Im out of ideas. This is a LONG post, but Im desperate.

[Domo]

My bike will not run consistently. I get it running a few days, then the idle degrades until it wont run long enough to make it more than a block or two of riding.

I'm out of ideas, and the only thing I can find wrong with my bike, is the left exhaust pipe. The bike always seemed weak on the left exhaust. The right side was always pumping healthy, but the left side was always weak, backfired, and sputtered when I shut er down. The baffle plate blew off in a backfire while I was riding, no idea where it is. Now somethings rattling around inside, and the bike runs really loud.

I've checked timing, compression, and tested for vacuum leaks and nothing ever shows definitive signs of trouble. I always fix/tighten everything I check anyway, but nothing ever shows any sign that it would be the problem. And the problems persist regardless.

Would a bad muffler cause the idle to degrade over time. Or, cause ANY issues with the carbs?

The only thing that fixes it, is either cleaning the carb, or re-syncing the carb. I do both each time, so I cant tell if its just one OR the other. But that gets it working..for a few days. Something is causing it to go out of sync. But, Im jumping ahead a little bit. Read on, I explain more..

An important thing to note as you read this post is; This has been happening, almost like clock work, for a few months. I am determined to fix this problem, but I need help diagnosing it, as nothing I've tried has worked.

It always SEEMS to work, for a few days. But it never lasts, and its usually as soon as I start thinking, its finally solved.

Here's my story.

What's happening to me is, I clean the carbs in the oxidization tank over night, then blow everything with compressed air. Every hole. Every jet. Then, I repeat. Despite the fact I've pulled these things, cleaned them, and SEEM to have fixed the problem by cleaning them. I have yet to see even the tiniest, microscopic suggestion of any dirt, grit, or grain. I wear a jewler eyepiece to inspect the jets as I go. They are ALWAYS, spotless. And each time, I resync the carbs afterwards.

Install them. They run like an effing top (can we swear here?) for about 4-5 days.

Then, it stops idling high on the most open choke if I stand the bike up from its side stand. The next day it doesn't high idle with any choke at all, side stand or upright. It just idles low. The fact that the choked idle changes at first whether the bikes is tilted or upright is strange, isnt it? Dont focus on this TOO much, as far as I can tell, its only happened once. Its more about the choke in general.

Next, the idle is messed completely. It starts to hang really high. I can feel the carburetor "coughing." Maybe not getting enough air. And if I adjust it, it takes several turns before it makes any changes, and all that happens is, its either too high, or too low. No inbetween. Just stall, or 4000 RPM idle.

This is always within a week of getting it running. Every time, it starts great, runs great for a few days, then within a day or two all this happens.

I have replaced the air filters (new OEM), float bowl gaskets, and some of the jets. I've re-routed my throttle to make sure its free from being pulled on turns.

My floats are not stock. Nor are they from an xs400. When I bought the bike, it had different floats in it, from an xs650. They are set to the manuals reco'd settings for float height, and I've read online that they're compatible with the xs400. I just received OEM floats, but I have yet to put them in.

So thats it. Clean the carbs, nothing comes out, at all. Re-sync, runs fine a few days. Slowly gets worse over the next 2 or 3. Then it barely runs, if at all.

Something is causing it to slowly degrade, and since the carbs seem to stay clean when I clean them, I can only start assuming the problem lies elsewhere.

Ive changed spark plugs a few times, the only thing they were telling me at the time, is that its running too rich.

remember, I've checked a lot of things. And nothing points to any hint of a problem. Im probably forgetting a few other things I've checked or tested, so Im open to suggestions still. I'll just say whether I've checked it, and how thoroughly I did so.

Oh, I also checked all fuses, and the battery. Both were 100%.

One more thing. Today as I pulled up to my house, and stalled. I noticed some white smoke coming out of my left airbox. I opened it and checked inside. Spotless. Could have been exhaust? Left side is the troubled side.... although the smoke was white.



I have a new 2-2 MAC exhaust in the mail, and need to install my OEM floats. Until then, I am WIDE open to any and every suggestion, or question.

Help me xs400.com, you're my only hope.

 

[arfstrom]

My first thought was valve clearance. Then my immediate second thought was do you have a filter on the fuel line? (in-line filter). A rusty fuel tank, with unbridled access to rust particles, being let loose onto your nice clean carbs. then coinicidently it runs great for a day or two, then its time to clean the carbs? With it backfiring I am still not convinced your carbs are clean. A clean carbs may included some time in the boiling bath of Lemon juice or vinegar, for up to 20 minutes to a couple of hours. (look up lemon juice on this site, I did) The weaker side comment made me think of the valves not set right. If you haven't picked up an feeler gauge, its time. I didn't see what type of floats you have in there? was it plastic or the copper? I had a problem with my copper floats, the floats would sink after a day or so, and never again. work right with out me opening up the carb again. That whole bit on your choke, (fuel enrichener) led me to think that the next time you clean your carbs, carefully separate the choke handle/all that other stuff. so some really seriousness boiling action of the carbs. Then something a friend I work with suggested, pull the boot off the spark plug, just to make sure that the plugs are working, equally on both sides, if it dies, then you know you have some thinking to do. Okay thats about it now, take care! hope that helps
Tobie

 

[Domo]

Hey Arfstrom, thanks for reading the whole thing.

I knew I was forgetting to mention something. Valves. I triple checked the valves, and had a mechanic check them after I was done. Dialed them in.

I don't have a fuel filter, but I'm putting that on the shopping list. Thanks. Im pretty sure I went over the plugs and checked the sparks, but I'll check again.

I've yet to see any actual grime, or grit in the carb when I clean them. I just do it because I keep thinking its GOT to be the carbs. I'll see if the fuel filter helps.

That helps explain the stalling and idling maybe....
But still, theres the weak left side, with damaged exhaust. Valves, compression and timing, are all good.

What else can cause my left exhaust to be outputting, seemingly 50% slower?

I read lack of "backpressure" can cause the exhaust not to clear the combustion chamber quickly enough, and flow back into the engine, or begin suffocating the combustion process.

Couldn't that be it? The exhaust isnt being forced out the muffler, so it piles up and starts back flowing?

 

[arfstrom]

I have no experience with the backpressure, so it could be. I am interested in knowing if the floats are really floating, if one float is shot, then you have a side that is not working right, but I suppose you dont have the rest of the symtoms, of a stuck/flooded float. there is no comment of smell of gas in your oil, or gas coming out your air box, or into your cylinder. I know when I finally sync my carbs, 2wice I screwed up, and let the homemade manometer suck ATF into the cylinder, I was able to bouce back. So this comment is just food for thought, good luck
Tobie

 

[bentwrench]

Not sure your model an year, but a couple of thoughts:

When you balance your carbs, are they out of balance? Every two weeks, how much riding do you do (is this a daily driver?) And are both plugs equally fouled when you pull them and they indicate rich?

Also, are you doing anything with the battery? Trickle charging at night, or charging while you clean the carb?

 

[JPaganel]

Have you checked compression? Is one cylinder significantly weaker than the other?

 

[HoughMade]

Classic dirty carb issue. Get the inline filter. Install it, then clean the carbs again (try shooting carb cleaner through each hole and jet so you can visually see the stream coming STRONG out each hole including a nice stream out the three holes above the throttle plate. Then reassemble. I make no guarantees, but I'm almost ready to guarantee this will fix it.

What is happening is clear to me- it runs well when the carbs are clean, then they (specifically the left) gets dirty again- schmutz from tank. Whether you can see it or not is irrelevant- use a paper based filter, not screen.

 

[campusxs400]

I had a similar situation and found that the charging system wasn't keeping up with the drain from the headlight and exciter coil. The battery would eventually run down and the engine would run rough, stall and back-fire on the left side. You should be reading 14 to 15 volts across the battery terminals at idle. I replaced the original regulator and rectifier with an aftermarket combined unit from electrosport.com I used part # ESR350 which cost $100. It was designed for the XS650 but works fine on my 1979 XS400-F2. I'm the second owner of my bike, bought it in 1981 with less than 100 miles on it. Rode it a lot in the early 1980s, then it sat in my mother's garage for 24 years. Three years ago I cleaned the carbs, replaced the gas, dropped in a new battery and it fired right up. After a couple of rides the new battery had run down and I became suspicious of the charging system. I don't know if it went bad back in 1985 or just deteriorated from age. One more thing, make sure that the passage at the bottom of the float bowl is really clear, it's very narrow and clogs easily. It provides the fuel to the up-take (brass) tube to the choke system. Good luck, I know how frustrating it can be to diagnose issues with old machines that have multiple symptoms that hide the real culprit !

 

[zvone]

I had the same problem.... Left cylinder didn't work at idling and have backfire from left exshaust, also I bought a new battery and it wasn't charged enough... But all troubles were caused because of carbs. At idling battery wasn't charged enough just 11,9 V, because it was to slow,when left carb stopped to work and when it let gasoline to cylinder it cames to backfire. I was checking the valvas, but they woro OK. Then I gave carbs to ultrasound cleaning, change membrans on both, make sync, put paper based filter for gasoline and now my XS runs very nice. The idling is good, charging voltage at idling is 13,4 V, only thing what I did a few times after I changed two gasoline filters because of dirt and now soon I have to take gasoline tank off and clean it again, even if there is no bad rust or dirt seen in it, But over the gas filter I see, that it does not get enough fuel at rpms over 6.000 min-1.
So by my opinion, like friends before says just CARBS

 

[xs400neophite]

Make sure you cleaned the idle circuit of the carbs. Check float height.
If muffler has hole, you can temp patch with muffler weld and once that is dry use aluminum tape until you get yer mufflers. Make sure your spark plug caps are screwed on tight and making good continuity -from coil wires- to cap -to plugs. check carb synch.
check points gap and timing if you havnt already. Also read up on drewciphers re-wire of coil wires/stator. Could be a open circuit overloading system?? Good luck

 

[Domo]

Hey guys. Thanks for all the replies, here's an update.

So, I cleaned the carbs out again. Again no signs of dirt, but as you guys have said, there doesn't need to be visible dirt for it to cause issues.

This time, I left them in the oxidization tank for 45 minutes. Blasted everything with compressed air, blasted every hole with carb cleaner until it was spraying out the other end, and then did compressed air again.

I also, drained the fuel tank to check for any dirt, and installed a fuel filter on the line going into the carb. So, I'll keep an eye on that.

My OEM floats came in, so put those in, and set them to manual specs.
Of course, she fired right up, and I did a sync on them to get them running right together. All good.

The mechanic helping me pointed out that he could hear one of valves wasn't set right, so I'll check that out as soon as I can next.

And my exhaust is still in the mail.

So far, everything is going the same. The bike dies, I clean the carb, get excited its working and then it dies again.

I'm keeping my fingers crossed, but not holding my breath.

 

[arfstrom]

Checking Valves isn't a hard thing to do. its just unfortunate that to do it, you do have to pull the one Yamaha name plate off, so that you can rotate the crank shaft, I tried to do it the wheel turning method, and that didn't work well for me, so I did the then its still was tuff to get the crankshaft to stop when you want it. You will figure it out fast enough, there is plenty of suggestions here too. any luck your gasket wont break on ya. Mine did, I replaced it with cardboard box that a gasket came in. (from O'riellys Auto for the Oldsmobile) Good luck

 

[kmac]

Checking in to see if you ever got through this problem. I discovered recently that I have a petcock that gets a little stingy with the gas at times. I had some issues just as you described. I have been running with the petcock in the prime position and she runs great. Easy thing to try if your still having problems.

 

[jmd_forest]

You've soaked and cleaned the carbs a number of times and theat helps temporarily. Have you chedcked for air leaks throughout the rest of the intake like the airboxes or the rubber fitting between the carb and the engine. You also may want to look at the throttle shaft seals, especially after soaking the carbs multiple times.

 

[drewpy]

did you clean out the float bowls? they have drilling's in there too