BS34 Jetting for Modifications

jpdevol

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Despite my bike’s radical intake/exhaust, I’m getting pretty close on jetting and the resulting drivability. The bike idles nicely for full stoplights and takes throttle well from low rpms through the range. I have a miss near WOT above 5K (guess). The plugs are “sooted” a bit still, but I can see some lighter browns beginning to form. The pic below includes some around town riding and necessary idling returning home.

The current set-up: 47.5 pilots

270-Y-0 NJ

“7F6” Dyno Tune JN

145 Main.

So, my next step was to install the Canadian 336-Y-0 NJ with 51X11 JN (just received).

However, upon examination of the needles (pics below with slots aligned), I think the new NJ/JN is going to change everything in a major way. The length difference concerns me. The stock 5HX12 is in the middle, the “7F6” to the right and the 51X11 on the left. Their length measures:

5HX12 – 53.41

7F6 – 57.38

51X11 – 51.38

The stock and Dyno needles are designed to run the original 3mm nylon spacer under the clip. I cannot find any info. whether the 51X11 uses that spacer (?). If it does not, then what about the hole left in the slide?

I think the proper move is to is to install that set-up AND drop the main back to perhaps a 135-137.5. The Canadian set-up is going to be way rich! ??

It appears to me that the Dyno Tune “Stage 3” kit relies on an oversize main, then compensates with a really lean (large) needle to get mid-range correct. Perhaps that was done to keep kit costs down and use the stock NJ.

Another alternative is to tune on my “lollipop” baffle (now full open) to attempt to send back more sound to exhaust port and affect the WOT miss.

I’m hoping someone has experienced switching between a “Dyno” set-up and a “Canadian” set-up to save some garage time.
 

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jpdevol

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Sorry, forgot pics of intake & exhaust - attached
 

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650Skull

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The 51XII and 336-YO is factory set up for most countries other than the US. Pic of my BS34 51XII as it came out of my Carbs.

it was set up with a 132.5 main and 130 small jet. had a set of MikesXS 26" Dunstall reverse cone mufflers


I'm in Australia, 80SG Oceania model. Go back to factory settings and work from there

P1000735 crop copyright.jpg
, P1000711 text.jpg


, P1000705 Text.jpg
 

jpdevol

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Thank you!

That's the first confirmation I've found of the "ring" on the 51X11; been to cmsnl.com, yamaha Canada, all over the www.

It will sit a good bit higher, away from the main than stock or current set-up.
 

650Skull

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Here are some BS34 Parts diagram.........
1st one is for the non US 51XII needle and 336-YO converted from the Japanese 4E3 model to English. Also shows the pilot screw, Doesn't show the pilot Jet, i didn't convert the second page.

4E3-3W6-3U6 TEXT copyright.jpg



US BS34 Carb parts Diagram
BS34 SH-parts-list 01.jpg
BS34 SH-parts-list 02.jpg
 

5twins

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When I measure needle lengths, I go from the top clip position (or only clip position in the case of the 5HX12) down to the tip. This is the "working" portion of the needle. The portion above the clip doesn't contribute to the fuel metering and can vary a little from needle to needle on needles of the same number. However, you should find the measurement from the clip slot down, the "working" portion, the same from needle to needle. If you compare this on the 5HX12 and 5IX11, you should find it the same, about 50mm. That means, set in it's middle (#3) clip slot, the 5IX11 would sit 2mm higher than the 5HX12.

There isn't much documentation on the Canadian or "world" BS34 carb settings other than that Japanese home market manual Skull posted excerpts from. And it gives no needle setting details, only jet sizes. Just because Skull found his needles set in the #4 slot, that doesn't mean it was the factory setting. These bikes got changed around a lot over the years by P.O.s. His 132.5 mains may have been swapped in as well because according to that Japanese manual, 130's were stock. And the condition of the screwdriver slot does indicate that main has been fooled with, lol.

Now let's talk about needles. Mikuni produces their needles in various series and the first number in the the needle's code denotes that (4, 5, 6, etc.). The length of each series spans 10mm. A 4 series needle would be between 40 and 50mm long, a 5 series between 50 and 60mm, etc. The length you posted for your 7F6 doesn't match up to this. It should be between 70 and 80mm long. Maybe it's custom ground for those kits?

If you can't sort the carbs with the parts you've got, there are some other needles available. They come in the rebuild kits from Cruzinimage. You get not just one but two different needles you can play around with .....

CruzinimageBS34Kit.JPG


The needles are a generic Y265 and Y267. The Y265 looks to be a longer copy of an XS400 5Z1, and the Y267 looks to be a copy of the '78-'79 BS38's 5O2 .....

KitNeedles.jpg


I'll also mention the air jets. These are a way to fine tune the idle circuit. You can achieve richness levels in between pilot jet sizes by changing the air jets. The pilot jet can have a big effect on plug color and your dark plugs may be due in part to you being 2 sizes up on the pilots. If you drop down to one size up (#45) and reduce the air jets one or two sizes (#135 stock to a 132.5 or 130), you'll achieve a richness level a little more than the 45 pilot alone but not as rich as a #47.5 pilot, and your plugs should lighten up some. I run 45 pilots and 132.5 air jets on mine and was also able to reduce the mix screw setting from around 3 to about 2.5 turns out. But, every one of these is different. My buddy has the same jetting but needs 3 turns out on his mix screw or the RPMs hang and it's slow to return to idle. We both run 137.5 mains and the stock fixed 5HX12 needles.
 

5twins

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Yes, that gave the non-U.S. models a slightly richer idle circuit. U.S. models got a #135 air jet. Combine that richer idle circuit with the richer midrange and a smaller main could be fitted (#130). This follows the same practice used on the BS38's where carb sets with larger needle jets and hence, richer midrange circuits, were fitted with smaller mains.
 

jpdevol

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Thank you both. I think I've got it headed in the right direction again: smaller jets are needed with world-wide JN/NJ. I expected that, but my hard-head would only let me drop to 140 mains on first try (because I need to be “3 sizes up”). Of, course that’s 4 sizes up from non-US spec. It was even richer near WOT with breakup occurring earlier.

Perhaps too, I'm overestimating the impact of the short, open exhaust and the V. stacks compared to previous experiences. The pipes are 1.5" x 26" though they do have two >90* bends. I am successfully running the Dyno Tune “Stage 3” set-up in the 650 Tracker. It has an exhaust that is 38” long with megaphone, but they start at 1.75” at the port then continue at 2” to the silencer.

In any event, I’ll go out to the shop and change to 45 pilots, 135 mains and have another go tomorrow.

Regarding that “7F6” needle; I don’t know its provenance, but many vendors offer the Dyno Tune kit – some specify that needle designation and one refers to it as a Heiden kit. It’s not currently listed on Heiden’s website. I did find it in my Mikuni manual, but as you state, it should be 72mm long and is intended for VM40-44’s.

I’ll report back…….
 

5twins

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Well, if you have that Heiden kit, most reports on it aren't that good. I guess it was designed for the 750 kit and also using Heiden's 2-1 exhaust. On less modded 650s, apparently it doesn't work that well. You also have to consider that it comes from Europe and I'm pretty sure they get different gas formulations than we do.
 
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