1978 XS400 Carb Clean help needed!

[arm]

Hi guys, I noticed my 78 XS400 is painfully unresponsive to the throttle and struggles to push any speed beyond 60km/h. My buddy who drove the bike back for me when I bought it told me that it for sure needs a carb clean.

I want to pull the carb and see how much some shops will charge since the process seems quite tedious and there are tons of tiny parts.

Im not sure how to pull off the carbs though, I found this video but in the video, the mechanic already has the carbs off

The trouble is I'm not sure how, I have a full tank of gas in the bike so I'm unsure if I have to empty the fuel first or if there is any setting that will shut off the fuel flow.

Please let me know if I can include any more detail or if there is a good video of someone taking them off!

 

[avemachina]

Hi there!

It's actually pretty simple, but annoying. You won't need to take the fuel tank off, just disconnect the fuel hose from the left-hand (shift lever side) carburetor, and the vacuum hose (from the right-hand, rear-brake side) intake boot.

For the fuel hose, make sure your fuel petcock is set to ON (if your options are PRI-ON-RES) or OFF (if your options are RES-ON-OFF). You'll lose a little bit of gas, so it's helpful to have a tray under the hose or something to catch it instead of spilling. If you're super paranoid about fuel dumping out, fold the fuel hose in half and use a little clamp (either a proper "automotive line clamp") or a big binder clip) to keep fuel from spilling out. I've attached a picture of what those look like. Mine are orange. You can also expedite the cleaning by draining the carb bowls, by unscrewing the little brass flat-head screw in the bottom-most part of the carb--use a funnel and that same catch-tray from before. The fuel will come from the tubey aluminum part that points straight down when you do this.

As far as actually removing the carbs, there are five points of disconnection: the hose clamps on each side of the intake boots, the hose clamps on each side of the H-hose (which connects to the air filters), and the throttle cable. The pic with the airbox diagrams illustrates this--you'll loosen #5 and #7 on both sides. The throttle cable can be wiggled out in-situ, just sit on the left-hand side of the machine, take a good look in, pull the cable out of the bracket guide, then spin it a little to free the end from the part that actually pulls it up/down (#53 in the exploded carb pic).

Once your hoses and cable are disconnected from the carbs, you can start wiggling the carbs themselves free of the rubber hoses--usually I wrestle them toward the rear and right-hand-side of the bike. You can find some clearance by rotating them around, and making sure the spot where the throttle cable connects doesn't catch the intake boot. THIS is the annoying part.

Regarding the actual carb disassembly and cleaning--don't be scared, and don't be shy! They go together like LEGOs. I've also attached a PDF schematic of all the parts and where they go (assuming you've got Mikuni BS34s, the default carbs).

 

[avemachina]

I thought I might add a couple of tips for the carb disassembly/cleaning:

- use a few cheap little plastic cups or a disused muffin tray to keep the parts straight.
- Be gentle with the rubber and plastic parts.
- The pins that hold the floats on can be stubborn, and the posts that hold them on can get brittle--be gentle and tap them out carefully if you need to.
- You'll see a million ways to actually clean the things, but if you can splash out for an ultrasonic cleaner ($90USD for a big one, maybe $40USD for a little cheapie), they're awesome.
- They make powder and cleaning solution and whatever for those cleaners, but I'm a fan of making my own--distilled water and a splash of Simple Green for the rubbery, plasticky, or aluminum bits, and distilled water with a splash of vinegar for the brass bits.
- Carb dip and spray also exists, but for whatever reason, it seems like everyone has to clean the things like three times before they're actually clean!
- The little BBs that hold the choke lever are a tiny, horrible nightmare. Be super careful not to lose them, and if you have trouble putting them back in, you can "glue" them in temporarily with a little vaseline while you work.

 

[arm]

I thought I might add a couple of tips for the carb disassembly/cleaning:

- use a few cheap little plastic cups or a disused muffin tray to keep the parts straight.
- Be gentle with the rubber and plastic parts.
- The pins that hold the floats on can be stubborn, and the posts that hold them on can get brittle--be gentle and tap them out carefully if you need to.
- You'll see a million ways to actually clean the things, but if you can splash out for an ultrasonic cleaner ($90USD for a big one, maybe $40USD for a little cheapie), they're awesome.
- They make powder and cleaning solution and whatever for those cleaners, but I'm a fan of making my own--distilled water and a splash of Simple Green for the rubbery, plasticky, or aluminum bits, and distilled water with a splash of vinegar for the brass bits.
- Carb dip and spray also exists, but for whatever reason, it seems like everyone has to clean the things like three times before they're actually clean!
- The little BBs that hold the choke lever are a tiny, horrible nightmare. Be super careful not to lose them, and if you have trouble putting them back in, you can "glue" them in temporarily with a little vaseline while you work.

Thanks for the detailed response! I just called a few shops and they said either the bike is to old and they wouldn't be able to find the seal kits or that it would cost $400 CAD!

I do wonder about the seals, in the video above the guy doesn't bug with the seals but the shop I called today said that once you open them you must replace the seals, so should I start looking for a seal kit before anything?

 

[avemachina]

Thanks for the detailed response! I just called a few shops and they said either the bike is to old and they wouldn't be able to find the seal kits or that it would cost $400 CAD!

I do wonder about the seals, in the video above the guy doesn't bug with the seals but the shop I called today said that once you open them you must replace the seals, so should I start looking for a seal kit before anything?

I'd look for a "carb rebuild kit", which is basically just a bunch of gaskets. They're a dime a dozen online (well, about $20USD), but the good ones generally have the jets, needles, carb float bowls, and a few o-rings. The seals they're fussing about might be the weird little guys on either end of the butterfly valve... axle? But I've successfully cheated that with some nitrile o-rings from my friendly neighborhood discount hardware store. They might also be bugging about the carb bowl gaskets which lol, no. I've reused them a handful of times, they're just that spongy gasket material we all know and love.

The picture is a typical rebuild kit. You might not need all the parts that it includes, but you will need two sets of these, one for each carb.

 

[arm]

I'd look for a "carb rebuild kit", which is basically just a bunch of gaskets. They're a dime a dozen online (well, about $20USD), but the good ones generally have the jets, needles, carb float bowls, and a few o-rings. The seals they're fussing about might be the weird little guys on either end of the butterfly valve... axle? But I've successfully cheated that with some nitrile o-rings from my friendly neighborhood discount hardware store. They might also be bugging about the carb bowl gaskets which lol, no. I've reused them a handful of times, they're just that spongy gasket material we all know and love.

The picture is a typical rebuild kit. You might not need all the parts that it includes, but you will need two sets of these, one for each carb.

Excellent!

Last question, do the carbs need to be "dialed in" after cleaning them? the whole process seems a bit intimidating and there are tons of tiny parts, so I'm thinking maybe handing this off to a shop since the carbs might only need to be cleaned once in my time with the bike

 

[tstidham]

The shaft seals are still available and are well worth replacing. I just did them on mine with a kit off ebay that included new butterfly plate screws, but it's a tricky job. However once done, the bike runs sooo much better and eliminated all of my decel popping I used to have.

 

[avemachina]

Haha, the butterfly plate screws on mine came from an old Macbook after I drilled them out, with some loctite. I know they say to peen the ends, but I've yet to see them budge. Kind of a Mad Max solution, but when you've got a tray of tiny salvage laptop screws and zero patience....

The shaft seals are part number 256-14997-00-00, but they were also used on Suzukis, as part number 13651-51010 (and probably some Kawasakis...)

As far as "dialed in", they will probably need to be synchronized again. The usual way I do this is bench syncing (once they're reassembled and joined together, roughly get the butterflies to the same amount of open using some kind of feeler wire like an unwound guitar string). Then, use an oil manometer, which you can build for a few bucks, or a real actual vacuum gauge if you have access to two of them.