Friend ran his motor with no exhuast

Discussion in 'General Questions' started by Rockjaw, Oct 23, 2014.

  1. Rockjaw

    Rockjaw Member

    Hey all

    Ive got a friend that has an 80cc bike.(brand new motor that he had running for a week. .he couldnt get it to start. I checked everything..put new carb, and it was getting spark..sucker wouldnt start for nobody..then i asked him if he ran the motor without the exhuast pipe on...he said yes ..that he drove it down the street with no pipe..I said no wonder.

    what really happens to these motors when you run them with no backpressure? I am going to say the motor needs new rings now. That correct? it lost compression?

    Any advice helps. Think the piston and cylinder need to be changed as well? or just rings.

  2. darwin

    darwin Well-Known Member

    As long as there was a good oil mix in the gas I wouldn't think the rings would be shot. Are you sure there's no compression? When turned over by hand is there any resistance? Or pedal start it and see how it feels. I'd be looking at the reed valves if there's no compression.
  3. butre

    butre Well-Known Member

    nothing. back pressure doesn't matter. if you ever hear someone talking about engines needing back pressure you can automatically assume that they've learned everything they know about engines from that kid in shop class who put a 4 inch pipe on his honda and couldn't figure out that the reason his car runs like crap is because his exhaust velocity is gone.

    on a 4 stroke, running without the pipe will melt the valves. on a two stroke it won't melt much more than your ears. something else is wrong with it, like blown seals or a vacuum leak
  4. crassius

    crassius Well-Known Member

    possibly, he has sucked some cold air back into the cylinder causing a 'cold shock' at the hot piston - this can sometimes crack or warp a piston
  5. jaguar

    jaguar Well-Known Member

    you can crack a piston by letting it have temperature extremes.
    in this case the outside air was reaching the exhaust side of the piston.
    I would replace piston and rings
  6. darwin

    darwin Well-Known Member

    I was thinking about this thread. If there's no compression then there's no suction for the oil/gas to enter the chamber. That could fry the rings and more. Seems like a chicken or the egg issue. Seems highly unlikely the rings were fried due to no oil, unless some body ran a tank of pure gas through it.
  7. jaguar

    jaguar Well-Known Member

    even with bad rings there is enough suction to draw in enough oil/gas to lube the piston/rings although there is little stress on them in an engine without combustion.