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1982 XJ750 Seca - Ongoing

Discussion in 'XJ Technical Chat' started by Tim O, Jan 29, 2018.

  1. Tim O

    Tim O Active Member

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    Exactly the four caps I just bought :)
     
  2. Rooster53

    Rooster53 Well-Known Member Premium Member

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    And that is what I did also, only I never used the 2,3 caps as the original Yamaha caps are still in spec. Hard to imagine 35 year old plug wires hanging in there, but they look good and since I ride very little I am not likely to get caught in a rain storm. The OEM coils are not cracked, and if I did need new plug wires I guess I would be tempted to use the splice rather than try the detailed coil surgery to replace the wires.
     
  3. Tim O

    Tim O Active Member

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    Frustrating Update!!!
    So my plug wire was taking forever to get here, and I just shortened the existing leads and added the new caps / plugs. All coils and caps ohm out exactly in spec.
    Took the bike out for a 20 mile ride... no issue, running well. Even floored it in the area it usually dies on the way home.
    Took the bike to work next day (23 miles) running well.
    Took the bike around town to a few appointments after work, fine...
    Then went home and 1/2 way there, same sputtering and uneven combustion as above, but 2-3 miles sooner than usual. It then dawned on me that this is happening at the same level of gas tank after filling... about 2/3 empty every time. I switched to reserve, and even though it made no difference the last couple times, because I was further out from home it had sufficient time to even out and start running normal again... for 2-3 miles, then started sputtering again a 1/2 mile from home.
    When I got into the garage immediately popped the plug caps off and measured the coils / caps resistance. Still perfect.

    So I'm guessing clogged petcock screen(s)? Might be able to check it tonight....
     
  4. kerriskandiesinc

    kerriskandiesinc Active Member

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    In line fuel filter...??
    i was having trouble with one of mine, switched it to a more bike specific one.....no more problems
     
  5. Tim O

    Tim O Active Member

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    The aftermarket petcock had really fine screens (Maybe the problem), and I haven't found one I felt was big enough that also fit under the Seca 750 tank... so no filter right now...
     
  6. kerriskandiesinc

    kerriskandiesinc Active Member

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    Len/Chacal sells one that fits, and works.....
     
  7. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Gas cap has an internal atmospheric vent passage and an anti-leak "valve" (basically a ball bearing within the passage) that can get stuck/clogged and create a "vacuum lock" within the fuel tank, thus fuel will not flow. This situation manifests itself as stalling/sputtering (i.e. lack of fuel) after a few miles of riding as fuel is drained from the tank, and finally the vacuum "lock" effect takes places and prevents any more fuel from flowing. Try opening the cap the next time this situation occurs and see of the situation resolves; if so then it's time to clean/rebuild the cap:

    http://www.xj4ever.com/gas cap.pdf

    The vent hole in the cap and the "ball valve" can be seen on the top of page 7.
     
  8. Tim O

    Tim O Active Member

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    Thanks,
    I opened the cap, let sit 3 minutes or so last time and it seemed to make no difference when restarted.

    Going out to the garage shortly...
     
  9. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Darn!
     
  10. Tim O

    Tim O Active Member

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    OK, I would have thought that this is a ridiculous question... but has anyone ever had it such that the gas stops flowing from one side of the tank to the other??

    I just drained the tank, zero sediment or crud. As I was draining, when flow diminished went to reserve... NO CHANGE. Popped the gas cap, no change. Looked inside, there was still 3"+ of gas on that side. And when I slowly drained most of the rest, I took the petcock out, no clogging, and tipped the tank to get the gas out of the other side and had to REALLY tip it over and work at it to get the gas to come to the petcock side.

    All this and I can see zero loose rust / paint, etc. Also disassembled the petcock and found no issues. There is some rust in the tank like all of ours, but it does not look excessive at all or like it's breaking loose...
     
  11. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Sounds like something has plugged up the back end of the tank
     
  12. Tim O

    Tim O Active Member

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    RRRrrrr…. so in my frustration, went back out to the garage put the petcock back in and shook the heck out of the tank with the inch or so of gas still in it.
    Low and behold... a 2" x 4" or so solid sheet of rust came forward into view, and must have been blocking the transition. Wonder what part of the tank it came from.

    Going to take the tank to work tomorrow, put hot cleaning fluid in it and shake it to death. We have a boroscope so I can see if it's really thin somewhere.
    Bummer because what you can see from the gas cap looks pretty darn good.
     
  13. Tim O

    Tim O Active Member

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    Cleaned out the tank best I could... got the pictured "stuff" out of it. Looks like the silver looking surfaces I can see from the gas cap are actually a botched liner job around the other parts of the tank.

    The items pictured are all silver on one side and that solid rust cake on the other. The largest being about 4" long.

    Also installed a decent size fuel filter, I probably have an extra couple miles fuel in the hose now from how roundabout I had to route it to get the filter under the tank. :)

    upload_2018-9-9_15-17-41.png
     
  14. hogfiddles

    hogfiddles XJ-Wizard, Host-Central NY Carb Clinic Moderator Premium Member

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    Oooo, so you have a larger reserve! Lol
     
  15. Polock

    Polock Well-Known Member

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    just throw a match in it
     
  16. Tim O

    Tim O Active Member

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    I may throw a match in it.... took it for a ride last night, started bogging down about 2 miles after filling...

    Think there's too big an air bubble in the long fuel line I just ran, and since it started bogging so soon, thinking maybe the gas cap vent really is stuck too.
     
  17. Tim O

    Tim O Active Member

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    I am at a loss....
    I have clear fuel hose on so I can see what's going on. There's a lot of air in the line between the carbs and the fuel filter which is at the outlet of the petcock (non vacuum, after market, been running fine for a year)

    When I take the tank off and elevate it so the whole line is nice and straight up, both with the petcock on normal and reserve, the air will not flow back into the tank and allow the hose to fill. Took the fuel filter off and tried the same thing, still will not fill. Both tests were also with the gas cap open and closed and each way tapped the hose, petcock, etc. to try and break the air loose, nothing.

    Took the tank off again and drained all the gas. While I did, gas cap open and closed, no difference in flow. Tank certainly drained in less time than would have at 85 mph so flow should be sufficient to keep carbs filled.

    Opened the gas cap guts, no restrictions, appears to close sufficiently to allow air in. And when I eventually put gas back in, heard air escaping when sloshing around and cap closed...

    Removed petcock and disassembled, found no debris or restrictions. Could easily enough blow air through tubes and ports in both directions. Only thing I could think of was flimsy screen over tube was collapsing in on itself. Gasket is thick enough and ports protrude enough that I don't think the gasket could possibly spin when opening valve, thus blocking port(s). And no wear or damage to that valve disk gasket.

    Inspected inside of tank from cap and now removed petcock, shined a lot of bright light through and shook around. Can't find any debris.

    Filled two feet of the clear hose attached to carbs with gas, steady level observed. Started bike and ran normally and smoothly and watched the fuel slowly empty out of hose, with all cylinders equally warming.

    Reassembled tank and petcock meticulously but removed upper petcock screen to try out theory, and re-installed filter to cover for that screen. Petcock looks like this guy:
    upload_2018-9-12_22-59-17.png

    Filled completely with gas so no issue with flow between two sides of tank.

    EXACT SAME SYMPTOMS... gas will flow to free air / gas can, but when hooked up will not allow air to bleed back to tank to fill fuel line. And again both with filter installed and not, and with gas cap closed or opened.

    Going to get another type petcock but why this is happening is absolutely infuriating and a mystery....
     
  18. Polock

    Polock Well-Known Member

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    did you take it for a ride? mine has a bubble in the filter/line every time it sets for a few days, when i get back the bubble is gone.
    if you just want it gone right now, open the petcock to prime and open a float bowl drain into a jar.
    the bubble should go right out to the empty bowl.
    it can't go back into the tank because it would have to turn into wee little bubbles to pass through the screen filter in the tank
     
  19. Tim O

    Tim O Active Member

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    Was gonna try that maybe... but weird because I took the screen off the petcock (Aftermarket) and the air still won't purge or move out at all.
     
  20. Polock

    Polock Well-Known Member

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    i think your fuel line's broke :)
     

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