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Looking for some advice

Discussion in 'The Garage' started by Jimmer, Sep 15, 2020.

  1. Jimmer

    Jimmer XS400 Member

    1981 xs400 special 2

    Visually as in the first pic. Left cylinder seems to be running hot based on the exhaust pipe colour. Seems to run ok.

    Popped in a new set of plugs. Rode it for an hour and then pulled them. Second pic clearly shows that the left plug is not burning fuel the same as the right.

    I would like some opinions as to troubleshooting.

    Thanks in advance
    20200915_122449.jpg 20200915_122837.jpg
  2. amc49

    amc49 XS400 Addict

    One side is likely eating oil.
  3. Jimmer

    Jimmer XS400 Member

    I was thinking about that, however it's not dropping oil levels and there is no viable sign of smoke in the exhaust when it's running. It's completely clean

  4. Jimmer

    Jimmer XS400 Member

    I think as step one, I am going to check the compression. Just ordered a tester which will arrive tomorrow. I will check back in when I have the results.

    If it's not compression issues I would certainly be open to suggestions

  5. xschris

    xschris a lifestyle not a trend Top Contributor

    Make sure your carbs are synced with a manometer. That will make sure they are pulling the same. I have noticed this also on some pipes on these bikes. Even with the double walled head pipe I think either a leak has occurred or some exhaust gasses have found it's way between them. With my bike that had this, it was on a set that came from another bike and the plugs looked the same and perfect. They never got any worse even after 3k miles.
    Last edited: Sep 15, 2020
    Jimmer likes this.
  6. Jimmer

    Jimmer XS400 Member


    Thanks for this. I will keep this in mind

    xschris likes this.
  7. amc49

    amc49 XS400 Addict

    '...there is no viable sign of smoke in the exhaust when it's running. It's completely clean...'

    Can mean nothing if the leak amount is small enough. Watch an old chainsaw that mixed oil at 20:1 ratio and a modern 50:1 ratio saw and where did the smoke go? If the ignition is firing hot enough and engine burns it then if the oil amount is lesser it will not smoke out tailpipe at all. Just shows on the plug.

    The only other choice is compression or something wildly wrong in a carb. Sync them like said, a lagger in the pair will be forced to drag up with the other rpm wise and slide not opening fully and then that side runs super rich because the vacuum has increased on the lesser open slide artificially. Higher rpm on a slide too low then pulls rich, it's unavoidable.
    Last edited: Sep 15, 2020
  8. amc49

    amc49 XS400 Addict

    On double wall header pipe, if the inner pipe is substantially smaller than the outer then the pipe does not run exactly in the center of the bigger one and luck of the draw as to if the smaller pipe touches the outer wall or not at the bends and the outer changes color highly based on how much of the inner is touching it to directly heatsink heat into the outer. I used to see Honda DOHC pipes color up differently all day long, it was not tied to carb conditions at all other than too rich won't color them like getting lean to make better hp. will.
  9. amc49

    amc49 XS400 Addict

    If the black plug and non-colored header pipe are the same side I'd be looking the carb over for issue. Compression test may say something.

    The valves getting off in clearance can do that too as can points if that type.
  10. Jimmer

    Jimmer XS400 Member


    As I mentioned I am going to run a compression test tomorrow. Start from there. If it's ok I will follow up with troubleshooting the carbs as suggested.

    The black plug and the hot pipe are both on the same side.

    Thanks for the input

  11. amc49

    amc49 XS400 Addict

    The black plug and the HOT (colored) pipe?????

    Compression test, you may be burning an exhaust valve. Compression drops to kill plug burnoff because it is leaking and the leak then overburns the pipe to color it. Set your valves NOW, and lean toward the loose side of spec on the exhaust.
  12. Jimmer

    Jimmer XS400 Member

    Man I love Amazon. Ordered the tester yesterday and received it this morning at 10 Am.

    Warmed up the bike and ran dry tests on both cylinders. Both are coming in at exactly 150. I think this is a pretty good result considering the engine is 40 years old.

    I think I will move on to the carbs now.

    Any other suggestions? Could points gap or timing cause this?

  13. xschris

    xschris a lifestyle not a trend Top Contributor

    Your bike is TCI so the timing is most likely ok. A timing light can verify it just to be sure. A valve adjustment check would also not hurt, just to be sure. Then I would look to carbs. If you have a IR temp gun, that can help with diagnosing problems. Checking head temps next to the exhaust side of the plug after a good long ride and see what you get. With having five of these bikes I usually get from 265-285 F on a summer day.
  14. Jimmer

    Jimmer XS400 Member


    Thanks for the feed back and info.

    I am trying to eliminate as many things as possible before I start pulling things apart. No sense in ripping carbs apart if the issue is not related to them. I will follow your suggestions and if nothing odd turns up then I will have a go at the carbs.

    Thanks for the reply



    Hey, Jimmer. U said you are looking for something easy first. Make sure your exhaust is not clogged. I've seen rusted, also hornets nest show the same symptoms. Place your hand behind the exhaust and feel the pressure of the exhaust. It's pretty easy to tell if one it's pushing harder than the other.
    If they're not even AND the exhaust is clear, then as xschris mentioned, carb sync. It's not as difficult as it sounds/looks/reads once you do it. make a manometer out of 2 bottles. Heat something up like a drill bit and push it through the plastic lid followed by the tube. Lay some goop,e6000 or equivalent if it didn't seal when hot. I like to use water balloon valves screwed on to a bottle as a makeshift gas tank hanging from the handlebars for easy access to the screw. Let me know if you need a pic.
    Good luck, man. You'll get it.
  16. Jimmer

    Jimmer XS400 Member


    Funny you mention the exhaust. I just did that check after I finished the compression test and restarted it. Great minds... Lol They are both clear and feel the same

    I am going to check the head temps tomorrow and if that checks out I will turn my attention to the carbs.

    If I run into any problems I will ask.

    Last edited: Sep 16, 2020 at 6:47 PM
  17. amc49

    amc49 XS400 Addict

    All good ideas.

    Just a thought...............when checking compression always check more than one time as the work is fraught with possible mistake. Throttle held wide open. And make sure you reset the gauge to zero before reading the other side, if not and you had 150 on the one side and then you test the other side and it has less you will still have the 150 making you think both sides are even when they may not be. Careful thoughtful work wins the day there. Usually if the gauge is a good one the sides will slightly vary, dead even is instantly suspect and run them again. Possible to be even but not likely. Force it to be sure.

    Adjust them valves, a tight exhaust can cook that pipe. Whatever spec is I always loosen up at least an extra thousandth, it will never hurt anything and the cam lobe lives based on the oil that gets through that clearance and why zero will kill lobes quicker than spit. There's almost always a variable of up to a thousandth too based on how the setting work is done and that extra space makes sure it is likely in your favor.
    Last edited: Sep 16, 2020 at 6:56 PM
  18. Jimmer

    Jimmer XS400 Member


    Hi. Yes I watched a few videos and read some stuff first before doing the test. Warmed up the bike a bit and then followed exactly as you just described. Got a solid 150 each time on both sides.

    I think we all have covered pretty much everything and it looks like I have to start digging a little bit deeper.

    Thanks for your reply
  19. amc49

    amc49 XS400 Addict

    Yes, to the next step.

    FYI, a weak coil or bad plug wire can make that plug black. The gaps need to be the same too.
  20. Jimmer

    Jimmer XS400 Member

    Hi good morning
    @xschris I ran a temp check with a gun this morning. Several times. Both on the head as you suggested and directly on the pipes where they connect to the head. Left side is around 270-275F right is slightly cooler at 265-270F. This after a good hard ride around the block a few times and then also just sitting and idling.

    I don't think that's enough to be a concern is it?


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