One Cylinder Just a Touch Too Rich

The diaphragms that were in it were fully stock.

The set I put in yesterday were on a stock slide but the diaphragm are some aftermarket ones from JBM industries.

My idle jet, needle, and main jet are all stock originals that set in carb cleaner for awhile that are nice and clean. The Venturi jet that the needle slides into is an aftermarket one from a carb rebuild kit.

It was running great (other than the richness). I switch the slide and bam it hardly runs in the mid range. Which makes sense because if these diaphragms has a pinhole or are sealed properly I guess it could cause some issues.
 
Is it this carb kit?
https://www.mikesxs.net/keyster-premium-nencho-carb-kit-xs650-special-1980-1984.html
200-1111_keyster-kit-xs650-special_01.jpg
 
No, it’s another one from mikes but when I first tried some of the jets they weren’t worth a shit so I put all the stock ones back in after a deep clean other than the jet the needle slides in.

But from testing alone, how would that one venturi jet be the issue if it was working great with it in two days ago but now with a new diaphragm it’s shot.

Or are you suggesting that the Venturi jet could cause the richness, and diaphragm could cause mid range power issue that started after inserting new diaphragms.

(Also I have another xs that came with this rebuilt kit and the jets inside it were so oddly spaced in size I don’t know how someone could use it)
 
The JBM diaphragms are generally well regarded so IDK about the slide issue, but you have slides that worked. The aftermarket rebuild kits are known to have quality control issues (whether marked Keyster or not). I suspect the needle jet is the source of the original richness when paired with a stock needle (guess). I'd put original needle jet back in (preferred), order genuine Mikuni needle jet, or use the aftermarket jet with its aftermarket needle and see if it straightens out.
 
Alright I’m following. I’ll give this a shot.

Original slides, original needles, original Venturi jet.

I’ll report back on this setup.


I’ll order some fresh Mikuni needles and Venturi’s as well.

Thanks for all your help!
 
I'm assuming along with the diaphragms, the slides and needles were swapped also?
Maybe have a close look and see if the slide and, more importantly... the needles are correct. There's quiet a few different size needles that will fit, only a few of 'em will actually work out of the box though.
Maybe put up some pics when you get the carbs off and the slides out?

Edit: only after I commented did I realize there's another page of comments. Oops.
 
Hopefully, the originals solve the issue. Here's a source of jets I've used with success although they're not Mikuni originals
http://www.nichecycle.com/ncs/categ...ha-xs650-80-84-y0-needle-jet-bs34-by-ncs.html

http://www.nichecycle.com/ncs/categ...ng-tuning/needles/102-3912-needle-by-ncs.html
Thanks!
I'm assuming along with the diaphragms, the slides and needles were swapped also?
Maybe have a close look and see if the slide and, more importantly... the needles are correct. There's quiet a few different size needles that will fit, only a few of 'em will actually work out of the box though.
Maybe put up some pics when you get the carbs off and the slides out?

Edit: only after I commented did I realize there's another page of comments. Oops.
definitely will grab some pics this go around of slide, diaphragms, needle and needle jet!
 
Will post pictures tomorrow with a more detailed update but in summary:

Left plug was fouled on the left cylinder causing to inconsistently fire. I put an old plug (that wasn’t fouled) and it ran great.

I still replaced needle jet with stock ones and stock diaphragms. Going to purchase some plugs and see what they look like after some miles.
 
Long Update Including Pictures:

Needle Jets: The first attached photo is the cheap needle jets I had in the bike. The second photo is a set of stock needle jets I pulled from a spare set of BS34s from the same year. The rebuild kit ones had a marking of "M31A" which did not match the stock which looked like "27...". The pattern seemed similar but when ahead with stock ones just in case.
IMG-3416.jpg
IMG-3417.jpg


Choke Plungers: The attached photos are the choke plungers from the carbs, they are clean, springy, and rubber seats seem to bee in good condition. I kept these for now.
IMG-3418.jpg
IMG-3419.jpg


Diaphragms and Needles: The first 4 photos are of a stock set of needles and diaphragms. They seem to be all in good condition with no excessive wear that I can tell. The last two photos are of my JDM diaphragms with stock needles, these also seem okay so I may put these back in as the JDM ones are basically brand new. On top of that, the JDM Diaphgrams and slides seems to take longer to come back down than the stock (stock 5-10 seconds and JDM 15-20 seconds) so I think that may be a better option. (Also I know these aren't good shots of the slides, but both sets fit snug in the top of the carb and slide smoothly)
IMG-3420.jpg
IMG-3421.jpg
IMG-3422.jpg
IMG-3423.jpg

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In summary, I found out I had fouled the left plug from running rich. I have been running this bike and testing with it for the last year with no avail and with basically the same 4-6 plugs (I know not the best testing techniques). I put a non-fouled plug in the bike and it fired and ran great. I purchased some fresh AP65 plugs (these are resistor plugs) but they should be good enough for now for me to ride on sunday and see if this rich cylinder is still an issue or if I just needed a fresh plug that isnt covered in soot.
 
Make sure your needles are assembled and installed correctly in the slides. There's a thick plastic spacer, a washer, and a spring, and they must be in the correct order or the needle height will be wrong .....

'80 Slide.jpg


Here's a pic showing the right order - thick plastic spacer under the e-clip, flat washer on top of it, and finally the spring. If you have the spring on the bottom, the needle would set too high and run rich .....

BS34ShimWasher.jpg


You can check for proper assembly without taking things apart simply by measuring how much needle hangs out of the bottom of the slide. It should be about 41mm. In your last couple photos, the ones of the JBM diaphragm/slide assembles, one needle looks like it's sticking out more than the other.
 
@5twins

I’ll definitely give that a check this afternoon. They SHOULD be correct but I’ve pulled these apart many times so that may be the problem!

Will report back on both sets of slides
 
New update:

Have not yet checked needle heights but wanted to do this for piece of mind.


First photo is steady state second photo is right cylinder, third photo is left cylinder.


So looks like right at 40% leak down.
 

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But if both cylinders are the same, this makes me thinks, yes the rings are probably worn but I don’t think that’s causing smoke.
 
Never did a leakdown (diff) test on an XS, so maybe someone who has can chime in, but.....

In the airplane world we used 80psi for testing. 20 to 30 psi wasn't even enough to properly seat the rings for accurate readings.
 
Check out my last post in the black and white spark plug thread.
Can you put the link to it in this thread?

I am PRETTY certain this is due to unbalanced carbs. When my bike is idling you can hear the e left cylinder not firing as well or consistent. Too much fuel, not enough oxygen = inconsistent sync and poor tune
 
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