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Missfire/backfire on right muffler

Discussion in 'The Garage' started by Joshfire, Jul 1, 2021.

  1. Joshfire

    Joshfire XS650 New Member

    Just recently bought a 1975 Yamaha xs650 that was sitting for sometime to assume. I cleaned the carb and out new plugs in. The old plugs were horrible with black carbon covering them probably from running too rich. No matter how I've tuned it stalls and won't run smooth and needs alot of help!
     
  2. XSLeo

    XSLeo XS650 Guru Top Contributor

    Points, Have you checked them? They need to be clean, gapped and timed right.
    You have two sets of points. One for each side. The side not running right is the one to check first.
    New points are not expencive. Around $20. Your repair manual explins how to do this.
    If that don't help it may be the coil. Both the primary and secondary sides. A multi-meter is your friend. Checking all the connections between the battery and points All these connections should be cleaned and tightened, right along with every othe connection on the bike. Start at one end and work your way to the other. A bit of dialectric gease will help keep thing clean.
    Leo
     
  3. bosco659

    bosco659 XS650 Junkie Top Contributor

    x2 for tuning up the ignition system. How did you clean the carbs? Did you set float heights? If jets aren’t squeaky clean, maybe throw a set in. Compression test wouldn’t hurt either.
     
  4. 83Kano

    83Kano XS650 Member

    Been chasing a misfire and plug fouling of my left cylinder for a while now. Pulled the caps off recently and found broken wires and corrosion in left lead where cap screws in. Replaced caps and cut both leads back to fresh wire and she’s running beautifully . May look to replace coil and leads at some stage…
    Certainly an easy check with a multimeter (not that I used one).
     
  5. Jan_P

    Jan_P XS650 Guru Top Contributor

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    Weak spark can also make plugs black
    It is about the simplest test one can do on the bike and one can do it with a straight back
    Measure charging voltage across the battery.
    We had a 78 or something a couple of months ago
    There was ignition off and leaking intake ( boots ) making it go poff in the exhaust.
     
  6. 5twins

    5twins XS650 Guru Top Contributor

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    If the plug caps are originals, besides checking the connection to the wire, you need to check the cap. They can and do go bad. They are resistance caps and when they go bad, that resistance starts to increase. Eventually it will get so high it starts choking off the spark. The original caps had a rather odd resistance rating of around 8 or 9K ohms. If you still do have the originals on there, best to just replace them with new NGK caps. You can get the 5K ohm resistor ones or if you are still running points, you can actually use no resistance zero ohm caps. The stock ignition is kinda wimpy to begin with so any little thing you can do to improve the spark strength is a good thing. Zero ohm caps is something simple and cheap you can do.

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