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Highway cruise behaviour of 2 strokes

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  • Highway cruise behaviour of 2 strokes

    My SDR (200cc thumper, based on TZR125, and what the DT200R engine is) has a flat slide Mikuni carb but gives the feeling of air pressure variation light surge from a CV carb behaving badly. No noticeable change in speed, digital tach is too slow in update to show twitchy rpms so can't say.

    I'm wondering what similar bikes might be out here and how they cruise. This is my first 2 stroke so assume everything is OK and I'm just assimilating.

    The light surging while cruising 110kph at ~6200rpm (power upto ~9000rpm) has a variation in power pulse feelings.

    The bike makes its power well enough, benchmarked to an RS125 everything in order.

    I haven't done much more than 5min cruising on the highway at a stretch, rest has been through the revs between suburbs where I don't really cruise or am bored enough to notice.

    I'm looking for experience whether this is "impending boom" (18kkm) and I should refrain from possibly Sunday country ride?
    Or perhaps it is normal behaviour for insolent oversize pocket bikes holding a fixed speed when they just want to rev through 6 gears and hold top speed?

    I've googled and found these typical suggestions:
    - racier tuned 2T dirt bike owners have light flywheels with this cruise surging feeling, some people add aftermarket flywheel weights to smooth out the power.
    - fuel/air mixture from air filter, fuel filter, carburettor internals

    Service/running wise I have:
    - a new fuel filter on inlet line, one on there before:
    - drained and cleaned carb with clean-r-carb by filling bowl by inlet line sitting overnight, running through on startup (started first kick to my surprise)
    - no problem with old plug, but have an iridium plug
    - tried a tank of 95RON after reading here a DT200R ran better, but I'm back to unleaded as I can't tell any difference, if anything I was more sensitive to change and noticed the cruise surging more than ever
    - drive chain on loose side, certainly not tight

  • #2
    My RGV really doesn't like to hold constant throttle at certain rpm ranges... from memory (been ages since i've ridden the thing now) around where it starts coming on the power hard, it wants to go go go (8k, 9k ish?).

    Made no real difference after rebuilding it.

    Soemone else can probably elaborate (rgvlee, jamathi) but its probably either something you can tune out, or "just don't ride like that"
    “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

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    • #3
      My RGV also does this a bit - around 8000rpm, I think. It might be partly because the powervalves are stuck down though, which would affect the airflow at 8000rpm upwards, when they should be opening.

      My KDX doesn't have this problem lower in powerband, but higher in powerband the throttle gets pretty twitchy, and at 100km/h it is high up in powerband in top gear.

      I have no idea if you can tune this out though.

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      • #4
        Thanks, if it is a characteristic I'll assimilate to 2-stroke ways and it won't bother me, I'm fine with imperfections. If I heard someone was cruising on the highway and didn't have this feeling, then it came on, then their engine seized... I'd be a bit more nervous going out to the back of nowhere.

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        • #5
          nevermind re-read your post.
          Last edited by xplosiv; 28-08-2009, 04:43 PM. Reason: re-read post

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          • #6
            The trouble with extremely tunes "race" bikes is the fact that they are a Pig to drive on the street in a legal fassion.
            same with a drag/race prepped car/bike of the fourstroke variety.
            RGV Apprilla 250cc are race bikes.
            so watch out what you want to do with them cause cruizin ain't happening.

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            • #7
              No problems cruising on my rgv; same with the rs250. modded rgv actually easier to ride/cruise on.

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              • #8
                ^^ hrmm mine probably needs a tune then.

                at about 1/4 - 1/3 throttle around say 9000 or so (i.e., gradual acceleration) it likes to surge...

                WOT is fine... below that rpm is fine...
                “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

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                • #9
                  yeah you can tune it out

                  mine gets antsy at those sort of rpms at low throttle posn, but you can hold it there and it will not surge. Crack the throttle and all hell breaks loose.

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                  • #10
                    two strokes are best for 2 things, speeding up and slowing down. not really made for anything else.
                    I'm Living the dream. Really I am.

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                    • #11
                      ^^ yeah what he said. well, 'cept with no engine braking, scrap the slowing down bit...

                      if you want highway cruise, you've got the wrong bike


                      pick the twisty way home and enjoy opening it up
                      “Crashing is shit for you, shit for the bike, shit for the mechanics and shit for the set-up,” Checa told me a while back. “It’s a signal that you are heading in the wrong direction. You want to win but crashing is the opposite. It’s like being in France when you want to go to England and when you crash you go to Spain. That way you’ll never get to England!” -- Carlos Checa

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                      • #12
                        Yep my RG500 did that

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