08 CRF450X -Slight stumble at mid RPM @ slight acceleration.
  • Orbitter
    Posts: 1
    Joined: Fri Mar 25, 2016 8:10 pm

    08 CRF450X -Slight stumble at mid RPM @ slight acceleration.

    by Orbitter » Fri Mar 25, 2016 8:36 pm

    Been working on it all day and can't seem to tune out the stumble, so here I am... Puzzled.

    Ok, so I have a bike in for repair. Customer says it surges bad at cruise and dies sometimes.
    I could not get it to die during the test ride, but did notice the surging/popping.

    Did notice during carb cleaning/rebuild that the previous owner had REMOVED the CORE of the muffler.

    It only happens at mid range RPM and very slight acceleration.
    Everywhere else in the RPM & throttle position range, it works GREAT.

    I have adjusted/replaced the jets but either way I go, it only gets worse.
    ONLY thing that changes, is weather the choke makes it better, or the hot start makes it better. But I can't seem to tune it out. :shock:
    The needle in the carb is non adjustable but I ground some grooves in it so I could adjust it.

    Initially, when test riding, the problem would get better when choked.
    So I raised the needle in the needle jet about 1/16".
    Not much change so I ground more grooves.
    Raised the needle 1/8", Worse...
    Strange thing is, when test riding at 1/8" raised, it got worse with choke and better with hot start.

    So I dropped the needle back down to stock and sure enough, got better with choke.
    Raised it back up to the first groove I ground @ 1/16" below the stock groove, and choke only hurts it. It only gets very slightly better with hot start pulled.

    But when the needle is set to stock height/groove, applying choke is a noticeable improvement. Yet just 1/16" higher in the jet needle, and it likes hot start.

    Is this thing really that finicky?

    Stock (145) Main Jet
    Stock (45) Idle Jet
    Stock NNEU Needle
    Stock Starter Jet
    Float Level At Spec


    I did put in larger main, idle and raised float level, and stuble got very bad.
    Lowered float level, stock jets & needle position, and it got worse again.

    It seems no matter which way I go, it is either to lean or to rich.
    BUT, ONLY at that very narrow RPM (Mid RPM) and very slight acceleration.

    Is it possible that the muffler being tampered with (Core Removed)is creating a tuned pipe effect at only that certain RPM and that is causing the problem?

    ...not so important details...
    Interesting part about that is, that the previous owner (Who removed the core) said that he never noticed such a significant problem/stumble. It was there but very mild. I would agree as I had the bike in for service before he sold it to the new owner and I didn't notice much of a stumble either.
    The new owner said he hasn't done anything to it and I would agree as he only had it a few days before bringing it back in to me for diag.

    Any ideas guys?

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