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XJ650 Sputtering and lack of acceleration after 5k RPM

Discussion in 'XJ Technical Chat' started by Justin1270, Apr 18, 2021.

  1. Justin1270

    Justin1270 New Member

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    Bike is an '83 xj650, bought recently as a project bike. Was not running properly from purchase, with some PO hack work. Checked valves and compression, all good. Ripped apart carbs, took them to church, fixed jetting, floats, and needle to proper specs. New plugs (had to buy BPR7ES due to BP's being discontinued here), and already had new coils installed.

    So the problem. Revs up to redline in neutral with no problem. Under load, will Rev up to redline in 1st with minimal sputtering, but 2nd through 5th after around 5k RPM will sputter and not accelerate. Thought it was a fuel problem, so tried running on prime, shortened needles, and adjusted fuel mixture. All to little to worse effect. Opened up airbox, and seemed to help little, but still only started to pull before sputtering out.

    So I'm out of ideas, any help appreciated.
     
  2. k-moe

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

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    Are you running those plugs with non-resistor plug caps? You should be.

    In the part where you took teh carbs to church; what do you mean by "fixed jetting"?
     
  3. Justin1270

    Justin1270 New Member

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    Unfortunately there are existing resistor plug caps on the bike. I know that technically it should be either one or the other, but the local bike shop advised that it shouldn't be too big of an issue.

    As for the jetting, I fixed the locations, as some jets were switched main/pilot. I checked all the sizing in the manual and confirmed the correct size.
     
  4. tj.

    tj. Active Member

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    Haynes manual has the pilot air jet and main jet labelled incorrectly...
     
  5. k-moe

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

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    Did you put the air jets in the locations cited by the manual, or did they get put into the correct locations?

    [​IMG]
     
  6. Justin1270

    Justin1270 New Member

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  7. Justin1270

    Justin1270 New Member

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    Is there another model of spark plug that is equivalent to the bp7es that is still in production?
     
  8. k-moe

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

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    BP7ES is still being made. All of my local parts stores have them in stock.
     
  9. Justin1270

    Justin1270 New Member

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    I can't seem to find them anywhere up here in Ontario, and even trying to ship from the states I don't see anymore than 1 available.

    Do you think this will be the solution to my acceleration problem, or just something that will help finetune the bike in the future?
     
  10. Simmy

    Simmy Well-Known Member

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  11. k-moe

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

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    It's not the cause of your current problem, but in a TCI system the total resistance on the secondary side ajusts the spark duration and does affect performance. If you can't get the plugs locally, then just get non-resistor caps (you can replace the resistors in the factory caps with brass slugs if cost is an issue).
     
  12. tj.

    tj. Active Member

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    Lots of variables and unknowns...If it were me, I would start with fuel/air first.

    Thinking in terms of throttle position...

    Is it only sputtering past 5000rpm under full throttle?

    Does it accelerate past 5000rpm under 1/2 throttle without sputtering?

    When sputtering does the use of the enrichment circuit smooth it out?

    Tried running with gas cap open?

    Is there an aftermarket inline fuel filter?

    Assuming a factory airbox...

    What kind of air filter is used? K&N? Standard pleated paper filter?

    Exhaust type? 4 into 1 etc.
     
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  13. Polock

    Polock Well-Known Member

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    5K under load sounds like the main jets to me.
    this is important
     
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  14. XJ550H

    XJ550H Well-Known Member Premium Member

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    i tryied to purchase ngk plugs on line at the auto parts store by make and model of bike did not show the ngk plugs and when i searched for the plug number it came up as in stock but did nor fit my bike the site said. they had them i bought them
     
  15. XJ550H

    XJ550H Well-Known Member Premium Member

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    upload_2021-4-19_21-27-5.png NipponDenso second plug type
     
  16. Jetfixer

    Jetfixer Well-Known Member

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    I run Autolite AP63 or just plain 63 these cross reference to the NGK . My local autozone does not stock the NGK 7ES nor BP7ES of course dude told me they didn't carry motorcycle plugs ...he was totally shocked when the girl showed him on his computer how to find them.:p The Autolite plug was the "HOT" ticket when I had my Virago 750 on the forum and they worked great.
     
  17. Justin1270

    Justin1270 New Member

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    Throttle position is irrelevant, will accelerate to approx 5k and then no longer accelerate, and sputters. Shifting to lower rpms will give instant acceleration again.

    I tried the enrichment circuit, makes no difference. Almost seems like lack of air, not fuel. Tried running with gas cap open, the seal seems to be mostly gone anyways, gas leaks out if filled to the brim. No aftermarket filter.

    Standard airbox, with a brand new hiflofiltro air filter. Exhaust is a newish 4 to 1 exhaust.


    Fortnine has been out of stock on those plugs for weeks. I may try those AP plugs, as after several hours of calling around ngk bp7es seem to be completely unavailable for the time being.


    Thanks so much for all the helps everyone, really appreciate it.
     
  18. Jetfixer

    Jetfixer Well-Known Member

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    A few other things to check , if your running a fuel filter try removing it ,could be a restriction and see if any change . Your synch maybe off my Johnny Cash bike idled perfect , the Morgan Pro motion columns were perfect , with them hooked up took off at wide open throttle number two and four were at top one and three were lower . Had to adjust them this has helped, but still playing with it got bike to 70 but it is no where like my yellow seca .
     
  19. XJ550H

    XJ550H Well-Known Member Premium Member

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    I found resistor caps and resistor plugs an issue .
     
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  20. tj.

    tj. Active Member

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    OK, the sputter is at 5K regardless of throttle position...only backing off the throttle makes it better.
    A couple things come to mind here.
    What needles are you using?...Y-10?...all the same?
    Floats wetset?...3mm?
    Clunk test OK? With the hats on, do they all return at the same rate?...diaphragm tabs in the correct place.

    Did you re-jet for the pipe? What main fuel jet are you using? Main air jet?

    The only other things I can think of that could block fuel flow are the screens in the carbs and the screen for the petcock, assuming that the fuel line is not kinked.
    BTW the enrichener circuit adds fuel AND air.

    That's all I can think of for fuel and air, other than a blockage in the airbox...something silly like a rag or mouse turds etc.


    Perhaps the PO had the same problem and replaced the coils?

    What kind of new coils are they? oem? aftermarket?

    Have you ohmed the new coils? pick-up coils?

    I don't know how to check a TCI box for functionality...other than swapping with a known good one. The correct TCI box (YICS 650 should be red 5G2...I think) shouldn't make a difference at 5000rpm...timing curves are the same above 4100rpm.
     

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