KTM 450XC - Carb Bog on Wide Open Throttle
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  1. #1
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    KTM 450XC - Carb Bog on Wide Open Throttle

    Recently I bought a 450XC where the previous owner had the quad sitting for around two years. The quad started and idled fine, but once you would stab the throttle it would bog badly. After some inspection, I noticed the accelerator pump squirter wasn't squirting at all. So I took the carb all apart, cleaned the jets, the passages in the bowl for the diaphragm and ball, as well as the passage to the squirter. I also adjusted the accelerator pump next to where the throttle cable goes. I put everything together and now the squirter would squirt when I stabbed the throttle, but it did not seem consistent and the bike still would bog when it's in WOT. I have some attached videos below of the squirter and the bog when the quad is running. Anyone have suggestions? Also bike seems to backfire and sometimes shoot flames, especially when revving up high.

    Video 1: https://youtu.be/qamIvOoIzJE
    Video 2: https://youtu.be/LvtK8EjG0lU

  2. #2
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    Another thing I noticed is that the quad runs excellent when choke is on. There's no backfiring, no bog, but once I turn choke off, it does exactly what's shown in the videos. It backfires and bogs in wide open throttle. Any ideas?

  3. #3
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    Well my guess is that your carb is jetted a little lean or that you’re at a different elevation than the previous owner? What is your elevation? Also did the previous owner put different jets in the bike or is it all stock?

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  5. #4
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    First thing first, in that first video it defanitly has a problem in the accelerator pump circuit. You don't have a consistant squirt and even when it does it looks weak. Possible bad diaphram, leak off or plugged passage. I'd take another look at it apart and see if you see a problem or try a new diaphram. You can rig up a fuel bottle so you can bench test the carb. Once you fix your root problem then do some reading on the rancho carb mods, or send it out to thumper racing or ct racing and get the mods done.

  6. #5
    Senior Member KTMATVHQ Guru
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    It is definitely running lean, and seems the accel pump circuit also has issues. If it's been sitting the first thing to do is thoroughly clean the carb. If it was not stored correctly the fuel will break down and clog the jets and circuits.
    built 400 ex 461, all done up, sleeeeper. 47hp, 32lb/ft... sold

    450xc, baldwin head w/Ti valves... motoworks full system... never done...
    i may ask a lot questions, but i also know some things.
    i dont capitalize, or punctuate or use spell check, but i will joke sometimes about spellcheck

  7. #6
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    Thanks for the feedback. The quad is a bone stock 450XC. I don't think anyone has ever messed with the jetting as it is all stock. I cleaned every jet and cleaned the accelerator pump passages prior to making this post. How could I test for a bad diaphragm? There aren't any visible tears.

  8. #7
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    If you are not sure about the diaphram they are pretty cheap just grab a new one, make sure its the same. Your dealer probably stocks them. If all the passages are free and the new diaphram doesn't fix it you may have a leak somewhere acting like a big leak jet. I don't feel like it is worth speculating any further on if the jetting is right or not until you get your accelerator pump fixed. If you hit a wall and just can't figure it out call thumper racing. They would be able to tell you where to go next and they carry all the parts.
    Last edited by Nat05gto; Yesterday at 03:35 PM.


 

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