jawa engine carb tuning

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

  1. samuelis

    samuelis New Member

    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
    I am using shittiest API TA mineral 2 stroke oil mixed 1/25 as it is still in break in. So i dont know.. are those spark plugs oil and carbon fouled or is it runnin rich ?

    thank you very much and sorry for my bad english
    Last edited: Aug 29, 2016

  2. jaguar

    jaguar Well-Known Member

    the black color is from the crappy oil.
    the carb is too lean.
    set the jetting in this order
    idle (idle jet or air screw)
    full speed (main jet)
    accelerating (needle position)
    accelerating from idle (slide cutaway)
  3. samuelis

    samuelis New Member

    Thank you for response!
    Idle is perfect so I am not going to mess with that.
    What size main jet would you recommend to use ? I have kit of 0.6 to 0.95. I am asking because changing main jet on this special creation is a pain as i have to remove whole engine then cilinder and only then i can unscrew carburetor.

    How could it possibly ran worse on 0.8 that I tried before ? Also I tried 0.6 today and I didnt really spot any difference. I mean when I remove spark plug everything is wet except electrode in the middle, are you sure that it is lean ?

    I can make a video showing how it runs if it would be helpful
  4. jaguar

    jaguar Well-Known Member

    check that the check valve for the float is working right.
    (fuel to carb, float bowl off, manually move the float up and down to see that the fuel flows and stops)

    make damn sure there are no air leaks (carb connection, crank seals, head/cylinder junction)
  5. samuelis

    samuelis New Member

    float mechanism works fine flows and stops ok.. also tried what happens when I cut fuel off so level of fuel gets lower - no noticeable difference in running, just stops when gas runs out.
    I am not aware of any leaks - I change paper seals every time I disassemle it (like 5 times now)
    Tomorrow a will buy proper oil and mix it 1/50 .. maybe it will allow to see sparkplug collor.
  6. jaguar

    jaguar Well-Known Member

    mix depends on type oil
    regular mineral oil 20:1
    semi-synthetic 25:1
    full synthetic 30:1