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).
Attachments
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
John

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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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