1978 Yamaha XS400 full tune up

[arm]

Hey guys, So I've been posting here for a while and some of you may know my 78 xs400 is a bit sluggish, I want to do full tune up on it and get it nice and reliable (probably shouldn't have went with a 50 yr old bike but balling on a budget) So far I changed the oil, oil filter, and cleaned the oil filter screen. The next things I see myself doing is

A MUCH needed carb clean, I'm a bit scared to though, I'm afraid ill break something or if I need to order seals or if I don't understand the severity of my carbs. I remember cleaning a kids dirt bike carb in my highschool auto class and it was brutal and never ended up working iirc. The alternative is taking it to a shop and paying $300 CAD ($222 USD). Im also concerned about how I sync carbs since I don't rlly have many fancy tools to do so

Clutch cable adjustment is also very much needed

whenever the bike runs it smells a lot like gas so I think its running rich (maybe this would be solved by cleaning the carbs?) not sure how to adjust this, The idle for the bike I think needs to be adjusted, its not the most stable and I'm not sure where the stock bike should idle around

I wanted to know if there is anything else I should take a look at? Im not insanely mechanically savvy and half the time I don't understand the words I read on here but I'm good with googling around and watching youtube tutorials. Do you guys think its risky to take on the carbs myself or hand it off to somebody?

 

[avemachina]

If you're truly, utterly terrified to do anything with the carbs, you could give it to the shop and consider that you're paying for peace of mind. Most everyone will recommend learning how to DIY just because it's something you might need to do more than once--if you've got a Philips-head screwdriver, a large and a small flathead screwdriver, a can of carb cleaner, and a little bit of time, you try tackling this the minimally-invasive way (no sync involved):

1) Drain the gas out of the carbs using the little brass screw at the very bottom of each carb body. Take the carbs off the bike. Flip the assembly upside-down and take the float bowls off the bottom of each (4 screws per). Clean any gunk/sediment out of the bowls with carb cleaner, and remove the (probably black or grey) gasket that the bowl was sitting on. You might need replacement ones if yours rip or are really old (easy to find online, just look for Mikuni BS34 gaskets)

2) Take a look at the underside of the carbs. The most obvious feature is the floats--two brass bubbles joined by a thin metal strip. They control how much fuel sits in the bowl. If you aren't comfortable messing with these, skip this step, but it's recommended: confirm that the distance from the top of each float "bubble" to the spot where the gasket was is 26mm. Gently bend the thin part in the middle (tang) of the strip connecting the floats to get your 26mm.

3) Look near the middle of the carb, for what looks like a big brass flathead screw with a hole in it--that's your main jet. Unscrew that, give it a good clean. Screw it back in and don't forget the washer that goes under it.

3) One last spot--look for a round black rubber plug. Jimmy it off, and you'll see the brass pilot jet--unscrew it with a flathead screwdriver, and clean it too. Screw it back in, and put the little plug back on.

5) Put the gaskets and float bowls back on, put the carbs back on the bike, connect the throttle cable, connect the fuel. Double-check to make sure you're not getting leaks anywhere. Start your bike, see if things have improved.

This is a very, VERY basic way to clean things up a little and help familiarize you with what's happening in there. You're definitely capable of twisting a few screws and spraying a little cleaner, so there's not a lot to mess up. If you're feeling brave, you can go back in and clear out the little passages, but at minimum this will introduce you to the process and help build some confidence. Give it a try before you hand it to a shop, you'll be surprised!

 

[arm]

dang, i think youve convinced me. I looked up a few video and wanted to get your thoughts

This video has a guy cleaning the same Mikuni BS34 carbs for the same bike

and this video went through a general walk-through

Also if I do go ahead with video one do I need to sync the carbs? I don't mind picking up a tool and doing it if its a matter of tweaking the carbs a bit and re testing

 

[avemachina]

Video 1 is pretty solid! Just a couple notes on differences you might find:

- the bracket he takes off first, around 0:22, might be a different shape. On mine ('81), it's just an L-shaped bracket on one side, not a hollow square like his. Same deal--just unscrew it either way. His advice at 9:42 on making sure the orientation is correct when you put it back on still applies.

- And at 1:55ish, the drain bolts he has are at the bottom of the bowls--mine are sort of on the side, and a brass flathead screw instead of a bolt. There are some holes pointing downward to let fuel out as you unscrew the flathead on the side. Have a little funnel or catch cup for the fuel if you want to keep things slightly less smelly. Same thing, just unscrew it.

Other than that, you're cleared for takeoff!

You only need to sync the carbs when you take them apart from each other--if you keep them in place, connected as a pair, you should be fine to keep whatever sync level you have now. If you want a "close" sync, you can bench sync them--try this method: bench-sync with a needle.

A more precise sync can be done with a fluid manometer, which you can build with a yardstick, plastic tubing, and some ATF or motor oil (here's a diagram from xs650 rider of how it's made with HoughMade's instructions here). He attaches it to a wall; I just hang mine from a wire off the handlebars. You can always dip into that at a later time, since you don't need to take the carbs off the bike to do it.

Good luck!

 

[CaptChrome]

You aren't going to make this worse running than it is now by trying a cleaning. And if you want to own a vintage bike, learning how to clean and adjust the carbs yourself is sort of the cost of doing business.

One serious bit of advice - Use only brake cleaner for cleaning until you get more skilled and knowledgable of this task. A lot of cleaning solvents will dissolve and kill the internal rubber seals and parts in the carb and leaving your carbs in worse condition than when you started. Those powerful cleaning solvents have their place, but until you know where all of the rubber parts are and how to remove them leave the carb cleaners, electrical parts cleaners, injector cleaners and other concoctions that your friends recommend on the shelf and only use brake cleaner to dissolve, flush and blowout any gunk inside the carbs.

 

[avemachina]

Captn speaks wisdom. Whatever you use, make sure it's labeled as safe for plastics/rubber (usually, the non-chlorinated brake cleaner). There are a few bits that can get brittle or chewy if you get too aggressive.

A little bit more background, so you know what exactly you're doing: when cleaning a carb, the actual point is to focus on the holes and passages, not so much the aesthetics. It doesn't have to shine, but the holes all have to be unobstructed. The little brass guys that look like flathead screws with holes in the middle, those are your jets, and they determine how much fuel goes where. There are a handful of little passages that route the fuel to various places, and those have to be goo-free too to make sure that the air and fuel are mixed in the right ratio. Any goo in the way will make the ratio off, or potentially make the moving parts stick, which leads to crappy throttle response, bad idle, etc.

Another little tip is to lubricate your throttle cable--you can usually just drop some motor oil or 3-in-1 into the space between the actual wire part of the cable, and the sleeve. Wiggle the wire part to help get the oil down until it gets to the other side. If you find any fraying on the cable, replace it. This helps it last longer and move better.

 

[arm]

Captn speaks wisdom. Whatever you use, make sure it's labeled as safe for plastics/rubber (usually, the non-chlorinated brake cleaner). There are a few bits that can get brittle or chewy if you get too aggressive.

A little bit more background, so you know what exactly you're doing: when cleaning a carb, the actual point is to focus on the holes and passages, not so much the aesthetics. It doesn't have to shine, but the holes all have to be unobstructed. The little brass guys that look like flathead screws with holes in the middle, those are your jets, and they determine how much fuel goes where. There are a handful of little passages that route the fuel to various places, and those have to be goo-free too to make sure that the air and fuel are mixed in the right ratio. Any goo in the way will make the ratio off, or potentially make the moving parts stick, which leads to crappy throttle response, bad idle, etc.

Another little tip is to lubricate your throttle cable--you can usually just drop some motor oil or 3-in-1 into the space between the actual wire part of the cable, and the sleeve. Wiggle the wire part to help get the oil down until it gets to the other side. If you find any fraying on the cable, replace it. This helps it last longer and move better.

Finally got em out! it wasn't to hard but it was a nice experience getting comfortable with the bike and its internals. How's the look of it at a glance?

haven't had much time but gonna disassemble and clean em soon

 

[CaptChrome]

How's the look of it at a glance?

They look pretty good, but all it takes is a spec of dirt inside the jets or a clogged internal passage and good looking carbs can run like crap. Congratulations and good job to this point. You are going to learn a lot by doing this.

One question for you folks with the Mark II carbs like on this 1978 - is this idle mixture spring/screw arrangement (circled below) normal? I was expecting them to be recessed into the casting like the ones on my 1981 Mark III carbs:

 

[tstidham]

That's how the mixture screws look on my 78. Makes tweaking them pretty easy!

 

[CaptChrome]

@tstidham - thanks! I learned something new. Yes, I am now jealous of how easy the pilot mix screws are to get to. I pretty much have to pull Trusy Rusty's tank off to see what the heck I am doing when I set mine.

 

[arm]

That's how the mixture screws look on my 78. Makes tweaking them pretty easy!

any tips on how to set them? I noticed my bike tends to run really rich and the left carb spring/screw seems a bit more tight then the right

 

[arm]

Here's a look at them opened up, I noticed that the carb on the left has a good bit of black deposit while the one on the right is clean. That's right, I have touched the one at the right at all.

I also want to get that last golden bolt out cause I saw there is a filter screen at the bottom of it.

Another point of concern is that one of the jets wont come out, it looks like part of the screw where the head is has broken off, should i just blast it with brake cleaner or dive deeper into pulling it out?

 

[arm]

Here's a video of what the top of the screw looks like (link)

I went to Home Depot and picked up a wrench for those golden bolts and even found a nicer flat head that better fits the screw in the video but im afraid im getting gold flakes on my screwdriver and not much give from the screw

 

[arm]

As it stands here what ive done,

I followed this video in the disassembly until the carb is ready to be put back together.

I pulled out 1 of the 2 jets and blasted carb cleaner through it, any vent I saw I blasted carb cleaner through it and made sure I saw it exit, cleaned the float bowls, cleaned the fuel filters, and overall sprayed carb cleaner in any hole or passageway I saw.

Something interesting was that in the picture below, the blue circle, I'm not sure what that part of the carb does. Still, I noticed a reasonable amount of gunk and built-up deposits, I put a rag over it and pushed it in with a small screwdriver and gave it a good wipe and there was a healthy amount of black residue on the rag, I'm not sure if that anything to write home about but it was something interesting I noticed upon inspecting the carbs.

Something I'm confused about is out to set the idle screw, sync screw, and the 2 screws on each carb.

Also, it doesn't feel like I did much, I did open it up and clean it but idk, I thought there would be some gunk that I could visibly see and get rid of, I am a bit pissed about the 2nd jet, I've left it in and blasted it with a ton of carb cleaner so I'm hoping I can get away with leaving it inside.

i did notice that some of my carb cleaner turned green when I cleaned around the float bowls, I'm guessing maybe its some old gas residue or something

 

[avemachina]

Great job so far! It may not feel like you did much, but all that goo, varnish, and residue adds up to crappy performance if you leave it in. Even if you didn't get to that second jet, you're way better off than if you didn't do anything at all.

The stuck jet might need to be drilled out eventually with an extractor bit and replaced (not as scary as it sounds, but when you're feeling ready, you can tackle this). Replacements are pretty easy to find. Since it's just the screw part that's a little damaged and not the hole, just clear whatever brass you can out of it, and you should be OK. You can use some compressed air to blow it out when it's dry to get the flakes out.

The circled-blue part is just a blank in the casting; this model carb was used in a lot of other Japanese motorcycles of the era, and you might see up to 4 of them connected together--some bikes have extra rubber hoses connected there. For us, it's just an easy way to tell which one is the left and right carb.

Now, for the screws you weren't sure about:
- The idle screw--this is the big thumb screw in the middle. You might notice that it contacts the same part that moves when you twist the throttle, and that's exactly what it does. Twisting it in or out sets the "bottom" for the throttle cable, so you don't have to keep holding the throttle at a certain point to get the idle RPM at 1200.
- The sync screw--this is the one that has springs sandwiched under it. This one adjusts one of the butterfly valves (big brass discs), so that you can have the carbs open the exact same amount for each cylinder.
- Idle mix screws--these are the little flatheads on the top of each carb. These control how much fuel relative to air go in. These can be a little tricky to perfect, but normally, turning them 2.5 turns out from a light seat will get you going.

 

[arm]

Great job so far! It may not feel like you did much, but all that goo, varnish, and residue adds up to crappy performance if you leave it in. Even if you didn't get to that second jet, you're way better off than if you didn't do anything at all.

The stuck jet might need to be drilled out eventually with an extractor bit and replaced (not as scary as it sounds, but when you're feeling ready, you can tackle this). Replacements are pretty easy to find. Since it's just the screw part that's a little damaged and not the hole, just clear whatever brass you can out of it, and you should be OK. You can use some compressed air to blow it out when it's dry to get the flakes out.

The circled-blue part is just a blank in the casting; this model carb was used in a lot of other Japanese motorcycles of the era, and you might see up to 4 of them connected together--some bikes have extra rubber hoses connected there. For us, it's just an easy way to tell which one is the left and right carb.

Now, for the screws you weren't sure about:
- The idle screw--this is the big thumb screw in the middle. You might notice that it contacts the same part that moves when you twist the throttle, and that's exactly what it does. Twisting it in or out sets the "bottom" for the throttle cable, so you don't have to keep holding the throttle at a certain point to get the idle RPM at 1200.
- The sync screw--this is the one that has springs sandwiched under it. This one adjusts one of the butterfly valves (big brass discs), so that you can have the carbs open the exact same amount for each cylinder.
- Idle mix screws--these are the little flatheads on the top of each carb. These control how much fuel relative to air go in. These can be a little tricky to perfect, but normally, turning them 2.5 turns out from a light seat will get you going.

Thanks man! I got the carbs rebuilt and put in, it was a bit tricky but some of the tips I read here like using a bar to pry it were super helpful.

the only issue im having now is the throttle cable and attaching it to the carb

Im curious how to hook it up to the butterfly on the carb, I know how it fits together but im not sure how I should set the throttle before connecting the throttle cable and the butter fly. I tried a few different ways and was never able to cave the throttle fully open maybe half open?

im not sure if this all make sense but yeah its confusing how to hook up the cable to the carb

 

[tstidham]

It looks like you're missing the top throttle cable housing bracket. It holds the end of the cable sheath so just the inner cable moves. It bolts on using the screws for the diaphragm caps.

 

[arm]

It looks like you're missing the top throttle cable housing bracket. It holds the end of the cable sheath so just the inner cable moves. It bolts on using the screws for the diaphragm caps.

AHHH righttt, there's a part on the carb where the cable runs through, totally forgot about that. now that I know that, is there any specific procedure I should follow? fully open the throttle then install? or is it just plug and play

 

[avemachina]

Ma'am

Once you've got the throttle housing bracket on, you'll push the end of the throttle cable into the part that opens the butterfly valve, and route the wiry part of the cable through the opening in the "arm". Then, you can pull the sheathing up a little and slide the rest of the throttle cable to rest onto the bracket.

 

[tstidham]

Yea nothing special, what avemachina said.