My xs400 has 2 into 2 'sport' exhausts on and I was wondering if I created more back pressure, would it run richer? I have a little lean issue. The carbs are all stock jets. Would a free flow exhaust cause this and can I fix it with added back pressure?
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XSLeo
XS400 Junkie
The best way is to adjust the carbs to match the exhaust and intake.
Often just adding a freer flowing exhaust can be adjusted by just adjusting the mix screws and slide needles.
Adding a freer flowing intake along with it almost always needs carb jetting changes.
This carb guide was written with the XS650 in mind but the BS34 XS650 carb and the XS400 BS34 carbs are very close.
www.amckayltd.com/carbguide.pdf. There is a section on tuning for mods. A very good read.
Leo
Often just adding a freer flowing exhaust can be adjusted by just adjusting the mix screws and slide needles.
Adding a freer flowing intake along with it almost always needs carb jetting changes.
This carb guide was written with the XS650 in mind but the BS34 XS650 carb and the XS400 BS34 carbs are very close.
www.amckayltd.com/carbguide.pdf. There is a section on tuning for mods. A very good read.
Leo
Recreateme
XS400 Addict
I have always associated that hang with either mismatched jet sizes or a lean mixture. I could be wrong though.
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You're not...
It is lean mixture. Or usually is. Plugged pilot jets/passages?
Any time the rpm hangs and won't settle down, it is lean. Two stroke dirt bikes are infamous for this.
However, if the float bowl level is low due to bad/misadjusted floats or the vent in the gas cap is plugged and causes the float bowl level to be low, it will do this. The portions of the needle jet that would normally be immersed in fuel, are exposed to air, so it is an alternate source of straight air.
fine wire (from a wire brush) or a guitar string. There are several passages you need to probe, make sure you get them all
yes, but make sure you take off all the delicate parts. Do a google search (top left on the forum) for carb cleaning and you'll find a whole bunch of guides on that
I prefer not to contradict, but putting wire and similar objects into jets and carb passages is at best a poor idea. There's even several outfits that sell welding tip reamers as jet cleaners, but I wouldn't do it. Many years ago, in tech school back when everything had carburetors, we were told in no uncertain terms by the instructor to never never never put hard metallic objects into jets. I tend to agree. I wouldn't do it unless there was absolutely no other way.
In a desperate pinch, when you have a heavily clogged pilot jet that will be replaced, but no spare on hand, you can heat it with a propane torch until red, then drop it into water. It will burn off the caked varnish, but should be replaced asap.
Not trying to start a fight here, but I've seen this advice for years, and having been professionally in this business at one point, I am very aware it is highly frowned upon. Quick and dirty, but not good. Sooner or later, it will come back to haunt you. Do yourself a favor and don't.
Put some solvent in the hole, then use high pressure air. Most spray TB/choke/carb cleaners will soften up the goo, the air will get it out. The tiny passages you see on the engine side of the carb in the throttle bore by the butterflies on the top: all four of those tiny holes are fed from the mixture screw and pilot jet passageways. They have to flow.
In the hole where the mixture screws go at the engine side of the carbs in the top front has the screw, the spring that keeps tension on it to prevent it from self adjusting, then a minuscule washer, then a small o ring. Don't lose the parts, and remove before carb cleaning, reinstall in that order when you're done. You can get the o rings from several sources. Most rubber parts don't much like carb cleaner, so remove the diaphragms and plugs in the float bowl by the pilot jets as well.
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Thanks Yamaguy for this input. I agree with what you are saying, but I can also attest to it also going well using fine wire. My carbs were extremely dirty, and after a lot of gentle cleaning I probed my jets with wire because I couldn't get the jets to open up. It worked for me, and the xs has been running great ever since. I fully agree with the fact that the brass jets can be severely damaged if you probe something of a harder material into them, since brass is one of the softer metals. And seeing as how loads of people here suffer from wrong jet sizes, I can imagine damaging the jets can easily cause bikes to run poorly.
Most of the ham-handed variety just cram it in there. And they'll use steel safety wire on main jets. Since there's no way to know who's who, I tend to go with the least damage.
You can put out a Class B (fuel) fire with water if you know what you're doing, but if you don't know what you're doing, you'll just make it worse. Which is why they tell you not to. Just sayin'
You probably wouldn't believe me if I told you what I've seen people do to multimillion dollar equipment because it seemed a good idea at the time. It wasn't.
Like John Wooden said: "If you don't have time to do it right, when are you going to have time to do it over?" I really think that applies to a lot of things.
FYI: I really don't ever want to contradict anyone unless it might cause more harm than good. Sort of a Moto-version of the Hippocratic oath: "First, do no harm." No offense meant, I'm glad none seems to be taken.
Whenever I get a few other things completed, I'm going to start on my winter project. I have the ratty (really ratty) 400 that my daughter learned on years ago. It will be a full to-the-last bolt strip. We're going to see about a complete transformation assuming no major engine problems. The GBR has to be correct first: it has to really be worth it when complete. (GBR= grief/benefit ratio) We'll see.
Go to the MotorcycleUSA.com page and look at the custom transformation of a Star Bolt. Finally, a custom that didn't fall out of the cookie cutter. Tank looks like a 70s DT. Nice work. If I proceed, you'll see my grand scheme soon enough.
Like you already observed, none taken 
Just a thought: if you're concerned about the GBR, why are you planning on a to-the-last bolt strip? I wouldn't try to fix anything that's not broken, nor take apart anything that doesn't need to be taken apart. You'll likely shear a few bolts, damage some gaskets and end up in a bigger mess than what you started with. But perhaps if you wanna do a nice job, you'll end up taking most things off anyways because you want to paint/polish stuff.
just my 0.02$ excited to see what it's gonna look like though!
Just a thought: if you're concerned about the GBR, why are you planning on a to-the-last bolt strip? I wouldn't try to fix anything that's not broken, nor take apart anything that doesn't need to be taken apart. You'll likely shear a few bolts, damage some gaskets and end up in a bigger mess than what you started with. But perhaps if you wanna do a nice job, you'll end up taking most things off anyways because you want to paint/polish stuff.
just my 0.02$
excited to see what it's gonna look like though!Like you already observed, none taken
Just a thought: if you're concerned about the GBR, why are you planning on a to-the-last bolt strip? I wouldn't try to fix anything that's not broken, nor take apart anything that doesn't need to be taken apart. You'll likely shear a few bolts, damage some gaskets and end up in a bigger mess than what you started with. But perhaps if you wanna do a nice job, you'll end up taking most things off anyways because you want to paint/polish stuff.
just my 0.02$
I've done many motorcycle and car/truck frame off restorations. It can be quite satisfying. It can also be a money pit of no bottom. years ago, I had no money, but was very motivated to get another street bike. While out deep in the woods on my dirt bike, I came across a pile of about eight mostly there XS650s that someone had dumped. I managed to get my pickup in there, and after I loaded the YZ, proceeded to put two frames and the best of the other parts in the truck. I had five engines, multiple wheels, several seats and mostly two to four of everything. Good thing I did, as when I came back for the rest a week later, it was gone. Anyway, I made one quite good bike out of all of that, and had enough stuff left over to replace whatever I wanted for several years to come. I really shouldn't have traded it in a number of years later. The GBR was perfect on that one.
This one is just good enough to see what I can do to it. It is quite corroded and a real cosmetic mess, so has zero sales value. The exposed aluminum is chalky, and the bolts are all rusty. So at the least, I'll make it attractive enough to sell. But you really can't buy a good 400cc bike these days, and even if you can, you're talking over 3K if you want to have it truly road ready with no issues. There are no 30 YO bikes with no issues. Someone has to go through them and resolve the problems.
The wife has been waffling about riding again, and if I can make something nice and reliable, for under the cost of a new one, but with exactly the features I want, why not?
Like Millennium Falcon, I want something that no one offers. So if the core is good, I'll make my own. I'm looking for function. After I get it running like I want, then I'll make it pretty. Even with a rebore and fresh pistons and new valves, it will still be less than 1500-1800 in my estimate. Where can you get a completely fresh bike for $1500-1800?
(I'm talking one that really is, not one that claims it is)
The grey or silver tanked one that shows up at the top on this site would be a nice look. Silver is an easy color to maintain, and it wasn't available stock. The goal is to look like it came from the factory that way, and hasn't been rebuilt, but maintained in a very thorough way. It will take a lot to get there, but I don't need it running anytime soon, so that's ok. We'll see. First I'll have to dig out my blasting equipment and stock up on soda and abrasives. Old guys like projects for projects. Young guys like projects because new is too costly, or they need a challenge. I don't need any of the above, but I am looking for something different, with a cool factor you can't get any other way.
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XSLeo
XS400 Junkie
On a wire to probe jets and such I think a guitar string is to aggressive. They are hard and stiff, that's good for probing some things but not carbs. I prefer the quick tie from a loaf of bread. Peel off a bout a half inch from one end. This leaves a fine soft wire. Much less aggressive than a guitar string.
Easy to find too.
Leo
Easy to find too.
Leo
unless if you play guitar, that is. Those old strings never seem to want to leave 
@yamaguy, I think that is a great way to go at the project. Just be sure to keep us all updated on the progress
@yamaguy, I think that is a great way to go at the project. Just be sure to keep us all updated on the progress
Old guys Rule! . . http://www.xs400.com/forum/showthread.php?t=10801
