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Running hotter on one cylinder

Discussion in 'The Garage' started by Thaddaeus, Jul 19, 2015.

  1. Thaddaeus

    Thaddaeus XS400 Member

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    I have had a slow leisurely project 1981 xs400 for the last two years. Purchased it non running, and slowly went through electrical system, carbs, and petcock. It starts/idles fine but the right cylinder runs way hotter than the left. With an infra-red thermometer the right header pipe (right next to the head) will be at 200+ degrees within 2 minutes, and nearly 500 Fahrenheit after idling for 5 minutes. Comparatively the left side will be around 90 degrees after 2 minutes and 140-160 after 5 minutes of idling.

    Several months ago I rebuilt the petcock, replaced the float needles, the float o-rings, moved to 145 main jets and 45 pilot jets. Stock Mikuni bs34 carbs, but with MikesXS filters. If i had the original airbox I would use it, but I never had it. This got it running okay. It had no voltage so I rewired the stator, built a new rectifier and switched to a small sealed AGM battery. The electric start motor didn't work so I removed it and the starter clutch, so it is kick only.

    In the last month I have:
    Soaked carbs in berrymans & blown out all passages with compressed air
    Synced carbs (with a homemade manometer & a snazzy Tecmate TS-81 VacuumMate)
    Adjusted floats
    Replaced pilot screw o-rings
    Swapped ignition coils to opposite sides
    Swapped carb diaphragms to opposite sides (problem has still stayed on the right cylinder)
    Adjusted valves
    Checked compression (155psi on left, 160psi on right)

    This is really leaving me stumped (and frustrated) at this point. I haven't been riding it at all because I don't want to hurt the motor with it getting so hot but I have taken it on some 5-10 minute rides, 30-40mph max but am still unsure if it gets that hot just from idling or off the idle circuit as well. Once warm it seems to have a bit of popping 4k-6k, but it is hard to say which cylinder it is out of. By this point I am assuming the right cylinder.

    One thing I didn't replace when I had the carbs apart for cleaning was the throttle shaft seals. Part of me is wondering if one is leaking air in, but I can turn the right pilot screw out to 4.5 turns and it still gets just as hot. Sometimes it smokes a bit. Also, I have discovered that a new exhaust gasket didn't seal and lo and behold it is on the right cylinder. Would that cause all of this?

    If you guys have any advice I would really appreciate it.

    Cheers,
    Thaddaeus
     
  2. xschris

    xschris XS400 Guru Top Contributor

    Exhaust leaks, air leaks or lean running carbs, over tight valves, out of sync carbs. What exhaust are you running? Any one of these thing will cause this. 500*F head temps will kill the motor quick. Should be around 270-295 at the exhaust side of the plug after a warm day ride.
     
  3. Thaddaeus

    Thaddaeus XS400 Member

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    Any tips for putting on exhaust gaskets? The left side sealed fine, but the right didn't. I have another gasket to put in so that will be my first priority, then I will double check the valves. The carbs are definitely in sync. Those temps were taken on the actual exhaust so next time it's running I'll get temps at the plugs.

    The plugs indicate running rich?
    [​IMG]

    The exhausts are unknown to me. They were on it without clamps when I got it and externally are a bit junky, but sound fine so I figured I would run them for now.
    [​IMG]
     
  4. bcware

    bcware Well-read Top Contributor

    Plugs are a bit rich, but rich won't make you hot; it makes you colder. The one on the right looks closer to normal. The one of the left looks a bit fat; the strap has excessive carbon especially. The strap should be nicely brown and change color on the 90-degree bend when everything is perfect. The threads are totally clean which, to me, suggests you're lean at mid to higher rpms. The dirty strap and tip suggest a rich idle if I am remembering everything correctly!

    Exhaust leaks at the HEAD are a bigger deal. Exhaust leaks at the muffler are less so; perhaps even negligible from what I have read. It might produce some popping and whatnot and perhaps affect the idle, but once you're on the gas the flow increases and it won't matter as much.

    Also, if you are NOT getting any backfiring/popping you probably do NOT have an exhaust leak. A major leak will pop like crazy and if it is close to the head likely shoot flames.

    Which exhaust gaskets are you referring to?

    If you have an exhaust leak at the HEAD you're probably measuring the temperature of the hot exhaust gases and less so the actual running temperature, but a leak here will definitely make you lean and hotter than normal.

    As I said before; a leak much further down the line after the headers is not as big a deal, but might mess up your idle. Lots of exhausts for high end race bikes are held on with springs alone; not even clamps or tight-sealing gaskets.

    A possibility is that you may be pretty OK at idle, but once you give it gas it is running lean. I definitely had similar issues before my carbs were all sorted out.

    The other wild card here is that using choke can mask signs of lean running conditions on the plugs. You may foul the plugs by using the choke suggesting a rich condition, but once choke is off you're now very lean again. The carbon stays on the plugs and you THINK you are rich, but really you are not. Reading plugs the way you are now is very tricky; it is easy to come to the wrong conclusion.
     
    Last edited: Jul 20, 2015
  5. Thaddaeus

    Thaddaeus XS400 Member

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    Exhaust leak is where the header pipe meets the head of the motor. I thought it was sealed fine, until I tried turning the pilot screw out to enrich the right cylinder and it started smoking a bit (from idling too rich) and some smoke was shooting out right at the motor. Tonight I'll try to get it to seal up with a new gasket and take a temp reading next to the spark plug instead of just at the header pipe.
     
  6. bcware

    bcware Well-read Top Contributor

    The way to get a good seal is to make sure the header and flanges are not damaged for one and when it gets put on it needs to be tightened progressively and evenly on each side. It also should not be OVER tightened; start conservatively and tighten until the leak is gone as needed.
     
  7. xschris

    xschris XS400 Guru Top Contributor

    The plug on the left looks like the gap is too small. They are definitely running rich. Make sure there are no old gaskets still stuck in the head before putting new ones in. This happens more often than you think:laugh:
     
  8. Thaddaeus

    Thaddaeus XS400 Member

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    Didn't have much time tonight so will try to post pictures tomorrow, but I took the right header/exhaust off and the gasket looked fine, but the flange looks a bit convex and one of the collars looks a bit concave on the end with the flange. Hope that makes sense haha.

    I'll try going one size down on pilot jets and see if that cures the richness at idle.

    Thank you guys for all your help. It's really keeping me from losing my mind with this bike :wink2:
     
  9. don't forget to check for outlet obstructions on that exhaust. A partially blocked muffler can contribute to all of the symptoms you have listed. GL
     
  10. Thaddaeus

    Thaddaeus XS400 Member

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    Haven't had much time to get on this lately but here are some pictures.

    Flange looks a bit bent?
    [​IMG]

    Collars are kind of fubar'd looking as well
    [​IMG]
    [​IMG]

    This also seems odd to me. The left side has about a 1/4" gap between the flange and head of the motor. I did not toque down the right side excessively, it went very smoothly and easy to this point. Maybe it isn't sealing up because the collars aren't long enough to put sufficient pressure down?
    [​IMG]
     
  11. bcware

    bcware Well-read Top Contributor

    The collars usually go on pretty tight for me; if they were even a little bent mine would not slip on over both studs. They also seen like cast steel or iron; VERY strong! The collars are a little mashed; probably over-tightened in the past.

    The flanges don't actually need to be perfect; they just need to be long enough to do their job which is to push on the lip at the end of the header and drive that lip into the copper seal; if it can do that you're fine. If something is preventing that from happening you're going to get a leak.

    My original exhaust/headers were pretty mashed up from crashes the bike had been; I just replaced it all with a mac 2-1. They also make a 2-2 I believe.

    You could start small and replace those flanges; they must be dirt cheap on here or ebay.

    If that collar on the one side is coming into contact with the head of the engine before a seal is made you'll have a leak for sure.

    I know the earlier (like '78 or so) bikes like mine have collars that go on the opposite way (flipped compared to yours), but I don't know if they use a different part for the collar and/or flanges. I was just wondering if you have a replacement part on the one side.

    Chris would know for sure.

    I'm not sure if you've removed both sides and compared parts or just the one side.
     
  12. xschris

    xschris XS400 Guru Top Contributor

    I would replace them. The square head bike use a different flange like BC said. Make sure they are going on the right way. It looks like at some point they where put on wrong and got messed up:(
     

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