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XJ750 Running fine but general issues.

Discussion in 'XJ Technical Chat' started by Maxwell Partridge, Jun 24, 2019.

  1. Maxwell Partridge

    Maxwell Partridge Member

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    Hey guys, sorry I've been silent, work has been kicking my ass, I'll grab those pictures today!
     
  2. Maxwell Partridge

    Maxwell Partridge Member

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    Tore down the carb again and actually replaced some of the parts in it from a spare carb I had, Apparently I had 3 carbs total, one is a Mikuni and the other two have the hitatchi logo on the side so I'm assuming those two are the correct carbs for the bike. I had to replace 2 of the main jets in 2 separate portions of the carbs since the ones originally in them were chewed all to hell, you couldn't take them off without locking pliers, and one of the carbs is missing a brass plate that goes under the rubber seal on the top portion of the carb, I have no clue what these are called so I will give a description now so you may know what it is: ""On the top after taking off the chrome-plated part there is the spring that comes out, the rubber flexible seal that goes around rim, and then the needle attachment to it. You pull that whole assembly out and then there is a brass plate screwed in with a single flat head screw also made of brass that covers two jet-looking screws."" So one of the carbs is missing that plate, and before rebuilding it using parts from the spare carb one of them was missing the actual jet located in there, I pulled it from the spare and put it in. So to my knowledge so far it has everything it needs now, except one single carb is missing that plate, I'm going to put it back on the bike tomorrow and see how it runs.

    How far out should I back the air-fuel mixture screws to start getting a baseline for how to adjust it? I currently have them backed out 2 1/2 (half rotation) turns from bottomed out. Thanks so much for the information so far!
     
  3. XJ550H

    XJ550H Well-Known Member Premium Member

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    plate not needed it was removed from the assembly process by yamaha

    2-1/2 turns from soft bottom is base line . open them all up to 2-3/4 turns

    you now have 4 hatachi carbs correct?
     
  4. Maxwell Partridge

    Maxwell Partridge Member

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    @XJ550H I don't have 4 Carbs as in 4 Carb Assemblies, I have a single carb Assembly that is 4 Carbs linked together that all run from the same throttle/choke cables and gas line.
     
  5. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    That's what he's asking.
    There are 4 carbs to a rack. WHat yoy wrote mesde it seem like you had a rack consisting of 3 Hitachi carbs and one Mikuni carb, which is physically possible.
     
  6. Maxwell Partridge

    Maxwell Partridge Member

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    Nah, Their all Hitachi and always have all been, but it was missing some parts as in that brass plate that he said isn't even needed, and one of the holes under that plate was missing the brass screw where (I am assuming) gas goes through.
     
  7. Maxwell Partridge

    Maxwell Partridge Member

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    Other than that, all I did was clean it out extra good again, and replaced some of the brass jets since the previous owner apparently destroyed them using locking pliers and completely crushed it. So I swapped some that still had the flat-head ends into the good carb from the parts carb. Now I'm waiting till tomorrow to put the carb back on the bike and try and start it up to see if it is running any better, I'll update you guys again then.
     
  8. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    The screw for that plate goes into a blind hole, so you don't need them in there either. Gas will never get into the upper part of the carbs. It'll go into the crankcase long before then.
     
  9. Maxwell Partridge

    Maxwell Partridge Member

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    There are three brass screws in there though?
     
  10. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    There is one screw to hold the unnecessary plate (both missing in the pic), and two air jets.

    [​IMG]
     
  11. Maxwell Partridge

    Maxwell Partridge Member

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    Oh alright, thanks for clearing that up for me! Like I said I'll post an update tomorrow when I get it running again!!
     
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  12. Maxwell Partridge

    Maxwell Partridge Member

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    When you guys say to back out the fuel mixture screws x amount of turns, do you mean full 360 degree rotations, or half that per "turn"?
     
  13. XJ550H

    XJ550H Well-Known Member Premium Member

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    full turns from soft bottom.

    the jets under the diaphrams are air jets . shown in photo above
     
    Last edited: Jul 2, 2019
  14. Maxwell Partridge

    Maxwell Partridge Member

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    Alright, before I put it back on I'll bottom out the air mixture screws again and back them out 2 2/3 turns using full rotations per turn. I know what the fuel mixture screws are I just didn't know what those jets under the rubber diaphrams were. Is it alright that when bottoming out the mixture screws they don't all end in the same position, I.E. not all flatheads look the same at dead bottom?
     
  15. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Yes. There is some variation as not all of the screws were cut exactly the same (e.g. the slots are not clocked to the threads).
    They are registered from the needle contacting the seat, so don't bother trying to match the slot orientation from screw to screw. Just count turns for each individual screw.
     
  16. Maxwell Partridge

    Maxwell Partridge Member

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    Alright, Thanks!
     
  17. Maxwell Partridge

    Maxwell Partridge Member

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    Alright, so I got another update. I got the carb back on the bike, it was struggling to start so I tore it back apart and adjusted afew screws along the bottom of the bowls and put it back together. It starts and idles fine especially once you get it warmed up but once you try to start going it bogs, like as if though it is getting too much fuel or something. Anything under 2-2.5k rpm makes the bike bog, you have to hold the idle at around 3-4k rpm to go from a dead stop and keep it above that when switching between gears, any ideas? I've tried adjusting the idle screws abit but cannot seem to find the sweet spot, I'm gonna mess with it more today once I get up later, I work night shift so I'm only now just going to sleep (6:20 AM EST).
     
  18. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    There are no screws along the bottom of the float bowls to adjust.
    The only screw-based adustments are the idle mixture screws (along the top of the carbs next to the enrichment plungers), and the throttle plate synchronizations screws (along the throttle shaft between carbs. The synch screw for #3 is the idle speed adjustment knob).

    What screws did you turn?
     
  19. Maxwell Partridge

    Maxwell Partridge Member

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    What I mean by I adjusted some internals along the bottom of the bowl was that I took out the jet s and put them back in to ensure nothing was clogging them after blowing air through them. I adjusted the air fuel mixture screws along the topsides of the carbs near the inlets to the head of the engine.
     
  20. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    How many turns out do you have them at now?
     

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