cares about the total annular area between the needle and needle jet, not how that area was achieved. A flat on one side of the needle increases that area and thus increases the fuel flow when that portion of the needle is in the needle jet. But, as hhh cautioned, this requires precision surgery.

That said, the geometry class you had in high school makes such adjustments much less hit-and-miss than they otherwise would be. Say the jetting was "perfect" everywhere but over a certain range of throttle positions, where you want it 5% richer. First, determine where the needle is with respect to the needle jet over that range. Then from the ID of the jet and the OD of the portion of the needle that's inside the jet in that range, calculate the annual area. Then calculate how much material you need to remove from one side of the needle to increase the total area by 5%. This paragraph sounds more difficult than it is (that is, assuming you were awake in geometry class), and will get you a lot closer to your desired final result a lot faster than the alternative method of crossing the fingers of one hand while filing the needle with the other.
hhh wrote:
Sun Nov 03, 2019 6:31 pm
I would recommend that anyone messing with Amals get a set of plug gauges to gauge needle jets. I've seen a lot of variation from what's stamped on them.
That's another excellent point. Wear of only 0.0005" is enough to make a bike very rich in the range where the needle jet dominates.

I have a Starrett split anvil gauge that measures needle jet bores to 0.0001". Ten or so years ago I bought a batch of 28 NOS Amal jets in sealed baggies and measured them. Taking the common .106, 17 of the 22 (77%) were within 0.0003" of the correct size. However, 3 were 0.0007" too large, which means they were worn out even though new. The other two were 0.0007" and 0.0008" too small, making them essentially .105 jets.

I measure all of my needle jets before I install them and record their size. Two years ago a friend and I took two of my Gold Stars on a ~1200-mile ride. When we returned the needle jet in one of them had enlarged by 0.0004"-0.0005". That is, it only took 1200 miles of the needle hammering against the brass jet to enlarge it to the point it was worn out. As hhh wrote, you need a gauge.

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Re: 1000-Series 2/4-stroke Concentrics

Post by hhh » Mon Nov 04, 2019 7:01 pm

Thanks for the clarification. Since stock needles are steel, I had assumed you wanted a stronger needle jet made of steel for it to work against. However, the wearing of similar metals problem might arise if the alloys are very similar.

So what you meant was to compensate for a worn needle jet with a bespoke needle that is fatter?

Does your Starrett gauge allow accurate measurement of a worn oval jet? I would expect that a standard plug gauge could very well indicate a pass although the hole has actually turned into a key hole from the needle tip wearing an indent smaller than the hole's radius.

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Re: 1000-Series 2/4-stroke Concentrics

Post by Magnetoman » Tue Nov 05, 2019 12:42 am

hhh wrote:
Mon Nov 04, 2019 7:01 pm
So what you meant was to compensate for a worn needle jet with a bespoke needle that is fatter?
No, making a bespoke needle to compensate for wear would be chasing a moving target and simply replacing a worn needle jet is by far the easier to do. The only reason to make a bespoke needle would be to tweek the mixture in some intermediate range.
hhh wrote:
Mon Nov 04, 2019 7:01 pm
Does your Starrett gauge allow accurate measurement of a worn oval jet?
The answer to that is 'yes', but with an explanation. All "split anvil" gauges only cover a limited range irrespective of the shape of the anvil, with the minimum end of the range determined by the minimum size of the anvil when fully retracted. My Starrett gauge uses a split cylinder and is specified to cover the range 0.100"-0.120". Although this might seem to be a problem because the diameter of a needle is 0.098" so in principle, at least, the depressions it hammered into the jet might not be any bigger than that. However, the actual measured minimum diameter of the cylinder is 0.096". This means that If I align the split in the anvil perpendicular to the oval of the jet it will give me an accurate measurement. Further, the correct anvil of my Diatest split ball gauge -- I'm nothing if not over-instrumented -- is specified to cover 0.091"-0.110" so that gives me a second way of accurately measuring the "ovalness" of a needle jet that has worn oval.

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