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  • #1

    two stroke ?'s

    My canam dirt bike says to check the timing at 5000 rpms. Two questions:
    1. Why would it change? Which it does at different rpms. It is the simple magneto style.
    2. What is a good source for an rpm guage that would work for this single cylinder?

    Oh and a third question while I'm here. What is a ball park compression reading that I should expect on this two-stroke motor. It just doesn't run the same ever since I had the cylinder resized to my new piston.

    Thanks all!
    "If it weren't for a budget I'd be bored."

    Thom
    79 xs11sf - Rented Mule
    80 xs11sg
    81 sr250t- sold to Pain
    Tags: None
  • #2
    2 stroke - half the power twice as often...

    Must have a centrifugal advance mechanism. Like the XS, which in addition to the vacuum "advance" has a mechanical centrifugal advance which is at full advance at 5,500 RPM.

    Most flukes or hand-held electrical testers have RPM setting. Not the first level $9 ones, the $19 dollar one I had had a tach.

    If I recall, 6.5 to 1 up to 8.0 to 1 is fairly typical 2-stroke compression ratio. There is some weird method of calculating bottom dead center to top dead center volumes that makes the ratio calculation a little different from 4-stroke.

    Not sure about the latest technology.
    Marty in NW PA
    Gone - 1978E - one of the first XS11 made
    Gone - 2007A FJR - the only year of Dark Red Metallic
    This IS my happy face.

    Comment

    • #3
      Nope no vacuum or centrifugal advance. Just straight magnet and coils. This motor is even simpler than a lawn mower engine. I guess I didn't get far enough in electronic school to explain to me why the timing would change at a higher rpm. Unless there is a lag due to the magnetism.
      I'll have to check the magazines for an rpm guage.


      Just when you get a grip on one thing the other is needed.
      "If it weren't for a budget I'd be bored."

      Thom
      79 xs11sf - Rented Mule
      80 xs11sg
      81 sr250t- sold to Pain

      Comment

      • #4
        Well the timing does have to change or you wouldn't get too far too fast.

        Might be in the electronics?

        30 years ago I knew a lot about 2 strokes, adjusting the 5 exhaust sections for optimizing power in a certain band or RPM, timing, port timing, 'shaving' piston heads to increase volume and change the timing, etc. Frankly I don't remember most of it anymore. I do remember that the timing is not as adjustable as a typical 4 stroke, and there is usually a very narrow band of power and that band is adjustable up and down the RPM but mostly you cannot widen it much.

        Some quick google search stuff:

        http://www.bridgestonemotorcycle.com...mpression6.pdf

        http://www.eric-gorr.com/techarticle...wo_stroke.html

        http://www.users.bigpond.com/jack_st...strokes_1.html

        Have fun
        Marty in NW PA
        Gone - 1978E - one of the first XS11 made
        Gone - 2007A FJR - the only year of Dark Red Metallic
        This IS my happy face.

        Comment

        • #5
          Excellent articles! It those kind of writings I spend way too much time searching. See...it does pay to be lazy sometimes. Thanks for including them.

          Well. I guess I have some more reading to do.

          Thanks Marty!

          Sorry... but I have to say that 30 years ago I was just figureing out how to use the toilet.
          "If it weren't for a budget I'd be bored."

          Thom
          79 xs11sf - Rented Mule
          80 xs11sg
          81 sr250t- sold to Pain

          Comment

          • #6
            Well, me too. Sally says I haven't learned yet...

            Actually it was more like 35 or 36 or 37 years ago. Yeah, I'm still learning math also.

            Hope the articles help. I just glanced them quickly to see if they might help. There are probably more and better ones.

            The one thing we did to jack up the power was redesign the exhaust. As mentioned there are 5 (I remember 5, anyway) sections, their sizes can be calculated to match where in the RPM band you want the most power. It always meant making the overall lenth longer, but the expansion chamber dimensions and all the other sections were fabricated based on either ratios or calculations. If you ever see land speed record attempts with 2 stroke engines you will see REALLY LONG - 10'-12' and longer - exhaust pipes. I think that was to get the power up in the very high RPM range.

            We messed with pistons also, and reed valve timing.
            Marty in NW PA
            Gone - 1978E - one of the first XS11 made
            Gone - 2007A FJR - the only year of Dark Red Metallic
            This IS my happy face.

            Comment

            • #7
              If the magneto says anything about CDI, it probably has an advance built into the pickup module. Most weedwackers and such that have a flywheel magneto nowadays have a built in advance.

              Steve
              80 XS1100G Standard - YammerHammer
              73 Yamaha DT3 - DirtyHairy
              62 Norton Atlas - AgileFragile (Dunstalled) waiting reassembly
              Norton Electra - future restore
              CZ 400 MX'er
              68 Ducati Scrambler
              RC Planes and Helis

              Comment

              • #8
                Stihl used to offer a handheld tach for dialing in there two stroke chainsaws. Bout the size of a calculator, you held it next to the spark plug. Should be able to still find it from a Stihl dealer or check with the larger tool distributors. Seemed to work fine up to and past 8 grand, bout as fast a stihl will go!!
                When a 10 isn't enough, get a 11. 80g Hardbagger

                Comment

                • #9
                  .. hey Thomas, this will give you more power
                  the price seems a little high but these parts are not getting any easer to find and i remember parts for my can am being real expensive in 1988, $500 ! for a piston kit for an 84 500 MX

                  http://cgi.ebay.com/ebaymotors/ws/eB...RK%3AMEWA%3AIT

                  Comment

                  • #10
                    Funny how I just left Ft. Myers yesterday too. Could have saved shipping. I notice the compression too. I am not getting that and the tht is the enduro model. I still need to double check my squish area first, but I think my machinist screwed up. Thanks for the info guys. I know I can get a .030 and a .060 over for pretty cheap. Just need to find out whether I want to go to .060 or not. The mx cylindes are pretty thick,but,,,
                    "If it weren't for a budget I'd be bored."

                    Thom
                    79 xs11sf - Rented Mule
                    80 xs11sg
                    81 sr250t- sold to Pain

                    Comment

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