Flat Spot off Idle

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Yup, those are them.

Still use the metal band clamps.
But, don't use the thin metal reinforcing shell.

If you're still using the stock air filters, then carb weight support isn't an issue.

If not, even tho' they don't use the reinforcing shell, I'd recommend some sort of rear support, like a bracket to the carb tops, or to the band clamp.
 
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Yup, those are them.

Got it. Just emailed to request a paypal invoice for a pair of their BS38CV "carb holders" AND a pair of their M-77 diaphragms.
I hope I followed their instruction puzzle correctly. At this point, I think my brain's leaking more than the carbs...


Still use the metal band clamps. But, don't use the thin metal reinforcing shell.
If you're still using the stock air filters, then carb weight support isn't an issue.

The bike's all OEM, so the backside of the carbs're supported by the stock air filter boxes, which're attached to the frame with two bolts.
I'll mount the carbs with the OEM clamps and leave off the metal reinforcing shells.
Again, many thanks 2M!
 
Update...

I installed new piston/diaphragms in both carbs. Now, the piston in the right (problem) carb takes ~15 seconds to drop whilst blocking the oval air port (i.e., it's perfect); but the left, which was perfect, now drops in ~4 seconds....similar to the way the right (problem) carb was performing with the original piston/diaphragm. And I seem to've rounded one of the screw heads whilst r&r'ing the carb cover to get the new diaphragm properly situated in the groove (long story). <insert frustration emoji here>

I started the engine and sprayed carb cleaner around the carb-to-manifold interface on both carbs. The engine sped-up both times! So I removed the right manifold and did a THOROUGH check for cracks, dryness, etc., but found none. It was pristine and flexible, inside and out. So I reinstalled the manifold and carb, which fit properly snug, tightened the clamp snugly and tested again....same thing--idle increases when I spray carb cleaner onto the manifolds. Obviously, in spite of their perfect condition, air's somehow getting in through that interface. Or, in spite of my having just replaced the throttle shaft seals, perhaps, it's leaking in through the shaft seals....though it's confounding how air's getting in through either, so....<insert frustration emoji here>

Although the manifolds look/feel pristine (I meant to bring a camera, but forgot....sorry), I'm awaiting the JMB "carb holders"...as I'm now well past the logic/reason stage and entering the replacing parts out of desperation neurosis.. <insert frustration emoji here>

That said, after turning the right (bad) carb's air screw out to ~ 4 or so turns, I've got the bike to the point where it's more or less drive-able. The right cylinder backfires just a wee bit on deceleration (when driving), but the idle's fairly smooth (a bit high at ~1400 rpm) and the flat spot's mostly gone. I say mostly because there's a slight hesitation upon initial acceleration, but, after that, it accelerates normally, doesn't appear to be over-heating and drops back down to idle without hovering.
 
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...I installed new piston/diaphragms in both carbs. Now, the piston in the right (problem) carb takes ~15 seconds to drop whilst blocking the oval air port (i.e., it's perfect); but the left, which was perfect, now drops in ~4 seconds....similar to the way the right (problem) carb was performing with the original piston/diaphragm...

With the "moon" port closed off, the only other air leakage is from the fitment of slide to carb body, and the enrichner (choke) plunger. If the replacement slide is slightly smaller, more air can escape. If the enrichner (choke) plunger is loose in its chamber, or not fully seated, or a wrong-sized replacement, or the enrichner body gasket leaks, air can escape.

Also, on the XS2 carbs, the enrichner (choke) only exists on the left carb, so more air leakage potential there.

...--idle increases when I spray carb cleaner onto the manifolds. Obviously, in spite of their perfect condition, air's somehow getting in through that interface. Or, in spite of my having just replaced the throttle shaft seals, perhaps, it's leaking in through the shaft seals....though it's confounding how air's getting in through either, ...

Might want to check the integrity of the enrichner (choke) crossover tube, and its fittings, for cracks, splits, ...etc.

...after turning the right (bad) carb's air screw out to ~ 4 or so turns, I've got the bike to the point where it's more or less drive-able. The right cylinder backfires just a wee bit on deceleration (when driving), but the idle's fairly smooth (a bit high at ~1400 rpm) and the flat spot's mostly gone. I say mostly because there's a slight hesitation upon initial acceleration, but, after that, it accelerates normally, doesn't appear to be over-heating and drops back down to idle without hovering...

Those aren't "air screws", they're "mix screws", deliver an fuel/air mix. Turning those out increases idle fuel, compensating for a possible air leak.

Do a forum search on "transfer ports" and "bypass ports"...
 
With the "moon" port closed off, the only other air leakage is from the fitment of slide to carb body, and the enrichner (choke) plunger. If the replacement slide is slightly smaller, more air can escape. If the enrichner (choke) plunger is loose in its chamber, or not fully seated, or a wrong-sized replacement, or the enrichner body gasket leaks, air can escape.
Also, on the XS2 carbs, the enrichner (choke) only exists on the left carb, so more air leakage potential there.

I think it may've been you who answered this earlier in this thread, but the dust cap on the choke (enricher) is torn, as can be seen in one of the photos I posted. Could that be the source of the air ingress or would a leak need to be due to a defective internal seal? I don't recall ever disassembling the choke or replacing any seals inside it. In fact, I've never thought to check for a rebuild kit for the choke..


Might want to check the integrity of the enrichner (choke) crossover tube, and its fittings, for cracks, splits, ...etc.

Is the choke crossover tube the short (~1.5") long fuel line that runs between the carbs? If so, it's new; I always replace it and its two hose clamps during rebuilds. If that's not it, I'm not sure what other tube you're referring to? The only other hose running between the carbs is the u-shaped one with a spring surrounding it and plastic fittings....but isn't that the float bowl balancer that carries fuel?


Those aren't "air screws", they're "mix screws", deliver an fuel/air mix. Turning those out increases idle fuel, compensating for a possible air leak.

I call it an air screw, but I know it's actually a mixture screw....and, having turned the screw on the problem (right) carb out ~4 turns is, I reckon, why I was able to get the engine in a more or less drive-able state. So I've gotta find/correct that air ingress.

Do a forum search on "transfer ports" and "bypass ports"...

Will do. Thanks.
 
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Update...

Today, FRUSTRATION...
I received the JBM carb holders and diaphragms (along with a new JIS BS38 carb screw set...none of which fit the top cover so I can replace the screw I stripped....but I digress). Anyway, I installed the carb holders without any problem and brought the carbs home with me to perform the diaphragm-ectomies (too dark to work at the storage place where I keep the bike), so I haven't yet test-run the engine to see if they eliminated the air leaks and/or improved the engine's performance.

However, there really isn't anything wrong with the old manifolds (carb holders) that I can see. Although I didn't photograph it, I tried stretching the material in search of cracks, but found none. They look nearly new, with pliable and solid rubber material (i.e., no cracks/damage). So, unless my recent and repeated r&r'ing of the carbs ovaled or widened the opening sufficiently to make it impossible for the clamps to create a seal around the carb openings, I'm not seeing much difference between the new and "old" sets of manifolds (carb holders).

As for the carbs, I remain mightily confused. Now, with the JBM diaphragms installed, I'm seeing the same functionality I saw with the other "new" piston/diaphragms that I just installed last week. The right carb--the side that was causing all the problems--seems to maintain a good, proper vacuum...if I raise the piston and block the curved port atop the intake side of the carb, the piston takes ~15 seconds to gently glide back down. Normal. However, the left carb--the side that IS running well--seems to have a questionable vacuum...if I repeat the piston-drop test, the piston drops in just a few seconds. Much quicker than the other carb. But, again, the side with the fast-dropping piston's the side that IS running well.
So, at this point, I don't know which way's up....I mean the good carb's got a poor vacuum and the bad carb's got good (normal) vacuum and both've been rebuilt multiple times with Mikuni parts (including new Mikuni needles) and JBM diaphragms. What's left? Ugh..

So, I guess today's question is: Should I be concerned with the differing piston drop rates between the two carbs--regardless of whether it's related to the air leaks--or just put everything back together, as-is, and see how the engine runs with the new manifolds? Or, if the fast-dropping piston in the one carb IS an issue....having replaced the diaphragms with JBM's, what else should I check that affects the piston-drop speed/vacuum in that upper chamber?
 

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what else should I check that affects the piston-drop speed/vacuum in that upper chamber?
On the BS34's the upper chamber is tied to the choke passages. You can raise the slide for the drop test and open the choke and it will drop like a rock. So a bad choke seal can cause slide problems. Not sure if the 38's are piped the same way. Anyone? Bueller?
 
On the BS34's the upper chamber is tied to the choke passages. You can raise the slide for the drop test and open the choke and it will drop like a rock. So a bad choke seal can cause slide problems. Not sure if the 38's are piped the same way. Anyone? Bueller?

Jim, you may be onto something, but I'll wait for someone more knowledgeable to chime in.

In the meantime, I tried the drop-test again, once with the choke off and a second time on. The piston drop was the same regardless of the choke position. So that tells me that either the choke is NOT related to the upper chamber vacuum or that you're right about there being a duff seal in there. However, I don't think rebuild kits for the choke are available anymore, so, if that's the problem....well, I may have another problem. But I'll wait for someone else to chime in...hopefully, based on my new drop-test results (with and without choke)..

p.s. The side with the choke's the side that's WORKING. So go figure....
 
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I would take the choke plunger out and examine it's end. It has a rubber plug embedded in it and that's what seals against the bottom of the hole it fits into. Maybe the rubber has deteriorated. Before removing it, you might try pushing down on the plunger while you perform the slide drop test. See if the added pressure will seal it and change the slide drop rate.

Yes, an original plunger is N.L.A. but Niche Cycle claims to have an aftermarket replacement .....

https://www.ebay.com/itm/Yamaha-TX6...r-Choke-Plunger-306-14171-01-00-/162606286799
 
I would take the choke plunger out and examine it's end. It has a rubber plug embedded in it and that's what seals against the bottom of the hole it fits into. Maybe the rubber has deteriorated. Before removing it, you might try pushing down on the plunger while you perform the slide drop test. See if the added pressure will seal it and change the slide drop rate.

Unfortunately, I already verified that the choke position makes NO difference re the drop rate. Tried it both ways and no difference at all.

Yes, an original plunger is N.L.A. but Niche Cycle claims to have an aftermarket replacement .....
https://www.ebay.com/itm/Yamaha-TX6...r-Choke-Plunger-306-14171-01-00-/162606286799

I actually ordered the plunger when (I think it was you) someone suggested that as a possible source of leakage a few days ago. But I suspect, as was described, that if something's leaking, it's gonna be a rubber-something and not the metal plunger. But I'll take the choke apart and reconnoiter further tomorrow.

Oh, and again, the choke components reside on the GOOD carb, not the side that's having problems. And the connecting line (that runs between the two carbs) is new. Is there a seal or something else within the other carb's body along the path leading into the other carb from the choke gizmo on the first carb that I need to check....if that makes any sense?
 
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You're not understanding things here. The choke plunger is metal, yes, but it has a rubber tip that does the sealing. That's why I suggested pushing down on the plunger, maybe that will make the worn or damaged rubber end seal.
 
You're not understanding things here. The choke plunger is metal, yes, but it has a rubber tip that does the sealing. That's why I suggested pushing down on the plunger, maybe that will make the worn or damaged rubber end seal.

Sorry, I may not've been clear. I did press down on the choke lever. I just tried again...No difference. I take it, then, that the choke position IS supposed to make a difference in the vacuum in the upper chamber. And since it isn't, then there's a problem with that choke seal. Have I got it now?

Note that the choke has been functioning correctly when starting the engine. It also helped mitigate the original flat-spot problem. That is, when I enabled the choke, the flat spot is mostly gone and the bike's ride-able. That's why I original came to the conclusion that the flat-spot's due to a weak fuel/air mixture or air leak. So if the choke is NOT working to affect the upper chamber vacuum, it IS working for its intended purpose--enriching the fuel mixture when starting the engine.

I removed the three choke screws, but the unit appears to have a gasket (I thought the XS2 carbs didn't use gaskets??) and it's not coming apart by hand. Are there other screws or something else I need to remove the take the choke unit off?
 

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Small Update...

I got the choke assembly off, but I left my metric wrenches at the storage unit with the bike, so I can't remove the plunger. However, I tested its sealing effectiveness by blowing through the port circled in red whilst lifting the plunger. When lifted, the air went through, when I let the plunger close, no air went through. I would kinda think that at least eliminates the likelihood of a bad seal...or choke assembly. No?

There is a paper-thin gasket between the choke and carb body and I was able to remove it without damage.
 

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Today's Update...

Disassembled the choke assembly, thoroughly cleaned its innards (very dirty/grimy!) and installed the new plunger and seal. Interestingly, the small, black rubber insert within the [wide] end of the old plunger was intact but VERY hard, so it was probably acting more like a stopper than a seal. Last night, lifting and lowering the plunger whilst blowing air through the side port DID open and close the airway, but I'm sure the more pliable rubber insert on the new plunger's doing a better job. At least, it feels like it may be sealing more positively with the new plunger than it did with the old plunger. Though, at this point, I may simply be positive-thinking myself into that conclusion. What I do know for certain is that the plunger's now sliding more freely within its channel than the old one did prior to cleaning.

Re the seal: Unless I've got it installed incorrectly (see attached photo), the seal slides up and down with the movement of the choke plunger, so it doesn't really seal the top of the choke plunger. Is that the way it's supposed to fit/work? I'm so used to seeing the old seal, which's been torn and tattered for the past coupla decades, at least, that I'm not sure whether it's supposed to cover the entire top or whether it's supposed to slide up and down with the motion of the plunger (when the choke lever's operated).

Anyway, unless anyone's got any further suggestions re checks for the carbs, I think--after having rebuilt them multiple times--they're as close to OEM spec as they're gonna get. Everything that's disassemble-able has been disassembled (and cleaned/renewed). Unfortunately, it's raining today and may do the same tomorrow, so I may not be able to reinstall the carbs and fire-up the bike 'til Sunday. But if anyone's got any last-minute recommendations....while the carbs're sittin' in front of me...please post them here.

The carbs'll be mounting onto the new JBM "carb holders", so I can't imagine where else air leaks could be coming from....if I'm still experiencing lean running and/or flat-spots.

p.s. One more question: I've never used any lube when installing the carb into the manifolds. However, some of the articles I've been reading reference grease. Should I be using that; and, if so, is wheel bearing, lithium or dielectric grease alright? Those're what I've got on hand. Actually, I think JBM recommended "soapy water". Thoughts??
 

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The rubber seal is supposed to snap onto that lip just above the brass hex "nut" shown in your 2nd pic. It should stay there with the plunger shaft sliding up and down through it.

A light smear of white lithium grease works well for lubing the intake boots and carb spigots. That's what I used to use before I got specific rubber lubes (red rubber grease or SIL-Glyde).

https://www.amazon.com/AGS-SG8-Lubr...1A60X714KPC&psc=1&refRID=TR97QTZKJ1A60X714KPC
 
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