its the plastic spur gear. my cen mt2 did the same thing after awhile the "ledge" on the inside of the spur gear wears down.after running for some time the pin that sticks up to grab the spur gear to shift into 2nd wears the ledge down and smooths it out so it wont shift no more! i would take the gear apart sometimes and use a hot flathead screwdriver to melt the flat edge back.:OMG: it works or the other option is to replace the 2nd gear.