Solved Motorcycle Mysteries - Yamaha XS400

My friend Laura's bike (Stevie) would run without a gas tank! Why, because there was gas mixed in with the engine oil, amongst other oddities.

[@simontartakovsky900]

Dunno if you are still looking for comments/advice but unsolicited advice never hurts. The engine breather is designed to allow the engine to burn off evaporating oil. Engines are actually designed to "use" oil and standard pod filters can't handle that kind of stuff. Keep on eye on your filter cause sooner or later it will get clogged up with oil. My guess as to why your carburators were having that problem is a bad float needle (the part that the floats push to block the flow of gas). Those carbs are notoriously hard to clean and are very sensitive to dirt. A way to tune this stuff is stick a barbed hose on the drain bolt and attach some clear tubbing. Pull the tubing up along the carb and then you can measure the fuel level in the carb. The servive manual gives a value for what this should be. From there you can figure out if that part needs replacing and any adjustments you need to make. Hope this helps

[@azurplex]

crankcase breathers are really common. Fix the stuck carb filler, don't change design features and cause other problems.

[@bradok3695]

Just came across your video. Haha I have a 1980 XS400 had the exact same problem. Glad to see I wasn't the only one thinking that was a weird design.

[@ovalsopal6174]

Haha, I have a XS400 Special as well. I love it! I bought it as a project, because I wanted something to tinker with during the pandemic quarantine, but it has had the domino effect, I replace one thing and then the next. To get an idea, I first replaced the Ignition, because someone took a screwdriver to it previously. afterwards I changed the headlight, someone had dropped it and dented it pretty badly, afterwards I installed the rear suspension, other suspension was to soft. Then I went and replaced the fork seals and checked the front suspension. With the suspension complete I set out to work on the electrical and fixed the brake light and the blinkers, took a while to find the proper flasher relay due to using a mixture of 12w led in the rear and the standard lights in the front. Also I had to solder the wires back to turn signal switch. Next I was excited to get riding and built a a seat and set it up with custom upholstery, that was a win. Now the next problem I have is a small oil leak somewhere, crossing my fingers its not a major issue, but I love my Special II, anyway your bike looks great!!! Take care

[@azurplex]

Bought mine for a similar reason. I like to tinker and I’m cheap. Although I am spending on her piece by piece. I figured I’d buy a old cheap runner and learn to ride. Now I say i might as well put a new tires, sprocket, chain and wheel bearings to be safe. Well she needs that oil seal to do it right. I find myself getting attached and want to treat her right and buy her nice things ;) Oh crap, I’m in a relationship! I was afraid of that. That’s why I got one the same age as my wife so she can’t complain that I’m seeing a younger woman. 😉

[@azurplex]

Sprocket and clutch seals are relatively easy to access in the front sprocket cover. I took a utility blade and chamfered the hard edge of the sprocket shaft opening. It was threatening to rip my new seal as I tried to seat it.

[@coleparrish9856]

Had this same issue found out the tabs on the floats needed to be bent up a bit (tabs that touch the jets

[@reidgk]

what size air filter did you get for the top of the engine? i have the same bike and need to replace that as well.

[@Yoda8945]

I recently went through the same thing because a float sank. These carbs do not have an overflow pipe so the gasoline will back up through the carbs and out the crankcase breather if the vacuum petcock is not operating correctly. It may have saved an engine problem leaking into the oil rather than hydrolocking a cylinder.

[@yves3560]

You´re not a mechanic you said, but you´re doing fine :) To solve this properly, check the tabs on the float as Cole said, if fuel keeps coming though, just replace both valve seat assembly (1J71419020 if I´m correct). Should do the job. Closing fuel cocks when not running should not be a issue if these valves close properly. In closed position check for leaking anyway. Enjoy !

[@jacobrodriguez1938]

what size is that cone filter you put on?

[@harrygrimley4352]

Am I correct. The whole rear wheel is newer? Disk brake ? Mine is drum. Have to figure out. Pulses with light pressure. Locks with hard.

[@SLTSAdventures]

The stinking NAME SHOULD BE STEVIE WONDER!... Cause it's a wonder it's good to go that quick!😊😅😅😂

[@empicard]

Has there been any issues since you put the air pod filter and plug? I just bought an 81 XS400 as a project bike and just watching videos of random issues and solutions with the platform

[@tims-garage]

Seems to have been ok so far, just needed to tune the carbs

[@IrnMaiden304]

Carb kits, rebuild them and get a new fuel tank petcock .... Problem mostly solved. Air doesn't mean those brass floats work with liquid, all it takes is a small pin hole in the solder joint and once the floats fill with fuel they sink to the bottom of the carb bowl and it will constantly flow fuel, you can find out by shaking the floats and listen for sloshing gas inside them or hold them under water and look for air bubbles. Usually age kills floats but really cold temps will cause them to collapse and break the sealed joint. Uneven or incorrect float height can also cause issues with fuel not stopping / no flow or insufficient flow, checking float height can be done two ways by either a long clear hose on the bowl drain port and holding it up past the carb and opening the drain to see how far up the bowl the level is or visually checking where the preload of the needle jet holds the floats up from a closed position and measuring the height and comparing to what a manual says

[@prospenda]

You can still get gas if your needle and seat is not good. Your gas can over fill the bowls in time and drain down into the intake boots sit on top of the Pistons and drain down the rings. Easy solution is just use your petcock when parked if it still works and doesn’t bypass.

[@Yoda8945]

There is no off position on those petcocks. They need to be rebuilt if they are passing fuel when the engine isn't running.

[@harrygrimley4352]

Wait. . no off position? Mine has four positions, fuel. Res. Pri. On. What is the difference between fuel and on? What is pri?

[@Yoda8945]

Have you tested four positions? On is the normal position when the engine is running and should not allow fuel to pass unless the engine is running. It is operated by vacuum. Reserve works the same way. If the bike has not been run for a while, the fuel in the float bowls evaporates and the bike will not start. Turning the petcock to prime allows fuel to flow to the carbs. It also allows the tank to be drained. If the petcock is operating correctly, it will not allow fuel to flow from the tank unless the engine is running. If fuel flows in reserve or On without the engine running, the petcock needs to be rebuilt.

[@cn9936]

I also need to push button starter switch thank you

[@MrToranaGuy]

Sounds like your needle and seats in the carby are not sealing properly, which will allow the carb to have too much fuel, which will leak back thru the crankcase breather and into the crankcase.

[@cn9936]

I just brought home a 1981XS 400. It hasn’t run in six years, could you please tell me where I can find the parts that I need to turn it up? Thank you

[@tims-garage]

Hi! I think that would be a good Yamaha Forum question. This one isn't mine...

[@cn9936]

@@tims-garage thanks

[@davidlvann]

I did the same mod (block off H pipe breather inlet, use crankcase breather filter) and it is a BAD IDEA. Gas will run through the H pipe but now into the left cylinder (lower side when on kickstand). It's possible to seize up the cylinder or blow something up (head gasket, rod, piston, etc.) if you try to crank it with too much liquid in it. Thankfully, gas will run past the rings if your carbs are only slowly leaking - causing fuel in the oil anyway, same as you are seeing here. The fix: 1. Test the petcock (tank fuel valve): if in the "off" position (on vacuum operated automatic petcocks, this is ON), see if fuel leaks out when the hose is disconnected. If it leaks, rebuild with new rubber inside, or swap it with a new manual petcock (ON-OFF-RESERVE) not the vacuum operated type (PRIME-ON-RESERVE). If the petcock is OFF or the vacuum bit is working, no fuel can overflow the carbs. 2. Replace the float valve rubber "needles". The conical tips of these valves are soft / rubbery to seal off gas when the floats rise limiting how much fuel can be in the bowl. After 40 years, any natural rubber product living in contact with gasoline is shot. 3. Ensure the floats are adjusted so that fuel stops flowing at the correct height. The best way to do this is with a barb fitting temporarily installed in the float bowl drain, and a clear hose held in a vertical orientation so you can use it as a gauge to see how high up the side of the bowl the fuel is. If the bike is level, the height of the fuel in the bowl and the hose will be the same. You may notice on this bike that using the centerstand avoids gas going in the engine (if parked where the bike is allowed to sit nose-up). But that's just insurance; fix the problems above first. Gas will exit through the airfilters, soaking and breaking them down in the process.

[@tims-garage]

Cool! I'll relay this to my friend!

[@RonKilgore]

I have the same bike, But i’ve rebuilt the carbs twice and checked every spark plug connection works fine, Left cylinder doesn’t run, First time i rebuild the carbs, the right cylinder didn’t run. I’m confused as to why it’s chnaged

[@IrnMaiden304]

Your carbs are not in sync or there is some kind of blockage or improper reassembly .... Sounds like fuel wasn't getting to one and now isn't getting to the other, if the cylinder was dead it wouldn't fire over that cylinder at all everytime not switch off to the other. That or maybe a bad head gasket pushing cylinder pressure to the other cylinder and a simple compression tester will show you but carbs sounds like it seeing as you had them apart twice

[@bc-mr1si]

did u get your bike running 🤔I have one of these bikes .drove me nuts .I found it was my float level not being set properly that caused the problems. all good now

[@RonKilgore]

@@bc-mr1si jet screw was torn apart from the previous owner not knowing how to properly take it out.

[@bc-mr1si]

@DashCamshortz yeah sometimes you get people who don't have a clue atleast u found it👍

[@stockton350]

Are you worried about gas going in the crankcase though?

[@tims-garage]

That's exactly what was happening!

[@musaabubakar-vx8cc]

hello i have this bike but i canot start it can you plz help me to start this bike

[@tims-garage]

Ummmm. That's a BIG discussion. Hard to diagnose over KZbin

[@bc-mr1si]

fresh fuel,new plugs .try this first .

[@pawerRanjerah]

Haven’t watch the video it’s the carburetor

[@cacavati]

It s a common cheat to mix oil and gasoil to make the engine sound smooth. Trick of scammers if you ask me...

[@user-es5mw6rz5r]

Puyyou down intesd