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New guy with a loss of power, please help

Discussion in 'The Garage' started by Woutair, Aug 27, 2019.

  1. Woutair

    Woutair XS400 Member

    Dear gentlemen,

    My XS400 has developed a serieus problem that has me completely at a loss. Driving 60mph my '84 Maxim would occasionally skip a few beats and continuing like nothing ever happend. The problem would only arise after doing a few miles at highish rpm. Sometimes it would lose a lot of power as I throttled down exiting the motorway, only to be fine again after the bike was parked for a few minutes. I took it in to my local mechanics since I was short on time (since one week later I was gonna take the bike on a trip) and he concluded that the carb diapragms needed replacing. He managed to source new ones and replaced them.

    Happy that it was fixed I set out on my journey, only to find within a few miles that problem was still there. I continued on, only to find myself on the emergency lane with a motorcycle that wasn't making any power and only capable of 40mph. After 15 minutes being off the motorway slowly driving to a relatives house, it would occaionally come back to full power before going again. it's been parked there ever since as I really have no idea what could have been the culprit.

    When the bike lost power it would be:
    -After doing around 20 minutes at 60mph
    -It wouldn't rev beyond 5000 rpm
    -It appeared to me that it was running on all two cylinders (Could have been mistaken in the heat of battle)
    -The problem completely disappeared after an hour's cooldown, and it would run normal.

    I have replaced the sparkplugs, cleaned the airfilter and the mechanic claims he cleaned the carbs as well as replaced the diaphragms. Oil changes and all regular maintenace have occurred as prescribed.

    Please help me as I have no idea anymore. As this motorbike is my only transport I really want to get it back up and running.
  2. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Check fuel flow and fuel filter. Seems like it could be a fuel flow problem.

    The carbs have specific settings too are you sure your mechanic did it right?
    buztabuzt likes this.
  3. Woutair

    Woutair XS400 Member

    That's what I thought too, but the weird thing is that the problem uccures during absolute stable cruise. I will try to look into it tho.

  4. Did he synch the carbs after he cleaned them?
  5. Woutair

    Woutair XS400 Member

    I don't think he did, but it wouldn't explain a loss of power after a significant amount of time I think
  6. markmakeitso

    markmakeitso XS400 Member

    Couple suggestions for issues like that:
    Plugged vent on the gas tank can cause fuel delivery problems after a fair bit of high load running. Sometimes it's bad enough that you'll hear air racing into the tank as you open the filler cap. That's pretty extreme though. After it sits for a bit the vent can catch up and your fuel delivery will appear fine for awhile.

    Flaky coils might act up when hot, and then behave better when cooled off. Suppose it's possible that electronic pickups could do the same, although that's relatively rare. A bad wire or electrical connection can also heat up and cause a voltage drop.

    I'm slowly looking into a similar issue on an 80 SOHC model. Fun times.
    buztabuzt likes this.
  7. keegan

    keegan XS400 New Member

    check the floats. the height might be off causing too little fuel to enter the bowls. I had an issue where my bike would lose power at higher speeds/RPM and found 1 side of 1 float was off by 3mm. problem went away after that. look up how to check and adjust floats. measurement should be 26mm on our bikes with BRASS FLOATS. I think plastic floats our different
  8. Woutair

    Woutair XS400 Member

    Did you find the issue? I still haven't. I might try replacing one of the coils and the leads.
  9. markmakeitso

    markmakeitso XS400 Member

    Not yet. Got some coils in the mail the other day, and ordered a timing light to set it after I replace the cam seal. This thing has a few problems, and my buddy is anxious to ride before it gets too cool. Sigh.
    buztabuzt likes this.
  10. Woutair

    Woutair XS400 Member

    Well, understandable! Keep me posted on the result of the coil change

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