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Tuning help or something more serious?

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  • Tuning help or something more serious?

    Check out the video here:

    I'm getting backfires in the exhaust. Is this a tuning problem or something more serious?

    I had bad CA ethanol gas in my tank for 9 months and when I went to start it up, my float valves stuck open and fuel poured out of the bowl overflows. My excessive cranking probably didn't help anything as I thought it would break free. It didn't so I tore down carbs and put brand new floats, measured my old one first and was right at 17 mm height. I also replaced the valves and polished the seats. No more overflowing bowls. I also did a complete cleaning of the carbs. I also took the time to put in a new inline fuel filter and new lines and vacuum caps. In my haste to get her running, I accidentally connected the fuel line to the bowl overflow! This flooded it again. After cleaning things up and realizing my mistake, I cleaned the carb again. I fired it up and noticed that it was running good but backfiring like crazy at anything around 25% throttle or above. It happens throttling up and also when letting off. Note that I've not installed anything new or other mods- the bike used to run great prior to flooding.

    Here is the setup:
    1. BC A.R.K.
    2. K&N pod filters
    3. I think I have 140 mains and cannot remember the idle jet size that came with BC's ARK
    4. Procom CDI
    5. BC Predators
    6. AI removed
    7. NBZT needle (Stock in my bonnie)
    8. 1 shim (it's about .76 mm)
    9. 2.5 turns out on idle mixture screws

    What I've tried:

    1. New plugs gapped and cleaned
    2. New oil after I'm pretty sure I got fuel in the old stuff
    3. I tried every tune (1 - 9) on the Procom CDI-- it makes a difference but I'm having trouble finding balance in timing
    4. Cleaned carbs
    5. Balanced carbs at idle
    6. Verified float height with ruler and also tubing to the drains looped up showed fuel right at mating surface of bowls to carbs
    7. New needle valves
    8. New floats
    9. I tried some custom tunes on the CDI and it definitely makes a difference
    10. I searched for vacuum leaks- I cannot sense anything

    Scarier possibilities:

    1. Maybe it hydrolocked and I bent something? I cranked it until it stopped before I noticed it was flooded. Where should I look first?
    2. ???

    Please help- I've read Jenks' tuning guide and didn't have trouble for the 3 years I've had this setup. This is perplexing me now and I think I'm just too close to the problem and need new ideas.
    Thanks in advance!

  • #2
    Aquinas... thanks for posting your question here. I have had some limited experience with my own setup similar to yours in the past. I believe the popping on deceleration is due to a "too rich" setting on the carbs, which allows unburnt gas to exit in the exhaust headers and when it reaches the BC Predators and expands in that chamber, the exhaust is hot enough to ignite those gases and you get the exploding or popping sound. I would look for any possible changes in your process that may have changed the air-fuel ratio to be richer. My best first guess would be the change in the floats since you replaced them - assuming everything else is exactly the same. Getting the new floats to be exactly the same as the previous ones is not easy. Also, once you are sure of the floats, you may need to experiment with different jet sizes given your setup. I ended up with 145 mains and 42 pilots. You can read more about my experiences here: --- Good Luck!

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    • #3
      aquinas did you manage to sort this out as I have also had a similar problem. After I had my ride dyno tuned it seemed to sort out this issue.