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left cylinder backfiring cant figure it out

Discussion in 'The Garage' started by Brock McGrath, Jun 16, 2017.

  1. Brock McGrath

    Brock McGrath XS650 Member

    I havea 1980 xs650 special and no matter what i throw at it i can't seem to solve this. After warm up the left cylinder behinds backfiring and surging getting worse and worse until it's essentially running on 1 cylinder. Leave it overnight and it's back to normal until it warms up and starts over again. I've replaced the plugs, wires, caps, coil, reg/rect, had the carbs off and cleaned and synced, all the fuel lines are good, as is the petcock. The headlight went out at about the same time as it started doing this and i know i read a thread on here with someone having the same issue but he never came back to say if he ever solved it, or if the headlight was a symptom or just coincidence. I'm running out of ideas and this bike is so close to being finished i just need to get over this hill so to speak. Thanks.
     
  2. figure8

    figure8 XS650 Guru

    What do the spark plugs look like? One quick simple test, switch plug wires to opposite plugs and see if you have the same problem. If yes probably a carb issue, you mentioned cleaning and syncing them. Positive your floats are good and also set correctly?
     
    Brock McGrath likes this.
  3. Brock McGrath

    Brock McGrath XS650 Member

    It must be a carb issue then, one of the first things i did was swap plugs both before and after the new set. I know the floats are good but i think I'm gonna recheck everything on the carbs. See if maybe i missed something
     
  4. peanut

    peanut XS650 Guru Top Contributor XS650.com Supporter

    Brock.. first of all welcome to the Forum . I hope you stick around once you get your bike running, this is a great community .

    As a matter of courtesy I think it is generally expected that new members of a forum that join specifically to get free help and advice from other owners should at the very least introduce themselves before asking for help.Its just being polite really. It also tells us a little about your bike whether it is stock or modified etc ;)

    Buying new components at random and fitting them rarely solves an issue and can be very expensive. its far better to use your noddle and do some simple diagnostics checks that will eliminate what is not relevant and narrow down the likely component/s causing the issue.

    Start by looking at what components only effect the running of the left cylinder ...it sounds like it might be a heat related issue!...;)
    Ignition
    To eliminate the ignition system components try using the known good components of the right cylinder with the left cylinder.
    Swop the spark plugs over
    Swop the plug leads and caps over
    Swop the coil Lo Tension leads and the high tension leads and caps over
    Substitute a good coil or test yours on another bike
    Carburettor
    Run the engine until the left cylinder starts to die then quickly apply a little choke about 1/4 to 1/2 choke . This will give the cylinders an shot of extra fuel . if the engine picks up revs then you'll know for certain that the issue is a fuel/air related one and not electrical or ignition.
     
    TwoManyXS1Bs and gggGary like this.
  5. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Didn't see setting valve clearance or a compression test done cold then hot mentioned.
    A knowing foot, throttle cracked, ignition off, can figure compression pretty close.
    if compression is suspect TDC on the bad cylinder, in high gear foot on brake, add some air in the plug hole listen to where it goes.
     
    peanut likes this.
  6. peanut

    peanut XS650 Guru Top Contributor XS650.com Supporter

    good shout gggGary . a very good diagnostic for this sort of issue
     
  7. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Stock, the headlight will only light if the alternator is charging, get a volt meter on the bike. Above idle you should see 13.5 to 14 volts. The stock TCI ignition is voltage sensitive and will mis-fire if it's not getting enough juice. Load test the battery new or not.
     
  8. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Like Peanut sez, hide the wallet for a bit, and pick up the testing tools.
     
    TwoManyXS1Bs likes this.
  9. Brock McGrath

    Brock McGrath XS650 Member

    I appreciate the concern for my wallet but those parts were all bought with the intention of replacing old or worn or incorrect a/m parts just to get this bike back to running. They just happen to help narrow down the possibilities with the issues I'm having. As for the bike it's stock aside from those small upgrades. It's rough cosmetically but it has good bones. And thanks for the advice ggggary i will give those a shot and let you all know
     
  10. peanut

    peanut XS650 Guru Top Contributor XS650.com Supporter

    Replacing parts on a vehicle as a means to diagnosing and fixing an issue is never the optimum troubleshooting method.

    Its an indication that there is no understanding of how a componant works nor how to test it .

    If you carry out just the 4x tests that have been suggested you will eliminate far more than bolting on untested replacement parts can do and it will cost you precisely nothing.;)

    The problem with substituting replacement new parts in order to diagnose a problem is that you have no idea if the new part works and also you are introducing numerous unknown components into the equation and possibly introducing further issues and complicating the matter.

    If you concentrate your attention on what makes the left cylinder function you will eventually find the problem . Obviously it cannot be anything that also contributes to the right cylinder working or both cylinders would be malfunctioning.;)
     
  11. Bob Kelly III

    Bob Kelly III Ranch Kid from way back,.... that got Old !

    Perhaps a valve lash setting is too tight... causing it to be held open all the time after warm up....
    the first thing I would do is loosen the valve lash alot and see if that cures it .... if it does then adjust them to the book's specks and see if it starts screwing up again...if it does then you have a cam or cam follower problem....
    .... check for spark after the engine is warm if it's constant and good spark ( blueish arc ) and not intermittent then the spark is good .
    it also might be an air leak on the left side that only shows itself when warm I think this more likely than anything else ....
    get it to the mess up stage and squirt wd-40 around the carb boots and head and see if you can find a place where it clears up for a second or gains RPM.... ....
    that's all I got !
    Bob...........
     
  12. Brock McGrath

    Brock McGrath XS650 Member

    once again i did not buy those parts to diagnose this problem they needed to be replaced for other reasons. I.e. before parts, bike no run. After parts bike run but not right.
     
  13. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Let us know how it goes with testing to narrow down the current issue?
     
  14. Brock McGrath

    Brock McGrath XS650 Member

    Absolutely, will do after i get a chance to mess with it again.
     
    Bob Kelly III and gggGary like this.
  15. Bob Kelly III

    Bob Kelly III Ranch Kid from way back,.... that got Old !

    I had back firing periodocally on my 83' XS650 it was caused by too lean of a condition the air screws were at 3/4 of a turn from their seat stock from the factory.... after I opened them up and did a dead cylinder test they wound up being almost 4 turns open !
    ..... you might look at the air screws ... just guessing here....
    ....
    Bob......
     
  16. peanut

    peanut XS650 Guru Top Contributor XS650.com Supporter

    once again i did not buy those parts to diagnose this problem they needed to be replaced for other reasons. I.e. before parts, bike no run. After parts bike run but not right.[/QUOTE]

    Apologies .......I must have misunderstood your first post.:)

    I've had no thanks from you for trying to help you solve your problem, just belligerence ...for my advice, so i'll bow out .
    Perhaps you should seek the help of a local garage or knowledgable biker before your money runs out
    good luck
     

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