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XS400E Cracked head or just cracked exhaust port?

Discussion in 'The Garage' started by tstidham, Nov 2, 2021.

  1. tstidham

    tstidham XS400 Member

    Hello everyone, new member here and I did do my best to search the forum for my symptoms, but I haven't found anyone posting anything similar. I just picked up a basket case 78 XS400E and went through it all and got it running (cleaned and regapped points, set initial timing, cleaned and reset carbs, etc). Now however, I seem to have an odd symptom, that I think only showed up now that I've "blown the cobwebs out". When ticking over there is an air leak hiss sound from the right cylinder AND if you put your fingers between the head and the spark plug, you can feel puffs of air. Looking in that area behind the plug, it is finned and hollowed out for cooling purposes, but I don't see any soot or black exhaust residue. Compression is 115 for left side and 100 for right, so low, but not terrible. That cylinder does fire, and once warmed, seems to be fine.

    So my questing is has anyone seen a 400 head crack from combustion side to that cooling hollow? Could it just be a crack in the exhaust port through to that area? Though I doubt this as you can feel the puffs of air with decent force and I'd imagine that exhaust port and open muffler would release that pressure pretty quickly. I guess the next step would be to pull the head and check it out?
     
  2. Buddha

    Buddha XS400 Addict Top Contributor

    Read your post, then read it again. I have to ask.....did you pull the spark plugs when you 'went through it all'? You don't mention that in your post.
    Any chance you have compression leaking past a spark plug?
    Mix up some soap and water in a spray bottle. Do a cold start on the bike and spray the soap and water in the suspect area, should tell you where your leak is.
    Just my 2 cents................

    My bad, you did a compression test, duh
     
    Last edited: Nov 2, 2021
    tstidham likes this.
  3. tstidham

    tstidham XS400 Member

    No worries and thanks for taking the time to reply. I did pull, clean and re-gap the plugs, checked spark, fresh gas, some oil into the cylinders, yadda, yadda during my going through it. I tried to spend as little as possible to see how she ran before shelling out big money by refurbing/cleaning everything that still seemed serviceable. Anyway, you can definitely feel the air coming from the head side open cooling passage towards the combustion chamber and not around the plug. It blows out from there towards the plug. So seems to be an odd place to crack and something I have not found anyone else mention.

    I suspect, due to the carb mounts being super cracked (again, I E-Taped them all up for test running) and the right side muffle seems totally devoid (hollow sounding) of any fiberglass packing and was ran lean on that cylinder and really overheated that side, maybe even melted the exhaust port. I'll have to pull the head to really look now I guess.
     
  4. Buddha

    Buddha XS400 Addict Top Contributor

    Yes unfortunately sometimes that the only way to see whats wrong.
     
  5. xschris

    xschris XS400 Guru Top Contributor

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    155-160psi (cold) is what these motors run at when in very good shape. Anything under 120 is very poor and in most cases wont even start or have very weak power. Sounds to me the motor is worn and also has a bad head gasket. There isn't any glass packing in the stock muffles on these bikes, just baffles. Post some pics of your bike so we can see what you have.
     
  6. tstidham

    tstidham XS400 Member

    Yea, I'd like it to be higher compression than what I have, but she does run, and surprisingly well considering the obvious issue. This one has some Dunstall Decibel mufflers on it, not the stock silencers and like I said, the right side one sounds VERY hollow if you knock a knuckle on it where the left sound is very solid sounding when doing the same thing. Don't know if they can be repacked yet or not. I uploaded two pictures, one of the bike, and a close up and augment of the head showing the direction you can feel air blowing out from when cranking the engine.

     
    Last edited: Nov 3, 2021
  7. xschris

    xschris XS400 Guru Top Contributor

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    This area has a very thick amount of metal around it. I would think the only way to compromise the top of the chamber would be to grind the ports very thin. Usually any leaks are plug/head gasket issues. Take it apart and see I guess.
    DSC03999.JPG DSC04000.JPG
     
  8. tstidham

    tstidham XS400 Member

    Yep, pretty much what I was thinking too. Thanks for the clean pics of the head surfaces. I did view some on ebay and came to the same conclusion, that there was no way the head surface/gasket could cause that without major cracks or holes in the castings being the cause.
     
  9. tstidham

    tstidham XS400 Member

    So with the low compression and what I believe to be an internal cooling passage crack, I picked up a used head and rocker box off ebay, cleaned and lapped the valves in, then did a re-ring and ball hone for the pistons/cylinders. It's going back together now and hope to have it running today.

     
  10. Buddha

    Buddha XS400 Addict Top Contributor

    So what did you find with the old head? I'm curious.
     
  11. tstidham

    tstidham XS400 Member

    My gut tells me it is a crack in the exhaust port into that central air cooling passage, but I'd probably have to cut up the old head to confirm this. The air is definitely blowing out from that area when cranking and running and if you get it all up to full temp, it almost stops. Meaning the head must expand and start closing off the crack.
     

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