HoughMade

That will be a fun project. XS400s are great first fix up bikes. The engines and trans. and pretty bulletproof so when you get the carbs sorted out, it will probably run decent. Sure you'll have to change out the fluids....actually fluid- oil. Check the points and clean them. Replace the plugs. Adjust the valves. All this is pretty easy. Just get the factory shop manual (available free here somewhere) or a Haynes and you will be fine. When you get that manual, just go through all the normal maintenance items and don't buy new parts on speculation- only by what you absolutely need! As for the carbs, they are really pretty simple. Knowing what I know now, I would just take them off, completely disassemble them (keep track of everything, especially the little bitty ball bearings that hold the enrichener rod in place- you will know whay I mean when you take them apart), and boil the carb bodies in vinegar water or lemon juice water for a couple of hours. This will remove everything from the passages about as well as ultrasonic- and cheaper. If you do this, turn the carbs into a different position every 20 minutes or so. While they are apart, order the parts you need after you have inspected and cleaned them. Don't order based on what the seller told you- see what you need for yourself. I made new float bowl gaskets myself and did not need any other carb parts- just a good cleaning. I have read online that small holes in the carb diaphragms can be repaired rather than replaced. Take heart! You can do this. Here was my winter spring project...and the smooth running, start on the first kick every time result (was sitting still for over 10 years before I got it). I didn't have paint to deal with, but that just gives you options to exercise your creativity!:

Sounds to me like the idle mixture is to lean. Turn the screws in all the way- lightly seat- then turn them out 3.5 turns, reset the idle speed, and tryi it again.

How did you check the cable?

Don't know where you are, but if in the U.S., I suggest using heavy duty oil intended for diesels. Shell Rotella T or Mobil Delvac are good. Auto oil has friction modifiers that will have a tendency to make wet clutches slip (even semi and non-synthetics have these). HD diesel oils do not and they have goos additive packages including plenty of zinc to prevent wear. If you use motorcycle oil intended for wet clutches, you can use synthetic- but that is much pricier than HD diesel oil.

What's the idle speed when idling in neutral? Does the clutch disengage smoothly? Does it tend to get slower and slower until it dies, or does it stop abruptly? I can't believe the air filters would affect this that much.

I used 15-40, seems good to me.

The petcock is vacuum operated. When the bike is not running, fuel should only come out on prime. It should not come out when not running in any other position. First, I would check your carb floats to make sure that the floats are set correctly, moving freely, and there is no schmutz on the float needle or seat. Only way to do this right is with the carbs on a table, not on the bike. Your float should block fuel from spilling into the carb regardless of the petcock position. It is possible your vacuum petcock could be letting fuel leak down when the bike is off, but then you should see fuel coming out when it is in the "On" position and the engine is off. If it does not come out, does not seem like a petcock issue. As for the stalling, get that fuel leak issue resolved first and that may solve it. It could be that the engine is flooding out. However, if that is not it, check the vent in your fuel cap. If it does not vent, fuel will not flow correctly and after a bit of time, it will stall. As for this place being dead, it is not a heavily trafficked board, but there are enough knowledgeable people here to get an answer. My suggestion is to post a new topic regarding a specific problem, citing the problem in the thread title. For instance, "82 XS400 Stalling- help!" and people will look at it and answer. When a thread starts with onle issue and then adds different ones later people who had nothing to add about the first issue generally will not check back in. Good luck!

Well, that's my apparatus. What it allows you to do is to measure the relative vacuum being pulled by each cylinder and adjust the balance screw accordingly. Here is the post that went with my pic above...and I do not take offense- there is no doubt, I am eccentric. As proof, below the quote about balancing carbs- I will post a pic of my other bike...and it's eccentric. It ain't a Yamaha...but it gets the job done...assuming the job can be done at under 40 mph:

The first pic shows the left carb idle mixture screw, the second the right screw. As I said in an earlier post, I have plastic caps on mine. The brass idle screw has the off-white plastic cover with a tab snapped on top of them.

There are 3 adjustments. This adjusts the idle speed: The screw on the spring-loaded shaft between the carbs adjusts balance. In the pic above, that would be right behind the idle speed screw I circled. The screw on each carb...on "back" as you say- the side facing the cylinder, adjust mixture.

^^^ Absolutely correct. I see that many people had to drill out a plug to access the idle/pilot mixture screws. I don't know if it is the year of my bike ('79 F model) or the fact it is U.S. spec., but mine had no plugs and never did. It is pretty clear the mixture screws were made to be accessed as the mixture screws have large heads on them that had plastic snap on covers with a tab on them. The snap on covers allow for about .25 to .5 turns adjustment without using a screwdriver, and they pop right off for greater adjustment. Anyone else have mixture screws like these?

Short answer- yes. Screw in to lean, out to enrichen.

If you have spark, my bet is on a carb problem- and not flooding, fuel starvation (unless you can see fuel pouring out). Test this by spraying WD-40 or starting fluid in the left cylinder when it is running. If the problem is fuel starvation, the cylinder will run with the spray. If that's it, take the carbs off and clean them both and reset the idle mixture. Personally, I would start at 3-3.25 turns out and go from there. Good luck!

If fuel is pouring out, your floats are not working correctly. When i went through my carbs I set the floats to what the manual said- 32mm and fuel poured out. Some years are supposed to be 26mm, some 32mm, but 26mm seems to be the safer bet, though I have mine around 29mm because the manual said 32mm. It could also be caused by dirt on the float needle seat. And yes, those plugs should be there. It forces the enrichener circuit (some people call it a choke, it is not) which is used on start up to draw fuel through the main jet. There is a passage between the two- check that to make sure it is clean. If you main jet was clogged, that passage and the entire enrichener circuit could be as well. Good luck!

Could be many things, but first thing I would look at are all the gaskets that have oil passages in them to make sure one did not get flipped, blocking an oil passage.

I look forward to seeing the pics.

That's good to hear. I had a similar issue (as you may have surmised) but mine was popping at idle. I balanced the carbs and richened the idle mixture up and the problem went away.

Yep, I would test it just the way you say. The seals are $5 for a set at Mike's XS if that's it.

Sometimes there can be air leaks around the butterfly shafts themselves when the holes in which they are mounted wear slightly out of round.

The first thing i would look for is an intake leak. Even if there is no air leak, the engine can hang at a high rpm when revved if it is too lean (or too rich), but with a lean condition, you would probably getting a popping at idle after the engine is warm. Third, synchronize the carbs if that has not been done recently. I really doubt that a butterfly is staying open- unless it's out of balance. You should be able to tell that by looking at both throttle shafts. after you let off the throttle.

I replaced the entire petcock. Here is a pic of the finished product: I got the adapter plate and petcock here. Notice that I had to notch the lever because otherwise it would not go full to the reserve position, but that was no big deal. https://www.partsnmore.com/cat_index.php?model=xs400&category=carb

To be clear, the fuel is never drawn by vacuum. The vacuum only turns the flow on and off and the fuel is fed by gravity. Rather than rebuild the petcock, I put on an adapter plate and manual petcock.

I have the stock loose bearings and once the races were right and properly greased, the feel and smoothness is fine. However, servicing those loose bearings is a real pain. The taper bearing, once the races are installed would be immensely easier to service. Also, I guess that there could be a little more resistance dialed in without damaging the bearings. For me, the cost of a Saturday morning spent with a magnet and sticky grease was much lower than replacing with taper bearings. If I ever have to replace the races, though, I will go with taper bearings.

Can't help you on the races, but I believe the bearings to be 1/4"....so not metric. I actually hand ground the races with a Dremel and it worked well.

Very nice...you spent more time making that picture than I spent building the manometer. ...and had I vacuum guages, I would use them too. However, for this particular task, $4 spent on tubing rather than $50 on vacuum guages seemed like the more fiscally prudent decision for me. - and I am curious about carb balancing technique with a feeler guage. If you mean just setting the throttle butterflies, that, in my mind at least, is more "pre-synchronizing." It will get it close, not doubt, but the more used an engine, the less accurate it will be. Good place to start, though. If something else, please share.