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ZXR250 stuttering

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Wade Wilson, Jun 22, 2014.

  1. Hi.

    I don't know where to begin, So I guess I'll start from the beginning.

    I own a 1994 ZXR250 that I put together from an $800 ZXR250 I found in my neighbors back yard.

    Ever since I got it, it's been having one major problem. It hesitates and stutters between 4k-8k RPM, but only when warm.

    When it's cold, it performs beautifully, not a single trouble in sight, when she gets warm, everything turns to shit.

    When it stutters, there is little to no acceleration, I have to clutch in, hold the throttle wide open and eventually it breaks through and returns to normal, but after about 5 minutes of riding it starts to stutter again.

    She used to be running rich, but I turned the pilot screws down to the stock 2.5 turns, I've readjusted to floats to 13mm stock settings. Replaced the spark plugs twice. Cleaned the carbs several times.

    The previous owner rejetted the bike and replaced the stock jets (122, 125, 125, 122) with bigger jets (125, 128, 128, 125), and strapped on an after market exhaust.

    I have no idea what could be causing it.
  2. Sounds lean, perhaps lift your needles a notch.
  3. I'd had the pilot screw from 1.5 turns to 3.5 turns.

    Still stutters.
  4. If it were lean, it would be worse when cold.

    If it has removable seats in the carburettor, then perhaps the o'rings which seal them have perished, allowing fuel to bypass the float needles, causing flooding. However this would normally cause it to run poorly below 4Krpm as well.

    When you changed the plugs, what did they look like, & were they all alike?

    Perhaps weak spark on one cylinder caused by a dodgy plug cap. Make sure they all have similar resistances.

    Low compression on a cylinder causing that plug to foul up.

    Perhaps the larger main jets are good for above 8Krpm, but are causing an overly rich mixture in the mid range. May need to drop the slide needles by moving the clip up one or two positions.
  5. The plugs were pretty dark and coated in carbon, I cleaned the plugs that are in the bike now with sandpaper.

    I haven't done a compression test yet.

    I just finished checking all of the resistances.

    Coils 1&4
    Primary windings: 4.1-3.9 ohms (the manual says 2.3-3.5 ohms)
    secondary windings are at 15.07 kohms (manual : 12-18 kohms)

    Coil 2&3
    Primary Windings: 3.9-3.7 ohms
    Secondary Windings: Failed to give a reading, the plug end was a bit worn so i cut it off and got some fresh wire exposed. 14.42 kohms.

    The needles aren't adjustable.
  6. The pilot screws only make any significant difference around idle. At those revs you're looking at main jets and partially blocked emulsion tubes. It only takes a few specks to block the tiny holes in the emulsion tubes and you'll get all sorts of weird behaviour that seems to vanish when there's any enrichment at all.
    Those bigger main jets might not suit the needle profiles, as already mentioned.
  7. Dark plugs indicate a rich mixture, which I assume is on all four. Carbon could suggest oil burning as well.

    As slide needles are not adjustable, then perhaps need to buy a jetting kit which comes with a few different sets of needles which are adjustable, so it can be tuned to suit pipe / air filter combo.

    Choke sticking on. Blocked air filter / air box. Main jets too big. Worn slide needles & nozzles. Slides not lifting properly - holes in diaphragms etc.
  8. The previous owner who rebuilt the carbs noted that the carbs are from a ZXR250A model and my bike is a ZXR250C model.

    From what i've read the change in carbs is fine, it's a bolt on mod. The needles are only 3 months old. the diaphragms are perfect. The carb has been torn down and rebuilt about 4 times now, the emulsion tubes are spotless, and the slides are working normally.

    After messing with the coils, I've noticed that the bike is not producing a strong spark. I might have to replace them before I continue.
  9. Before replacing the coils, do a spark gap test to confirm the spark is weak. The spark should be able to jump a 6mm gap. If not then check the voltage going into the coils, that there is little difference between battery voltage & voltage at the coils. Also that the battery voltage is good - 12.6v + with ignition off, and 13-15v when running. Also sometimes a faulty CDI is the cause of weak spark & not the coils.
  10. So today I went out and I bought 8x CR9E's and 8 CR8E's (they're supposed to run hotter or something) and 2 meters of spark lead.

    I red did all of my ignition electronics (coils, leads, plugs etc) and the bike started first time. I used a can of aero start around the engine to find any vacuum leaks.

    I patched up the leak that I found and took the bike for a ride. Everything seems to be running perfectly.

    So I rode her for about a half hour and the bike started coughing and stuttering between 6-8000 rpm.

    After limping her home I removed the spark plugs to reveal this...


    I think i can safely assume that it is no longer an ignition problem. That carbs seem to be running super rich, but I've already adjusted the floats to 13mm (stock) and the pilot screw is turned out by 2 turns (stock is 2.5), and I've bench balanced them and they're still running rich enough to destroy spark plugs.

    What do I do?
  11. Setting the float height only works if the needles and seats aren't worn and the floats aren't damaged or binding. To truly check the fuel height in the float bowls you need a piece of clear plastic tube to attach to the drain nipple on the bottom of the float bowl and bend up the side of the carby. Slowly open the drain screw and the fuel that flows into the tube will show the actual fuel level in the float bowl. Naturally the bike needs to be upright to do this. Your other option is that the enrichment system isn't closing (or in the case of choke butterflies opening) properly.
  12. Is it the same on all cylinders?

    a bad float needle/seat will generally show up on one cylinder, or two, not all 4.

    Also, it will run rough on idle, all the way to the clearing point. So I'm guessing not that.

    How does the emulsion tube look? if these are oval, then you will never get it to run right.

    Has it still got the original idle jet size?

    Oh, and it still could be ignition. poor spark can look like over rich, on a spark plug.

    Have you got the right temperature plugs in there?
  13. Well, I finally fixed the buy...

    By buying a new carb.

    I have no idea what was wrong with the old carb, but swapping out the carbs seem to have fixed it.
  14. I finally took apart the old carbs and discovered the root of all the problems. The stutter, the failing to start when the bike was warm, the running rich, the difficulty starting in general.

    It was these IUVm4tp.

    The diaphragm on the left is missing the small brass spacer, the one on the right has the spacer. From what I can figure, vacuum pressure forces the slide open, pulls the needle out of the jet and allows more fuel to pass through.

    Without the small spacer, vacuum pressure would build up over the time of a short ride and it would just hold that pressure, keeping the slide pulled out.

    Just something I have to keep in mind next time I check the carbs.

    I hope this helps someone out there who's having the same issues I'm having.
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