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XJ750 running on 3 cylinders.....

Discussion in 'XJ Technical Chat' started by maxsauve, Mar 26, 2020.

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  1. maxsauve

    maxsauve New Member

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    Hi y'all I finally got my XJ to start! It's a bit cold here so it needed some extra juice from the battery to crank it up good.

    However I've got issues with the bike, again.

    The bike will only start on choke, it's about 6 Celsius here so I get it, and it will only run on 3 cylinders.

    Ever since I got it to start, I wanted to take it for a little ride. I let it warm up with the choke slightly open, get on the bike, turn the choke off and it either dies or idles and dies if I give some throttle. However it doesn't do it all the time...

    next day, same procedure but I noticed, while warming up the bike on choke, that cylinder 1's exhaust is cold compared to the 2,3 and 4. I check the plug and it's wet. Ckeck if I have any spark and it does. I also checked the compression and it's pretty good (125 + psi). I decide to take the carbs of, and go through every hole with a can of carb clean in carb #1. reinstall and same problem.

    Now my question is: How come I have fuel, compression, spark but no fire in cylinder #1?

    Now I got it to idle a bit, and it seems as if the #1's exhaust got hotter, however that could be from heat transfer of the engine being super hot.

    Could my enricher circuit be clogged? Could I still get spark, but not a strong enough spark to get it to fire?


    Any help would be greatly aprreciated

    Carb set up:

    - float set (haven't wet set, will do)
    - carbs synced
    - 2.5 turns air/fuel mix screw
    - stock, airbox stock, stock muffle
    - valves checked
     
  2. Toomanybikes

    Toomanybikes Well-Known Member

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    Bad spark plug cap? And have you done church of clean on carbs?
     
  3. maxsauve

    maxsauve New Member

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    Yes I did the church of clean, I event sent my bike to a mechanic to sync them. The first night I got the bike all cylinders where working, all the exhausts where hot the and the spark plug from #1 was dry and had a white tip.

    So It might of got clogged up since then (which 3 days ago) or the coil might of gone bad?
     
  4. Jackson Reeves

    Jackson Reeves New Member

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    I'm no motorcycle mechanic (but I am an auto mechanic lol), but when you mention that the plug is white and dry, that usually means that that cylinder is running hot. Could be that it's lean, so you might have to check your carbs and make sure that everything is actually tip top there, not everyone knows how to work on carbs nowadays. Also, your coils or spark plug could be bad, I'd recommend pulling the plug and grounding it to the engine, while trying to turn the bike over. See if you get spark. If you don't have spark, check your ignition system and go from there. You could also shoot some lubricant right into the spark plug hole to make sure that your piston rings aren't rubbing on the cylinder wall, which causes the cylinder itself to heat up and cause some serious problems (spoken from experience). Good luck!
     
  5. SQLGuy

    SQLGuy Well-Known Member

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    You pretty much can't.

    If your plug wires were swapped between 1 and 2, then you would have no fire from either 1 or 2, but you would have spark on both.

    Otherwise, your fuel to #1 must be way off.... way too much, or way too little for it to consistently fire.

    One thing you could try would be to spray a bit of carb cleaner into the #1 carb when you've got it running on 3. If you have pods (which mostly complicate things, but simplify this part), you can spray carb cleaner into the #1 pod filter. If you have a proper air box, you can attach a line to the #1 vacuum port and spray a bit of carb cleaner into that. If this helps things, then you still have some sort of blockage in the #1 carb, or maybe something wrong with how it was assembled.
     
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  6. maxsauve

    maxsauve New Member

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  7. maxsauve

    maxsauve New Member

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    I just swapped the #1 lead with the #4, and nothing changed. Also changed the spark plug and still same issue.

    So know it's clearly a carb problem, and I think it's running too rich, is it possible that the float valve is stinking? Which could flood into the cylinder and just not burn?
     
  8. Simmy

    Simmy Well-Known Member

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    If you’re doing all this on the sidestand it might be any of the 4 floats flooding, #1 is at the bottom. Verify you’re on the centre stand.
     
  9. SQLGuy

    SQLGuy Well-Known Member

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    1 and 4 share a coil and fire at the same time, in series. It's pretty difficult to lose spark on just one cylinder in a series fired setup. A bad plug is about the only thing that will cause it.

    Could be flooding on #1, though. How wet is that plug?

    I don't think it's possible to sink these floats, but it could be hung up or otherwise not sealing its pin correctly. You would see that in the service fuel level and also because the carburetor would probably start overflowing fuel.
     
  10. maxsauve

    maxsauve New Member

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    God dam I hope that's not it ahahah. It was on the side stand the whole time. I'll try next time.oj the side stand
     
  11. maxsauve

    maxsauve New Member

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    So I just took the carbs out of the bike to check if the needle valve leaked and it did not. I even added pressure and nothing leaked. I also checked the fuel level in each carb and it was all in spec. I also swapped leads from 1 to 4 and no changes. I checked the compression and it's the same across the cylinders. Also bike was on center stand. What should I look for now???
     
  12. tabaka45

    tabaka45 Well-Known Member

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    Have you checked for an air leak on the engine side of the carb? I had a similar problem when I bought my bike and found that about 1/2 of the gasket between the intake manifold and the engine was missing. Before I replaced it I had to open the pilot screw on that carb almost 5 turns to get it to fire. Have you tried opening the pilot screw? I agree with Jackson the it is very lean or maybe no fuel at all.
     
  13. maxsauve

    maxsauve New Member

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    I'll check that out. I've also noticed that when the bike runs on 3 cylinders at idle, I would get backfires from the #2 carb (the one not fonctioning right). Would that tell me anything?
     
  14. tabaka45

    tabaka45 Well-Known Member

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    I have been told that very lean will lead to backfires because the gases build up in the exhaust and ignite in the pipes--or at least that's what I think I heard. In any case, I think that cylinder is very lean. If it isn't an air leak, then it has to be in the carb. Just curious, but does the cylinder fire when you add "choke" or rev the engine?
     
  15. maxsauve

    maxsauve New Member

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    No it doesn't fire when revving or on choke. It's completely dead. Spark plug gets wet and it still sparks
     
  16. maxsauve

    maxsauve New Member

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    However I noticed, while the bike was idling on 3 cyl, that i could feel air being sucked in through where the throttle shaft seal sits (under the white plastique cover and c-clip). Is it possible that could create too much of an air leak to not even be able to fire? If I put my finger their I can feel suction. I'm gonna put some grease and see if that makes a difference
     
  17. XJ550H

    XJ550H Well-Known Member Premium Member

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    You could spray with WD-40 and you would hear a result or starter fluid carb cleaner
     

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