New Guy with a few questions!


XS400 Member
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Boulder Colorado
Hi everyone,
My name is Zach, and my father and I just picked up our first project bike, an early XS400. Im not 100% sure what year/ model it is, as it seems it has some conflicting equipment on it. I know it is an early bike because it has wire wheels and drum brakes, but it also has electric start w/ the kick backup. I was under the impression that teh "economy" models with drum brakes didn't come with electric start. It has ~7800 miles on the ODO, but the speedo cable broke some time before we got it so who knows if that is acurrate or not.

I did have a few questions about the carburetors though. I am fairly mechanically inclined, since I have 3 years experience as an Audi Technician, however bikes and carbs are somewhat new territory for me. (The downside of growing up and working in an age of EFI)

This bike seems to have a few quirks to it, which I'm pretty sure are fueling/ carb related. The bike fires right up, and seems to settle into a nice idle around 750rpm(im guessing, the Tach is not functioning right now either) after a few minutes of warm up. However once it gets warm, it starts backfiring and popping at idle every 100rpm or so. If I give it some gas it seems to rev up just fine, but then settles into an idle aruond ~2200rpm and won't drop below that, even if I snap the throttles open then closed(they do seem to be closing, and not hanging up on anythign.)

To me the first issue sounds like either the pilot jet is clogged, or the main jet/ both jets are wrong. Does anyone know what size the stock jets are? I can find the parts online, but it doesn't list a specification with them. I do live at altitude, approx 5300ft above sea level, which could have an effect on what size jet it should have. Anyone have any ideas to help me out? As far as I know the bike is stock, with the black plastic airboxes and chrome exhaust (which looks factory to me). My father and I do eventually want to put a 2-1 exhaust and Air filter pods on it, which im sure will affect what size jets we should put in it.
Thanks for all your help. I am stoked I found this forum. I will try to post a picture later tonight when I get home from work.
-Zach J
Here's a pic of the bike, Its not very detailed, I will try to get some better ones soon.


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750 rpms is a bit low, it should idle around 1,000 - 1,200, but I believe it is a good sign that it idles stably so low.

Popping/backfiring and hanging rpms are symptoms of a lean condition. As the engine becomes hotter the incoming air is also hotter and therefore less dense and leaner.

The carbs transition from pilot jets to main jets with some overlap, but at lower rpms the pilot jets are the primary contributors of fuel. No one can tell you what the stock jet sizes are without and accurate identification of the bike; they vary from year to year.

Before worrying about jetting inspect the intake tract and carbs boots for leaks. I use propane gas from a small torch for this. Spray gas on different areas of the intake and carbs while the bike is idling. If the idle goes up you've probably found a leak. Don't run the gas for long obviously; brief spurts and shut the gas off.

Thoroughly inspect the exhaust for leaks including the underside. had all of the jets in pairs. I bought every size available because they are reasonably cheap, but shipping is not. Ignore the carbs until you are totally certain there are no air leaks or exhaust leaks.

Once you've done all this move on to the carbs; and don't worry, they are pretty trivial and actually fun to work on. I can offer more advice when you're ready.

Based on your photos I'd say it's a '79 or later, but the wheels may have been a swap for looks. My '78 came with drums all around and NO electric start, but I think that style gas tank started in '79. Do you have vin numbers on the steering stem/frame and engine?
Thanks for the timely reply! I will be honest though, I was just guessing at the 750rpm figure. As soon as I am able to get the tach working I will report back with a more accurate number. On it looks like all the jets are for a xs650, but I believe I read the the carbs are the same BS34 mikunis?

The boots are brand new, and all the fittings/ clamps are tight, but I will double check with the Propane in the next couple days.
Thanks again
Yes, caters to xs650s, but some of them use the same bs34 carb. The bs34 jets are the ones I bought. "Main Jet - Large Round Type, Mikuni type - Fits:1980-84 Stock
XS650 Mikuni 34mm."

If the boots are new I doubt you have any leaks there, but just double-check that the bolts holding them to the engine itself are also snug. If the exhaust is fine too pull the carbs. If you have the nice older ones with brass floats you probably won't need to replace anything, so don't buy a rebuilt kit or anything yet.
So I pulled the carbs off today. Turns out I do have the ones with the Brass Floats. I noticed in another thread you had advised someone to bend the arm holding them so that the float sits at 26.4mm off the base of the carb. I am wondering if this is something I should do as well? I pulled out a jet on the intake side, and It has the numbers 155 on it, which doesn't make sense to me. I thought I saw somewhere that the stock ones are only 137.5? If anything, shouldn't the bike be running super rich? I will post pics of where I pulled out the jets from.
Just looked again. It looks like the pilot air jet is the 155, the pilot fuel jet is a 42.5, and the Main jet has no markings on it other than the letters JET PO. Is any of this making sense?
So heres another question, should there be a gasket between the boots and the cylinder head? I havent pulled the boots off yet, just the carbs, but Im wondering if i should put a thin bead of silicone or make a cork gasket where the boots bolt onto the cylinder head. I am trying to eliminate all sources of un-metered air, which pretty much is limited to where the carbs connect to the boots, and the boots (brand new) attach to the cylinder head.
Ok so I disassembled the crabs completely tonight and had a good look. A few things struck me as odd. First, I have no idle adjustment screw. It looks like it never had one. heres a picture of what I'm talking about, if I'm right, this is where they should be...

Second, I read somewhere on here that one should bend the floats up to 26mm. does that mean that the joint should be 26mm off of the gasket surface at rest upside down? there are 2 tabs on each float. One seems to push in a needle (priming/starter?) the other limits vertical travel of the float. Which do I bend? should the needle tab be flush with the rest of the float? Here is a picture of my floats.

Thanks for your help guys I really do appreciate it.
the adjustment for pilot is under that plug with a little indent, its an anti-tamper plug.

26mm is right, adjustment is the brass tang that has the "wire" wrapped around it.
So will that plug pop off with a pick tool, or is that something I am going to have to drill out? And with the adjustment on the float, do I bend the floats so that the tang with the "wire/spring" is in the same position and the rest of the float is higher, or bend the whole thing so that tang stays straight with the rest of the float. If I'm correct I think I understand it to be the first option. Thanks Drewpy.
I.Think you need to drill it out, although try a self tap screw and pull. You bend the tang with a Small flat blade screwdriver

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I may make a video of carb cleaning and adjustment tomorrow. Why not? I have the entire bike in pieces and the day off. Stay tuned; I will post it here or maybe make a thread.
@Drewpy, Got it, I will try a self tapper tonight. I have some stinger scrws with some nice big threads that I think will work well for this.

@BCware, I think a video like that would be awesome. I look forward to seeing it.

@Jayel, Thanks for the link. That PDF clears up a bunch of other questions that I had. I might print it out at work and throw it in a binder to keep in the garage.