2 smoke DS's on pavement......?

Discussion in '2 smokers' started by MItrnsplnt, Aug 27, 2012.

  1. peter650

    peter650 Been here awhile

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    Oct 18, 2009
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    492
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    Sydney
    So what your saying, is if your bike is geared for 140, & you are doing 120 your main jet is doing FA :ear
    #41
  2. peter650

    peter650 Been here awhile

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    Sydney
    I am just trying to understand this FR700. You got a KTM300EXC max power @8500revs geared for about 145klm = about 17 klm per 1000revs @ 120klm you are at 7000revs but you are not on your main jet. At what revs would you go, onto your main jet? I must of been jetting 2 strokes wrong for the last 30years. Anyway never to old to learn. Cheers Peter
    #42
  3. barryadam

    barryadam No Value Added

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    Jun 11, 2008
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    Southern California
    I get a lot of surge and other unpleasantness on the KX500. :D
    10 miles of pavement is enough for me.

    The KDX220R is more civilized on the road. But I wouldn't plan a long pavement ride with it.

    Barry
    #43
  4. FOXedupONE

    FOXedupONE Long timer

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    C'ville,VA.
    Did Shenandoah 500 dual sport ride no problem on my plated wr144. Top recorded speed on gps was 68mph. But it and I seemed to prefer 45-55 on the road sections.[​IMG]
    #44
  5. rxcrider

    rxcrider Long timer

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    Apr 19, 2007
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    N Ridgeville OH
    I had this concern when I picked up a 2005 300 EXC a few years ago. I only found a few people who had actually had a piston seize and they occurred when running at or near WOT and then closing the throttle. Here's why:

    The engine and exhaust are hot with little excess fuel and oil like you have at lower RPM. Close the throttle and the engine is still spinning along, but the fuel and air flow just got chopped. Now, when the piston comes down, there isn't a pressurized charge in the crank case to get pushed up the intake ports. This lack of incoming pressure allows the pressure pulse from the expansion chamber to push hot, dry exhaust gas farther back than normal. (go read here http://www.702sportbikes.com/showthread.php?14147-Two-Stroke-Jetting-The-Wet-Oil-Line-Method or google wet line two stroke jetting) Normally, the pulse from the expansion chamber pushes most of the fresh, wet charge back into the cylinder leaving just enough to keep the piston, rings and power valve cool and lubed. When the pressure pulse pushes the fresh charge too far back, hot, dry exhaust gas comes in contact with the pistong and rings at the exhaust port, heating them up and burning off teh oil film. This is when the engine will seize.

    The solution I heard that made the most sense was to slow down with WOT engine braking. If you push in the kill putton and twist the throttle to full open, you are pushing a fresh, wet charge through the engine, without burning it. After a couple seconds, you will have filled the expansion chamber with a wet fuel / air mix as well. The next step needs to be done in the correct order to keep from blowing the expansion chamber off the exhaust flange. Doing it backwards won't always lead to a big bang, but it can. Close the throttle, then release the kill switch and the engine will return to life. Then resume riding normally. After being at WOT with the ignition off (where you will observe more engine braking than you are used to from your two stroke) the pressure pulse in the expansion chamber can push the fresh charge as far back as it wants because gas in the expansion chamber is also cool and wet.

    The issue is that a perfectly jetted two stroke at WOT won't seize, but when you close the throttle abruptly without reducing the engine speed as quickly, the jetting is no longer perfect because the pressure in the exhaust is much greater than the pressure coming from the crank case.

    This can happen as easily in a long sand wash as it can on a road section.

    I first learned about this subject when a friend began dual sporting a CR250R. We made him get an FMF Q because the shorty silencer someone had put on it was way too loud. What we didn't realize was that by going to a more restrictive silencer, the pressure pulse would become stronger and he would need richer jetting to accomodate it. Having only dealt with four strokes at that point, I figured a more restrictive exhaust would just make it run a bit richer and the worst that would happen is a fouled plug. I was dead wrong and he seized the piston when he closed the throttle to slow down for a turn on a road section.

    EDIT: and I'm now dualsporting a 1999 Husky WR250
    #45
  6. sam2

    sam2 Adventurer

    Joined:
    Jun 1, 2012
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    31
    I have a street legal 1982 husqvarna 430wr which I rode on the streets for quite a while without problems until I seized it about a month ago doing 100 mph, now i have to rebuild it (getting my piston in this week). There are no trails near my house, I always rode it about 30 miles out to the pines before I hit the first trail. I usually took it easy on the road, but I got stupid and decided to see how fast I could go down a back highway. Moral of the story, take it easy and you will be fine on the road. resist the urge to see what your offroad bike can do on flat pavement.
    #46