Glad to be here and the XS that was meant to be...

I *think* the oil leak... or leaks... have been located. I placed some cardboard under the bike and got a solitary drop of oil a few hours later. Following it directly up, there was a bit of oil on the underside of the crankcase. Gravity seems to point to a bolt on the underside of the crankcase. I wiped it clean for the photos; the "washer" has seen better days for sure; looks like the bolt has eaten into the rubber-ish material.

Can you guys help with identifying this bolt? It seems to be part 90105-06069-00 or the "washer-based bolt". It is directly underneath the neutral sensor.

What function does this bolt serve beyond keeping oil in the case hahaha?

I will say the oil leaks more liberally when the bike is running and warm. When cold, maybe a drop or two a day. It sat a week and had just a small oil spot.

Tomorrow we will sync the carbs AND see if we can observe the oil escaping the case. It drops right down into the path of the rear tire so no bueno.

Also the wires coming out of the stator are slightly oily. Not "wet" but covered with a slight film. Normal or no? I can see where a PO may have made a "repair" to the rubber wire pass-through gasket, but the oil drip did not appear to come from the wires.

See photos... these are looking from the shifter side of the engine. Second photo shows the bolt, a (blurry) oil sensor in the foreground, and some oil-damp wires:)

Thanks all; we're getting close!


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On the positive side, I followed the advice @Buddha gave and found some better solder for cables. I ordered a new clutch cable but wanted to shorten it as the bars are lower. I shortened the sleeve and cable by roughly 145mm, re-used the original cable end, and made a nice new clutch cable with proper solder. I used a solder pot this time.

The new cable had a flange that was just a bit larger than the OEM so I re-used the original flange as well. Lubed it up, followed the adjustment procedure, and all is well. You can see in the photo below the new flange is slightly larger... large enough to not interface with the clutch cable holder. This photo is before I smoothed the surface of the cable end.

I used the Niche brand clutch cable. The wire is robust and appears to be the same size as OEM. I'm going to re-purpose another as a brake cable as I used improper solder the first time around.

On further inspection... the leak is coming from one of the various seals on the sprocket side of the engine. So we're going to remove the sprocket, clean everything, and see what's leaking. We'll most likely replace all the seals while it's exposed. Fun!
On further inspection... the leak is coming from one of the various seals on the sprocket side of the engine. So we're going to remove the sprocket, clean everything, and see what's leaking. We'll most likely replace all the seals while it's exposed. Fun!

On that side you got main shaft seal, clutch rod seal, shift shaft seal, and the o-ring on the neutral switch. There are also the wires to the stator, but it's less common for that to leak..
On that side you got main shaft seal, clutch rod seal, shift shaft seal, and the o-ring on the neutral switch. There are also the wires to the stator, but it's less common for that to leak..
We’ve got two leaks: the clutch rod seal and the neutral switch o-ring. Parts ordered. I’m going to wait on replacing the other seals for now.
As far as the carbs go, a sync didn’t solve the hanging throttle issue. I did discover that the bike is running rich. Very rich.

We’ll check the jets; there’s a chance the PO installed larger jets as he was planning on pods and a free exhaust.

Carbs will get another cleaning and we’ll check the slides/diaphragms and the shaft seals. If the rubber is 40 years old… time to replace.
Took the carbs completely apart and remembered... we didn't clean them, we just installed new bowl gaskets. So they'll get a good clean now.

The PO didn't do us any favors; one of the Starter Jets was loose in the socket. And one of the bracket screws was terribly mangled.

The other thing I discovered is the slides, whilst in good condition, do not drop under their own weight. The diaphragms are stiff. That might explain the hanging throttle issue.

Also one of the washers on an outside butterfly shaft seal was missing (see below).

I guess the carbs require a total of 4 butterfly shaft seals; I thought it was only 2. Oops.

Also, can you guys help with a few questions?

I've seen different jet sizes listed for these bikes. Ours (1981XS400 Special II) currently has a BS34 with the following jets:

155 Starter Jet a.k.a. Pilot air Jet
42.5 Pilot Jet
135 Main Jet

1) Are these the correct stock jet sizes?

2) I'm looking for a part number that isn't on any diagram I can find:
The OUTSIDE of the butterfly shaft is secured by a circlip, underneath which is a thin brass washer. I can't find the part number for this washer anywhere. Our carb was missing one. It will be easy to replicate but I love getting actual Yamaha parts if possible. See photo below of a dirty carb and the washer in question.


The jets sizes you have listed are the stock ones for a 1981XS400 Special II (XS400H).

I don't have a part number for the washer at hand as none of the exploded parts diagrams seem to show them, but I would just measure up the washer and get something at least as thick. You can then rub it on a file or stone to get the exact thickness (my two cents anyway.)
On that side you got main shaft seal, clutch rod seal, shift shaft seal, and the o-ring on the neutral switch. There are also the wires to the stator, but it's less common for that to leak..

Seals delivered.

The location of those seals makes me wonder; the slow oil film under the neutral sensor would indicate we need to drop the oil before swapping... true? Or maybe just a 45 degree right lean for the swap? Any thoughts from experience? Dropping the oil is no problem, but if can be avoided...
Waiting for install:
4 butterfly shaft seals
1 clutch rod seal
1 neutral sensor o-ring.
Found some M8x14x1 brass washers for the butterfly shaft retaining clips. Had to buy a multi-pack but I'll have brass washers for years haha. They'll need to be thinned just a bit.

Carbs are fully apart (all rubber bits removed for cleaning) and cleaner then they've been in years I suspect. When we get time they'll be reassembled.

Once again, I appreciate all of the help!
Carb Diaphragms: the old ones were intact but quite stiff, so I removed them from the slides.

I have replacement diaphragms.

The two plastic washers which sandwich the diaphragm… I’m fairly sure they are not supposed to come off. There’s an aftermarket diaphragm company that requires destroying the plastic, but there must be a better way.

On other bikes, the plastic washers have a bit of space for the diaphragm… but these BS34 slides have very little space for the diaphragms to slide in.

Anyone with experience?
Well I solved the carb diaphragm issue. It was nice and hot in the garage so I sat down and began gently opening up the space between the plastic diaphragm holders. Slow and steady. After a while they had just a bit of separation. I then used some Hoppes and got them nice and slippery. The new diaphragms went on much the same way… very carefully and slowly. I used the dull side of a butter knife to slide them into the groove that is on the bottom of the two plastic rings.
The first diaphragm took a while; the second was easier, maybe due to the fact that I soaked the plastic in hot water prior to working the diaphragm.

Now I have two new, ultra flexible diaphragms! I’m almost worried that they give zero resistance. We’ll see.

Carbs have been thoroughly cleaned. Time to reassemble and then address the oil seals on the engine.


An old, stiff diaphragm and the new ones. I did make sure all of the old material was removed from the slides.
Took a bit to replace the clutch rod seal and the neutral sensor o-ring. Used a highly advanced technique pictured below, that, whilst making the repair easier, may not have been necessary.


If you've ever wondered, here's what lies behind that plastic neutral sensor cover. Up and to the left is the new clutch-rod seal as well. Much less lateral play with the new seal.

Honorable mention: some nicks in the casing and all of the other very dirty seals and extraneous gasket material left by a PO. The seals aren't allowing any oil to escape but this winter they'll get replaced. Now that the clutch-rod seal isn't leaking I'll wipe it all dry, button it up, and see if my repairs solve the oil issue.


Highly advanced technique using a box of books, blankets, and a rubber floor mat. All weight is on the frame. Mostly.
No leaks under the bike, which is a good sign. Running it for a while will be the real test.
Carbs have been re-assembled and are back on the bike, along with the NEW air filters.

We rebuilt the petcock but it still drips fuel; I've fiddled with it a lot and it seems the piston isn't fully sealing the fuel orifice. I've seen guys use spacers and extra o-rings to solve the problem but since we have an aftermarket non-vacuum petcock at the ready, we'll do that for now and eliminate the vacuum hose.
Progress report:
Even after a carb overhaul (new diaphragms, shaft seals and washers, a thorough cleaning etc) I was still getting the hanging idle. Ugh.

The throttle cable is new, lubed and with no kinks. It snaps back with authority, at all handlebar positions.

Carb is synced. Float levels set at 26mm

Bike starts with the first kick. If I engage even the half-choke, it stumbles. That would lead me to suspect a rich condition, and the plugs would suggest that as well. Everything is stock, except for Hi-Flo brand replacement air elements. The only thing I haven't looked at is the idle mixture screws. So I pulled the carbs and drilled the caps.

To my surprise, the screws were set at much more than the OEM 1 to 1.5 setting. They were at 3-4 or more turns out. So I'm wondering did someone tweak them without damaging the brass plugs? They were untouched before I drilled and removed them.

I pulled and cleaned them and put them back, this time at 1.5 turns out. Sprayed the carb out with air and put it back in the bike.

Bike fired up first kick as usual, and the idle once again sounded fantastic. I could listen to it all day:)

I blipped the throttle and the engine came back to idle so much faster. Still a bit slow to return but they didn't once hang. Ok... maybe now I'm getting somewhere. I'm going to turn them in by .5 and see what happens. The Haynes says factory setting is between 1 and 1.5 turns... so we'll see.

If anyone has thoughts as to what I may be missing, I'm all ears!
Checking in... success with the carbs:)
Thanks to a few members here... especially @tstidham for some good ideas as I sorted this issue.
My idle mix screws were not properly adjusted; the bike was idling off of the main jet and never touching the idle circuit.
At least the carbs got yet another cleaning... and I learned a bit more about these BS34's in the process.
Now my idle doesn't hang and when I roll off the throttle the engine drops down to idle speed as it should. I don't have it dialed perfectly yet but it's close. My fuel level, as measured by the tube method, is still low. BUT... I adjusted the floats higher and it flooded. I'll have to find the middle ground there. I still have a random oil leak but it might be as simple as the o-ring on the oil filter basket being not quite up to the job.

BUT... the bike actually feels like a bike now. Onward.