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Carrrrrrbbb pop pop carrrrb pop.

Discussion in 'The Garage' started by AussiXS, May 25, 2018.

  1. AussiXS

    AussiXS XS650 Enthusiast

    If the title didn't give much away, let me explain.

    1978 xs650, with Hugh's hand built complete kit. Timing is pretty much on point but does kick back every now and then.

    My issue is my carbs pop every now and then and stall the engine. Then other times they just pop and I'm on my merry way popping. I can't take the bike outside of my suburb because it stalls. Why causes the popping?

    I have no filters on at the minute because I keep taking my carbs off to clean and diagnose. But I do have foam free flowing when I have them on. I bought pilot jets of 35. Stock exhaust (why change such a sexy thing?) My plugs are also quite black. I create a manometer (2 Gatorade bottles and lots of hose), the problem with that is as soon as I plug hose into the right hand vacuum it sucks all the fluid very quickly. What would cause that? I have adjusted the two small screws either side an it doesn't stop the major sucking.

    I have not done a leak test but I will tomorrow. anyone have an idea what's happening here?
     
  2. grizld1

    grizld1 Grumpy old man Top Contributor

    The fluid is getting sucked out of the right side of the manometer because vacuum is higher on the right side than on the left. This is going to sound backward, but here it is: your right side throttle plate needs to be opened: tighter restriction=higher vacuum reading. Black plugs could be from oil burning, could be that your fuel level is too high, could be your mixture screws are backed out too far, could be your PJs are too rich, could be a combination of all the above--insufficient information even to guess.
     
    Robert Reed likes this.
  3. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    robinc likes this.
  4. 5twins

    5twins XS650 Guru Top Contributor

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    You're 3 sizes up on your pilot jets. Why? That's an awful lot for just one mod (the pod filters). You may not need any change, and at the most, only one size up. So, all that popping could be related to this jet size thing, or to the carbs being badly out of sync, or it could be timing related - or any combination of these things. These bikes like their timing set exactly right. With a PMA, that can be difficult because of the lack of timing marks, or poorly located ones made by the owner. Yet another strike against PMA use in my mind.
     
  5. AussiXS

    AussiXS XS650 Enthusiast

    I dropped the pilot jet down to the stock 27.5 and the bench synced the carbs. It was really handy blowing against the butterfly's to test pressure. Once that was done, no popping at all!
     
  6. AussiXS

    AussiXS XS650 Enthusiast

    After a good old ride around for the first time in 2 years of problems and pain, its great to get out an about on the bike!

    I did some playing around and found that the carbs would pop as soon as I took the choke off. When its on i can drive for hours with no problems. What would cause the carbs to pop when the choke is off but work fine when on?
     

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