1. Hello Guest. You have limited privileges and you can't "SEARCH" the forums. Please "Log In" or "Sign Up" for additional functionality. Click HERE to proceed.

XJ750 Running fine but general issues.

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

  1. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    When I put them out to about 3 1/4 the bike was struggling to run and was bogging alot, when I put it to about half that it runs but bogs at low RPM (2k and under) but once you get higher in rpm it runs fine. Just for clarification I am talking about the screws above the butterflies on the carbs, not the screws attached to the throttle bracket.
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    the jets you swapped in were they from a set of hatachi carbs? mikuni jets are not marked in mm size but flow rate.

    did you confirm the jet sizes you swapped in from the other spare carbs?

    when you took jets out and blew air though them did you do the passages they cover as well?

    and are jets in the correct location? main jets larger with washer under the aspirator tubes with smaller
     
  3. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    1. Yes from same carb
    2. I do not have a way of making sure they are the same size but by eyeballing they looked exactly the same.
    3. The passages were already fully cleaned and had air blown through them last time I cleaned it, I've done a complete teardown and cleaning twice already.
    4. I put them back together the exact same way they came apart, I've worked on carbs before and am good with intricate details (Thanks US Navy) and under the bowls of both carb sets look exactly the same, so I'm assuming they are correct. The smaller jet has the brass washer under them.
     
  4. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    Hatachi carbs correct
    bright light magnifiying glass jets have numbers on them
    ok good
    hatachi xj750 jet.jpg

    pilot jet smallest on left. main jet largest on right with washer
    if reversed this is you issue

    Rule #1 is never trust the P O if he had his hands in it all details need to be checked in your case jet location
     
    Last edited: Jul 7, 2019
    k-moe likes this.
  5. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    After alittle more diagnosing me and my neighbor were able to figure out that Cylinder 1 is not firing fully, there is very weak power coming from it (too dark and engine was hot at the time of testing so couldn't take out plug and see if it is just weak spark).
    I'm pretty positive that my carbs internals look exactly the same as the picture above.
     
  6. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    TCI systems have visibaly weaker sparks thanother systems (the spark is not actually weak though).
    If #4 is firing than #1 shoudl be too, unless something is wrong with that carb, compression is low, those valves are out of spec, or the sparkplug is bad.
    Swap the #1 and #4 sparkplugs and see if the problem follows the plug (they fire from the same coil).
     
  7. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    Alright, I'll do that tomorrow, Thanks K-Moe!
     
  8. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    Alright so update! I know it takes me time between posts and I apologize. I swapped plugs from 1 to 4 and the low power from cylinder 1 completely disappear and didn't show up in cylinder 4 (confused). I'm rolling with it, I vacced the carbs and adjusted the throttle screws between carbs like this: ""Adjusted main throttle idle screw on underside of carb to get RIGHT BEFORE you get hanging idle when blipping the throttle, then adjusted each carb afterwards to match the vac reading from the main carb (carb 4) and now the bike is purring like a kitten (unless cold with choke)."" Now for the mixture screws I pulled them back about 3 3/4th turns and the bike would still sound like it was bogging and sputtering at low RPM, giving me a signal that it is running rich still, continued backing them out slowly between test rides till their all about 4 3/4ths full turns backed out and the bike seems like it is running perfectly now. Only question I have now is I have read other people saying that their 750s get up to 145MPH, mine struggles to go anything faster than 75 (not to mention it wobbles like a bitch when it gets past 70) so new tires and a balance are in it's future.

    Thanks for everyones help so far!

    EDIT: Figured I should also mention when I checked the spark plugs they were black and wet, so I know it was running rich.
     
    Last edited: Jul 13, 2019
  9. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    the 750 does not go 145 mph top speed is around 125 mph. wobble could also be steering neck bearings, wheel bearings (front and rear) swing arm bushings.
     
  10. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    Any particular reason my 750 might be getting on 27 mpg, is this normal?
     
  11. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    that's kind of low.
    Im getting 36 mpg on a set of carbs and still have to dial them in just bench sync and mixture screws set to 3 turns have only set idle.

    of course riding style affects fuel consumption too
    mix screws at 4-3/4 turns more fuel. until you resolve the rich running your fuel mpg will be low

    you sync to carb 3 not 4

    you are approching the point where you need to look at size of your pilot jet


    fix the wobble or slow the f down
     
    Deathboner727, Rooster53 and k-moe like this.
  12. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    Fix the wobble.
    Check the headstock bearings, tire pressure, tire wear, and rear shocks.
     
    Deathboner727 likes this.
  13. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    Also I just checked the picture from before how the jet should look in the carb, I don't think my carb I have on it actually has the correct jets in it. They look more rounded compared to the picture previously shown.
     
  14. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    got a pic or 2 of the carbs? get used to taking photos as you work.
     
  15. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    They could be the correct jets, but damaged.
    +1 on pics
     
  16. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    The jet size that was in it was size 110, I swapped in 120. What is the correct size?
     
  17. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    The jet size that was in it was size 110, I swapped in 120. What is the correct size?
     
  18. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    http://xjbikes.wikidot.com/carb-specs

    Years & Models: 1981-3 XJ750 Maxim, Midnight Maxim, and Seca USA models
    Carb Manufacturer: Hitachi
    Carb Series: HSC32
    Carb Model ID: 15R00 (Maxim, Midnight Maxim)
    Carb Model ID: 5G200 (Seca)
    Main FUEL Jet Size: #120
    Pilot FUEL Jet Size: #40
    Main AIR Jet Size: #80
    Pilot AIR Jet Size: #225

    check them all while your in there.
     
  19. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    Alright thanks! I took a video and will upload it to YouTube as a private video to show you guys the issue I'm having now. The fuel jet is 120 now, but it still spots and sputters at under 2k rpm, like it has no power. It acts like it wants to die but will eventually get up and going once you get past 2k with tricky throttle control.
     
  20. Maxwell Partridge

    Maxwell Partridge Member

    Messages:
    115
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Pennsylvania
    I guess I should also add real fast that it has a brand new sealed battery, it's an everstart from Walmart.
     
    Timbox likes this.

Share This Page