72 XS2 carb issue I can't figure out

HIGHRPM

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Hope everyone is doing well. I need help, in more ways then one, lol. I am just finishing a total build on my XS2 and am having an issue I have never had in owning and working on these bikes since 71. Bike has all new ignition, dwell set to spec, timing on spec, advance on spec. Charging system is perfect , new battery and all wiring connection triple checked for proper continuity. Carbs have been ultra sonic cleaned multiple times, all passages blown out with air multiple times, petcocks rebuilt , new fuel hoses with filters. Both carbs have new kits installed , floats tested and set to spec , choke enricher plunger replaced and tested for proper operation with air. Bike starts ok, warms up ok , produces great power from start to full red line. I have checked the carbs air flow with a meter that I use to balance carbs. Now the issue. The LH side blows white smoke, the RH side is always clean. I have tested the cylinder head temp and exhaust temp on both sides to make sure they are very close to the same. I have tested the exhaust output with a CO meter and the LH side is defiantly running rich. I have reset the LH carb float anywhere from 23 mm to 27 mm with no difference. The pilot jet and main jet are stock size, the needle is set on the number 2 highest ring, both diaphragms work properly when compared. I read where someone said it might be valve seals, so I swapped the carbs over side to side complete with throttle cables and the problem switched from LH to RH which only verifies in my mind that it's not the valve seals, it's the carb . I am lost and going in circles, as I can't fault anything in the carb that I can see. I am in DESPERATE need of help !!
 

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The LH side blows white smoke, the RH side is always clean

Both carbs have new kits installed
If the kits are aftermarket, the next step for me would be to get genuine Mikuni NJ/JN, mains and pilots from reliable source. There are reports of incorrect parts and who knows about quality control.....

Does that smoking condition persist after some miles and in all throttle ranges?

PS: Super nice bike!
 
Yes, I would, at the very least, replace the mains and pilots with genuine Mikuni ones. There's no telling how accurately made the repops are, even if they are marked with the right size. For the needle and needle jet, you're kinda stuck. Repops are all that's available because Yamaha no longer carries them. All I can suggest is to watch eBay, N.O.S. ones pop up occasionally. Luckily, those two items aren't high wear items. If you still have the originals, maybe swap them back in to see if it fixes the problem.

If you are running the needles in the 2nd slot from the top, that's very lean. Stock setting was 4th step from the top, a 2 step difference. Are you running the generic Y-22 kit needle by chance? That could explain it. The length is about the same as an original 4JN19, but who knows about the taper.
 
If the kits are aftermarket, the next step for me would be to get genuine Mikuni NJ/JN, mains and pilots from reliable source. There are reports of incorrect parts and who knows about quality control.....

Does that smoking condition persist after some miles and in all throttle ranges?

PS: Super nice bike!
The smoking persists, gets way worse just off idle. The carb makes it look like I am running a 2 stroke.
 
Yes, I would, at the very least, replace the mains and pilots with genuine Mikuni ones. There's no telling how accurately made the repops are, even if they are marked with the right size. For the needle and needle jet, you're kinda stuck. Repops are all that's available because Yamaha no longer carries them. All I can suggest is to watch eBay, N.O.S. ones pop up occasionally. Luckily, those two items aren't high wear items. If you still have the originals, maybe swap them back in to see if it fixes the problem.

If you are running the needles in the 2nd slot from the top, that's very lean. Stock setting was 4th step from the top, a 2 step difference. Are you running the generic Y-22 kit needle by chance? That could explain it. The length is about the same as an original 4JN19, but who knows about the taper.
What exactly is a Y-22 needle , any certains years ? I have reset rod clip to #4 position. Was set too #2 to try and figure this issue out.
 
The Y-22 is an aftermarket needle included in some of the rebuild kits. That number should be stamped on the needle just below the clip slots. If it's original and genuine Mikuni, it will be a different number, a 4JN19, and also have the trademark Mikuni "square-in-square" emblem stamped in it .....

Mikuni Emblem.jpg


The 4JN19 needle used in the '70-'73 carbs is quite short compared to the needles used in the later carb sets. It measures about 40mm from the top clip slot to the tip. It's so short because those early carbs had a raised floor in the main bore. None of the later needles will work, they're too long .....

BS38Needles.jpg


So, I would check the number on and measure the length of the needles you've got. Maybe someone swapped in ones that are wrong, too long maybe.
 
I was out on a little train trip.....Anyway back to your carb issue. I'm wondering if during the ultrasonic cleanings, the needle jets were removed and verified clean (perhaps ID #)?

A generally accepted - around here - source for genuine Mikuni brass is
http://www.nichecycle.com/ncs/categories/carburetor/mikuni-parts-by-category.html

They're not genuine Mikuni, but Niche does also offer a house brand of needle jet (Z6) and a needle (4JN19) that they spec as replacements for '72 (306 model) BS38 carbs that are perhaps better than other aftermarket (I have some for pending resto). However, will reuse original if possible.

http://www.nichecycle.com/ncs/catalogsearch/result/?attr=0&q=256-14941-86-00

http://www.nichecycle.com/ncs/categ...ng-tuning/needles/102-3650-needle-by-ncs.html
 
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I was out on a little train trip.....Anyway back to your carb issue. I'm wondering if during the ultrasonic cleanings, the needle jets were removed and verified clean (perhaps ID #)?

A generally accepted - around here - source for genuine Mikuni brass is
http://www.nichecycle.com/ncs/categories/carburetor/mikuni-parts-by-category.html

They're not genuine Mikuni, but Niche does also offer a house brand of needle jet (Z6) and a needle (4JN19) that they spec as replacements for '72 (306 model) BS38 carbs that are perhaps better than other aftermarket (I have some for pending resto). However, will reuse original if possible.

http://www.nichecycle.com/ncs/catalogsearch/result/?attr=0&q=256-14941-86-00

http://www.nichecycle.com/ncs/categ...ng-tuning/needles/102-3650-needle-by-ncs.html
During cleaning, needles were removed and measured with a caliper to verify if it meets original spec and it- they do. Will look into Niche cycle, thanks. Apparently there were four different Y22 rods, all with different measurements, but mine specked to stock specs. Would going one size smaller on the pilot jets help out? I have moved to sea level, so not sure where I need to be. Thanks for any guidance .
 
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The part that I was asking about is labelled #1 in the diagram below as it is not included in most kits and often overlooked. It can hide crud if not removed.
BS38 XS1 CARB.gif
 
I'm not aware of 4 different Y-22 needles. If a needle is marked Y-22, then that is what it is, a Y-22, and any marked that should be the same size (length and taper). So, are you saying that's what you have in your carbs, Y-22 needles? Now, while the length may match the original 4JN19, I don't know about the taper. And the taper on a needle is very important. Besides the length, that's how you "tune" with needles, use ones with different tapers.
 
IDK what the kits in question are, but I have "Keyster" kits (BS34) that do come with 4 "Y503_" needles and they reference 51X8 (a Mikuni # with LOA 52mm):shrug:
20221025_234935 (2).jpg
 
IDK what the kits in question are, but I have "Keyster" kits (BS34) that do come with 4 "Y503_" needles and they reference 51X8 (a Mikuni # with LOA 52mm):shrug:
View attachment 228371
With all the researching on the Y22 needle, A pic came up that showed four different versions of the Y22 and four different sizes. Which only confuses me when people say there is only one. The only kits I have ever used are KEaster.
 

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So far as I know, Y22 is a "made-up" designation by Keyster (as is Y503 for BS34's) - Mikuni has no such designation AFAIK. If you want to compare them to the specs for a genuine 4JN19:
4JN19 SPECS.png

Not that I think that's the cause of your issue; I suspect it may be the needle jet or a passage that's not "good" as it's only one side's carb.
 
So far as I know, Y22 is a "made-up" designation by Keyster (as is Y503 for BS34's) - Mikuni has no such designation AFAIK. If you want to compare them to the specs for a genuine 4JN19:
View attachment 228449
Not that I think that's the cause of your issue; I suspect it may be the needle jet or a passage that's not "good" as it's only one side's carb.
I was told needle jets are not available. So if it’s faulty wth does a person do ? If its a passage , I am screwed !
 
Your early carb uses the long version of the Z-6 needle jet and there is a repop available. The short version of the Z-6, as used in the '74-'75 carbs, is not available.

LongZ-6.jpg


NeedleJets.JPG
 
If the kits are aftermarket, the next step for me would be to get genuine Mikuni NJ/JN, mains and pilots from reliable source. There are reports of incorrect parts and who knows about quality control.....

Does that smoking condition persist after some miles and in all throttle ranges?

PS: Super nice bike!
white smoke generally means oil burning. Prob needs new rings.
 
white smoke generally means oil burning. Prob needs new rings.
I read where someone said it might be valve seals, so I swapped the carbs over side to side complete with throttle cables and the problem switched from LH to RH which only verifies in my mind that it's not the valve seals, it's the carb .
I would generally agree, except for the OP's statement above:umm:
 
white smoke generally means oil burning. Prob needs new rings.
Generally, sometimes yes, sometimes no. Being the plugs are never oil fouled, just wet from unburnt fuel. Bike has been tested multiple times with a CO analyzer it shows the left hand side very rich compared to the rh side. If I switch the carbs side to side, the problem shifts side to side, so not rings, not valve seals. Virtually equal compression side to side. Am in the process of ordering all new internals , including the needle jet which is original.
 
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