Engine Trouble comptession test

Discussion in '2-Stroke Engines' started by determined, Apr 29, 2014.

  1. determined

    determined Member

    I can see slight day light between the head and gasket so i tightened really hard and its still the same so i did a compression test at jogging speed and got just over 120 . Is this good compression ? Im new here
     

  2. butre

    butre Well-Known Member

    120 is good, I suspect the combustion chamber itself is sealed and you're seeing light some other way.
     
  3. determined

    determined Member

    Good to know now if i can just figure out why im not getting spark. I changed to a new spark plug and am going to purchase a dmm to test but im new to this so any advise is much appreciated. It was running fine then just suddenly died while riding but i think it could be because it got slightly rained on the day before. It wont even try to start anymore and i got less than fifty miles on it. I dissconected the kill switch that was hooked up to the blue wire
     
  4. butre

    butre Well-Known Member

    take off the magneto cover. if its full of water it might be shorting out. if you're lucky the magneto isn't fried.
     
  5. crassius

    crassius Well-Known Member

    daylight around the head gasket is good - it means your compression ring on the head is concentrating the torque of the head bolts onto a smaller area to get more pounds per square inch
     
  6. jaguar

    jaguar Well-Known Member

    unscrew the 4 screws holding the stator coil onto the engine and sandpaper the area that the screw heads will contact. That metal to metal connection is the ground path for the ignition system. Being loose or rusty makes resistance or an open circuit.
    If that don't work then disconnect the coil from the CDI and test for around 30 volts
    AC when pushing the bike to turn over the engine. best to first remove the spark plug
     
  7. determined

    determined Member

    Can you please tell me how to test while moving the bike? And do you mean to disconect the spark plug wire at the cdi ? How do i use the dmm.?
     
  8. jaguar

    jaguar Well-Known Member

    you can tape the voltmeter to your gas tank
    have the voltmeter connected to the wires coming out of the engine (that you disconnected from the CDI)
    push the bike (with spark plug out) and read the voltage
    no voltage or low voltage indicates a bad stator coil
    good voltage means the CDI is probably bad
     
  9. determined

    determined Member

    I appreciate the help can you tell me what probe wires go where on the dmm ports and on the wires? And also what setting ? I know thats a lot but im new to all this and id be very very greatful for your expertize.
     
  10. jaguar

    jaguar Well-Known Member

    read your instruction manual for the DMM
    set the voltage range higher than 30 volts
     
  11. determined

    determined Member

    Well i tested everything according to instruction and i got nothing on volts and nothing on ohms is it possible that my cdi and mag went bad? Just prior to it dying i was going kinda fast
     
  12. jaguar

    jaguar Well-Known Member

    if it shows infinite resistance or something in the mega-ohm range that means the coil has a burned wire and open circuit.
    its ****ed so throw it away. get another one.
     
  13. crassius

    crassius Well-Known Member

    on a 2K scale, you can also try between the blue wire and the tiny bit of copper wire that comes out of the top of the coil to a soldered connection on the steel armature - if you get a 320 - 360 reading there, you can often resolder that connection and be good again
     
  14. determined

    determined Member

    I appreciate the intent but thats a little unclear to me on what to exactly do with the mm.
     
  15. crassius

    crassius Well-Known Member

    you could check google for how to use a meter

    once you get it set for some range that is about 2K or close to that, you just put the leads where you want to check (mag can be off the bike & sitting in your lap or on the bike or anywhere)

    nothing else to do - just touch the leads and read the meter
     
    determined likes this.
  16. BigBlue

    BigBlue Active Member

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  17. determined

    determined Member

    Wow thats really good info!!! I appreciate all the ecpertize. Im considering a quality cdi replacement .is it really that much better? And dont i have to get a higher quality coil at the same time in ordrr to reap the benifits of a good cdi? Im not sure i can even find a performance coil
     
  18. jaguar

    jaguar Well-Known Member

    At JNMotors you have two options:
    1) buy the CDI alone and buy your own coil (the best being a Honda coil for their CR motocrossers)
    2) buy the combo of CDI and coil
     
    determined likes this.
  19. crassius

    crassius Well-Known Member

    for standard riding, a standard CDI & mag are just fine
     
    determined likes this.
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