XS 400 Carb Rebuild

Hi folks,

I posted an introduction in the new members forum.

I'm here to learn about my new motorcycle. I've read a number of posts here and you guys seems like a real knowledgeable crowd.

I'm posting because I'm not having too much luck with my carburetor, the left carburetor. It's overflowing into the air filter. It's been doing so since I got the bike. Twice, I've had the carburetor apart. Both times I opened the float bowl, the float was not moving freely. The first time, I didn't take the float assembly apart, but I did wiggle it around until it moved freely. After reading a bit online and a hint from a friend, that the float is likely to be the source of the problem, I took it apart again. I removed the float, and cleaned the needle valve and seat. The rubber tip did have a crusty layer on it, so I thought it would help. The right carb, which leaked at first, but doesn't now, was much cleaner. I think I'll take some pictures to post next time I've got the thing apart, which will hopefully be tonight.

I'm a complete newb when it comes to understanding how the carburetor works. I'm tempted to turn it over without the air filter on so I can see where the fuel is leaking from. I'm afraid this will allow dirt to get into the engine though. Is there a way to tell if the float is working without install the carb back on the bike.

Oh, one more thing, can I use anything stronger than a teethbrush to remove the varnished gasoline from the jets and valves and seats. The carb cleaner doesn't take it off without some scraping.

The other interesting thing, is that the bike has started twice, but not run for very long each time. I've definitely used the choke to get it going, but I'm not sure if the leaking fuel is associated with the choke being open. Is that likely?

I looked at the spark plugs, and they had brown deposits on them. Can I sand them a bit with a high grit paper or emory? I also read yesterday, that I should check the gap? I think the post recommended around .028" clearance. Sound right? I'm assuming new spark plugs would be a good idea. Can I get replacement plugs at your typical Advance or Autozone?

Also thinking of changing the oil if I can get the carb to stop overflowing. Guessing I'll have to special order the oil filter? Since I likely need new carb parts anyway. I'll order them all together.

How do I tell if I need new carb holders? I don't see any glaring cracks or anything.

Also, I have to keep the bike outside, which limits my ability to keep things clean. So far I've been covering the intakes with rags while I work on the carbs.

Sorry for the long rambling post. There's a few specific questions in there, but mostly I'm hoping somebody will take petty on me and read the whole thing and ask a few questions.

Thanks for your help.

Luke

Partial success tonight.

Tried kickstarting, after reading that the starter can sometime draw too much current, resulting in a weak spark. It worked!

It's only running on the right cylinder, though. And it cuts out as soon as you twist the throttle. It won't start with the choke open either. I changed the right and left spark plugs and it didn't run then either. Switched the plugs back and it ran again.

I have a video with sound but I can't figure out how to post it. Maybe I can put it on one of those free blogs or something.

Will I damage the engine by running it on 1 cylinder for 10 seconds at a time.

I didn't have the carb overflowing problem this time. But I didn't crank it over for extended periods either. I think the float valve is still messed up, so I'll order a new valve and seat. Should I do both carbs?

It seems the next step is to clean the pilot jet and pilot air jet again on the left carb. Also replace the spark plugs, since it seems the left plug is bad. Can a plug just go bad? I've attached a picture of the tip. Also checked the gap and it's between .028 and .030".

I'm pretty excited. I've been on the fence on whether this is a worth while project. With a little success, I'm more optimisic. The bike is really clean and relatively rust free. Just need to get the heart running.

Oh, I'll need to change the oil as well. My owner's manual says 10W/30 type SE for above 50 degrees and 20W/40 for colder. Without starting a free for all, can I just use SM.

By the way, the bike has 7800 miles so it was used. It just sat for three years with the fuel in the carbs.

Attached a pic of the carb holder cracks. I don't know how to tell if they are bad enough to need replacing. I've read some cracks can be siliconed. I'm looking for a judgement call, would I need to new holders to get the bike running properly. If I can get it running and on the street, I've no problem replacing the parts.

Also, all the carb cleaning articles, I've read recommending cleaning with compressed air. I don't have an air compressor. Anyone got any clever ideas?

i don't know much, but i do know that spark plugs can go bad. don't bother trying to repair a spark plug, just buy new ones. new plugs are a good idea when getting a new(used) bike anyways.
as for your intake gasket cracks, if you remove it pour water or something on the crack, and any liquid can seep through the cracks, they're clearly goners. or, if you can get 'er running, if you spray some carb cleaner on the cracks and listen to the idle speed, if the speed increases when you spray the crack, they are toast. i don't see a problem with siliconing them, but i'm pretty new myself. hope this helps!

visit h-tee-tee-pee://lucashurt.blogspot.com/ to see the video

Thanks Jonny. I'll pick up new spark plugs.

Well, to start with, sounds like you have a whole host of little issues, but never fear, once you get it working - IT WILL be a worthwile project :)

To start with, you really need to replace the cracked rubber that I see in that picture - and anywhere else you have any cracks in rubber. See, much of these are part of a vacuum and if there is a hole, guess what, no vacuum.

A good way to tell if the float is working is to leave the bowl off and hold the carb vertically (like it would sit when it is hooked up), then slowly turn it over to where it is vertical the opposite way, see if it moves smoothly. I am by no means an expert, but am pretty sure you cannot really check it any other way.

So the whole issue you have is with the left carb? Here is what I would do:

Go to Autozone (or favorite run-of-the-mill parts store). Get a "paint can" of berrymans carb cleaner. While it says to soak it for a short amount of time, I would soak it for at least 3 days - be sure that all the gaskets are removed, carb cleaner does not like them!

Then take it out and scrub anything that looks like it need to be scrubbed. Next, use NYLON FISHING LINE to clean out all the tiny holes in the jets. I emphasize fishing line because anything harder than that and you will, most definately, scratch the hole or make it bigger, throwing off the whole system.

I would also order a gasket replacement kit for the carb - do this before you start soaking it, that way by the time the carb is clean, you will have them (hopefully). Also, if you determine that your float assy. is shot in that carb, get another one of those too (besides, like I say with my 73CB350 - at least I know that it is new!). Put her all back together and see what happens.

Being a relatively new rider myself, I know this encorprates two elements that are probably not appealing - extra money and waiting. However, with such a serious (and utterly bike parking) situation, you need to start somewhere and this is the best start. Good luck to you and keep us updated on the results.

By the way, an easier way to upload videos, if you wish, would be through Youtube. You do not have all of that extra work that comes along with a blog site.

Mike

First things first, the plug you pictured doesn't look bad. Maybe just a tad rich but it's hard to really judge that for certain.

You can use a brass bristle brush on some of the parts in the carb, like the inside of the bowl and even across the jets. Take it easy over anything that's brass though.

Compressed air is a good idea when cleaning a carburetor - all the jets and ports should flow freely. You might even consider purchasing and replacing those jets in your carburetor, but at the very least they must be clean.

Your test for the needle and seat:

1) Get a straw, attach it to the nipple on the carb and blow

2) With the bowl off, gently raise the float arm. You should be able to modulate the flow of your breath with the float arm. A very slight sticking once closed isn't a problem, but if it requires a great amount of force than you may in fact need to replace the needle and seat.

Now another factor you should check is how to set your float level. You should look at your shop manual to figure the procedure. If you want to try a "seat of your pants" method than bend the arms of the float down a little (toward the bowl), and see if that changes engine performance. You only need to make some pretty subtle adjustments to make a difference. Find a way to accurately measure the float level so you can set both carbs the same. I wouldn't start out by screwing around with the float level, but you could start by seeing if both carbs are set the same. You just measure the height of the float arm from a fixed spot on the carb with a ruler or preferably a machinist's rule, and then do the same on the other side.

Next, I would definitely silicone up those mountings. They could be a part of your problem. I would definitely order new ones, but silicone can probably be used in the meantime.

What about choking the bike. Does it still run like crap and die?

Are you certain you have a good spark on both cylinders? Just kicking it and getting a spark sometimes isn't enough - sometimes a faulty condenser can cause one cylinder to intermittently have no spark at RPMs much above idle.

Is there a mixture adjustment screw on the carb? How about turning it clockwise into it's seat? Does that help? Counterclockwise? A rule of thumb is to seat the mixture screw and then back it out 1.5 turns. Your ideal mixture adjustment, assuming correct jetting is normally between 1 and 2 turns back from the seat.

Those are just some basic things I would look into myself. It is a positive sign that the bike at least appears to be half running. Doesn't sound like you're facing any serious engine problems at least from what I could hear.

Well, I got a little excited and ordered a two carb kits and the carb holders.

cdg - I like the idea of the straw. I think it long run it's worth replacing the float valve, but I'll experiement with the straw to "get the feel" of the how it works.

I was also thinking about purchasing new pilot jets. I've got the BS34 carbs and the jets are something like $4 as MikesXS. I've just got to get the size right.

The funny thing is that the bike don't run at all with the choke on. I'll have to experiment more with this, because it doesn't seem right.

I purchased new plugs last night, so I'll give that a shot, but I really think I need to clean the carbs again.

I'm going out of town this weekend and I won't get the new carb holders for about a week. But next week while waiting for the parts, I plan to take the carbs apart . . . for the last time. I'll post some pics and take my time. I'll make sure I identify the jets and take lots of notes. My brother in law has an air compressor that I may be able to borrow. I also thought of using a syringe with carb cleaner.

For now I've passed on the carb soaking, mostly because I don't want to deal with all the chemicals in my basement. I'll use some old film canisters to soak the smaller bits and clean the larger parts the old fashioned way.

I guess, I'm starting to understand what clean really means when it comes to carburetors. Plus, I got pretty energized looking at some pics of restored bike. I'm mentally over the hump of whether or not to fully jump into this project.

I'll try to give the bike a cleaning next week too. I'm sure it'll shine up nice.

Thanks for all the help. I'll post again when I've got some news.

Luke

Just be prepared to spend more than what the bike is worth in terms of time, parts, and tools you might or might not have to buy.

I still also say you should check in on the spark problems.

If you can't get the straw to work, a piece of fuel or vacuum hose will work just as well, provided it's clean enough you're willing to put your lips on it. :)

Be creative, be resourceful, and be extremely meticulous. It sounds like you're doing well so far. You'd be amazed at what you can accomplish in a basement (or dorm room) if you just take your time and use your common sense.

I would buy a shop manual for it if you don't already have one. My general jetting rule is:

1) Assuming stock or mild aftermarket exhaust, use factory jets up to about 3500-4500 feet MSL. Every bike is different.

2) Assuming the same, but at altitudes much above 5,000 MSL, go with about #5 or #10 leaner than you have. Basically shoot about 10-20% richer (larger).

Pilot jets seldom have to actually be changed from factory settings, so you can probably remove the one from your carb, get a magnifying glass, and find the number stamped on it. Or just look it up in your shop manual and hope your manual is correct.

I would also put new condensers in this bike. I don't know if I mentioned that already. They're cheap, they do ocasionally fail, and it would be a good experience as far as you getting to know how the points work on your bike. Unless this one happens to be new enough to use CDI.

Hey cool, another XS400 owner. Glad you're getting new carb holders, those look as bad as mine were and mine were shot.

If the bike is leaking gas into the air filter, that tells me your fuel petcock is leaking internally. It should only flow fuel when the bike is on. You can get petcock rebuild kits but I think they're a waste of time; I got an aftermarket manual fuel valve and an adapter plate which is much more reliable. Had to enlarge the opening in the tank with a Dremel though but I think it's worth it. Alternatively, you can install an inline shutoff valve, but they are a pain.

Most of the problems with these Mikuni BS34 carbs come from the pilot screws. The various other jets are probably fine unless a previous owner screwed with them. Screw the pilot screws in and count the turns so you can return them to the same settings; then remove them and hit those holes with carb cleaner, compressed air, carb cleaner, compressed air, etc. You'll want to clean and blow the rest of the jets too of course but those are the critical ones. Also find the passage that leads to the three little transition ports just forward of the butterfly and shoot carb cleaner through it until all three holes spew cleaner freely.

The straw test for the float needles should work fine. Be aware that even brand-new needles can stick and flood the first time they are used; some healthy tapping on the bowls from a screwdriver handle can fix that.

One thing to check while you have the carbs off are the diaphragms and the pistons. Inspect the diaphragms under a bright light and pray you don't find holes or cracks because new ones are SPENDY. Also, when assembled, lift the pistons and make sure they fall quickly and smoothly. If the pistons hang up you need to polish them and the bore they ride in.

All that done, you should be able to get it to idle. To really make it run well, though, you will want to synchronize the carbs. This can be done with 8-12 feet of aquarium airline tubing with a bit of ATF in the middle. Hook both ends up to the nipples on the carb holders and use a stubby screwdriver to adjust the sync screw until the ATF stops moving left or right.

The XS400 is a fun old bike. They're easy to work on because you have plenty of elbow room, and once you get through a little learning curve, they're not too hard to understand too. I was totally clueless about carbs when I started, but over time I've finally got my bike running like a champ. She's kind of homely with the dented flat black gas tank, but that's the way I like her. :)