Home Forums Tech Talk CR125 strange engine behavior

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    • #35488
      Brad Richtsmeier

      I have a stock moto CR 125 shifter kart that I play with on track days.  The last couple times I’ve been to the track, it will run run normal at the beginning of the session.  Then,  in slow speed corners when I’m backing off throttle or feathering it at low RPM’s, the rev’s start to pick up on their own (like someone is tugging on the throttle cable).  Sometimes the symptom will  break on its own and I can drive through it, or sometimes its so drastic that I spin and stall.  It has acted this way in hot humid weather and yesterday in cool dry weather.  The entire kart is relatively new with very low hours.  I’ve checked over all systems with my father, who is a professional mechanic with significant experience in two stroke engines, (though no kart engine application experience) and we can not figure out a solution.  We have taken the carburetor apart and it looks brand new inside.  The throttle cable, springs, and slide all operate perfectly and appear to be free from defect.  We put a new spark plug in it.  I run Sunoco 110 fuel.  We have examined the fuel system and it appears to be functioning properly.  There is a little fuel in the loop of the overflow line on the carburetor after each session.

      Our initial theory was that pressure might be building in the fuel system over the course of the stint, and then it causes the motor to get too much fuel and it essentially “pushes” the throttle independent of my input.  That being said, there is no evidence of  a venting problem.

      Has anyone experienced anything similar or have any advice?





    • #35503


      This is gonna sound silly, but I must ask

      Do you wear a racing suit, or regular jeans?

      I you do wear jeans, or your suit is too loose, or too wide at the bottom of your legs. It will push the throttle cable where exposed by the pedal and bracket area.


      Tuck your jeans under your socks, so that there’s no left over hanging out jeans, or if the suit is to wide, put some zip ties around your leg sleve.


      Try that and let us know what happen.

    • #35504
      Brad Richtsmeier

      I do wear a tight fitting race suit.  The first couple times I experienced it, I assumed that I was bumping the throttle cable.  I did take care to analyze it, and I’m definitely not.

      I should also mention, when the engine does this, the kart in general starts running differently.  The engine will bog down in places that it wasn’t a lap ago, and be kind of erratic with the power delivery.

      To me, it seams like the kart is running too rich, so maybe I should change the jetting to lean it out.  However, why does it run amazing during the opening laps of a stint most days?

    • #35519

      You may have an intake leak, could be carb intake booty, crankcase seal, etc.

      Check your boot first, check your pick up fuel line inside the tank, sometimes they could be partially cracked.

      Sounds like you may have a vacuum leak, and when it leans out engine revs.

    • #35609
      Noah Stark

      Fuel tank vent…. sounds like you most likely have one suck line, and one return line with no vent or too small of a vent.  That would cause this exact problem after a few laps.  The less fuel in the tank, the more laps before it happens.

    • #35614
      Brad Richtsmeier

      Thanks for the advice so far, guys.

      For my last session, I did disconnect the vent tube attached to the gas tank to see if it was restrictive.  It did not seam to make a difference.  This tube was attached to small metal vent tube exiting the top of the tank.  The kart is a 2010 CRG Road Rebel if that matters.  Is it possible to get a larger size vent exiting the tank, or is that metal piece CRG standard  one size with the amount of tank venting being controlled by changing the plastic tube that attaches (the one I removed with no results)

    • #35618

      Did you check your fuel pick up, inside of the tank for cracks?
      Did you check for vacuum leaks at the intake rubber booty?
      How is the tension on your throttle cable? Too tight? Give it some slack.

    • #35679
      Sanford Mitz

      As long as you are checking things, pull the throttle cable all the way out of the sheath and make sure it is not unraveling somewhere. Very lightly coat it with anti-seize compound and put it back in. If you have a right angle in to the carb be sure and check that area.

    • #35696
      Dale Daugherty

      I’m sure you would have probably noticed if the engine was sliding back, but is the engine sliding back? I forgot to set my engine stop once and after a few laps it began to pull itself back under acceleration, which of course pulled the throttle open. Very unnerving to have your kart develop a mind of its own and go full throttle when you aren’t demanding it. Double check and make sure your mount is tight and the stop bolt is set.

      Probably not your issue, since it does not really fit the symptoms… but does not hurt to check :)

      2010 Spirit [Birel] SP32 // SRS CR125 - Stock Honda [#08]
      Hill Country Kart Club - New Braunfels, Texas - http://daledaugherty.wordpress.com/

    • #35704
      Evan Fraser

      I just thought of this last night.. do CR125’s have reed admission?  I had a similar issue with my Rok GP where when it was cold it was fine but it would pick up randomly, and it would idle really high at the end of the race.  Turns out I had a dime-sized hole in one of my reeds.  Replaced the reed and it was fine.  If it has them, have you checked them?

      Evan Fraser

      #43 - Formula K FK01 / Briggs LO206

      West Coast Kart Club

    • #35800
      Brian Degulis

      Boging and high RPM at idle sounds like your losing fuel and running lean. Check your fuel system from tank to carb.



    • #37399
      Michael LaGrone

      Yes I agree with Brian,

      It is NOT a rich condition it is or could be a LEAN contition. Try going up two sizes on your main jet. or adjusting your needle. The main is for WOT and needle is just off idle, Pilot is bottom.

      Try first the needle clip position, the main jet.

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