trouble tuning carb - homebrew jawa/simson hybrid

Discussion in '2-Stroke Engines' started by samuelis, Aug 29, 2016.

  1. samuelis

    samuelis Guest

    While ago i found a 70cc cilinder and head kit for simson for cheap. And as I had old Jawa stadion motor wihout cylinder that has same crankshaft stroke (44mm) i decided to make something out of it. I have pretty good mechanical skills so I machined the blocks to fit this cylinder and put it toghether that was the ez part.
    but there my skills end.. and now I am suffering with engine not running right.

    The problem is that the engine runs pretty good in low rpm - 1/3 - 1/2 throttle, but when I give it full throttle in low rpm engine bogs and loses power and has deep "dull" sound. It generally runs better at about 1/2 throttle.
    As it is in high rpm I can give it wot and it will pull just fine. Also when revs perfectly fine when on neutral.
    Starts on first kick every time..even without choke. Idles perfect..

    Everything seems that it is running rich.. black sparkplugs and it smokes quite a bit but when try to cover 1/2 of carburetor intake with tape it seems to run better.. I dont understand. What should I do next ?
    I have AMAL 19mm carburetor with 0.7mm main jet and medium needle..
    When i have tried 0.8mm jet it was worse, when tried long needle (supposed to make it leaner) also ran worse.
    those symptoms are so contradictory that I dont even know...

    Images in attachment: 1. sparkplug after cca 15km trip
    2. sparkplug after startup ad about 15 seconds of maximum rpm uphill run then shut down
    3. for general ilustration