if it's a 2 speed there is the clutch set up in the center diff, it has a little arm that swings out and catches a notch on the 2nd gear. Either the little piece in the clutch isn't swining out or the notch on the second gear is stripped off. I would take it apart and see what is going on inside. It's also possible you may have your one way bearing in backwards. If you really want to make it a single speed transmission you will need the following parts:

Part # 08023 Moderate Gear Set (Center Transmision single speed)

HPI 14, 15, 16, 17 or 18 Tooth Savage Clutch Bell and Bearings single speed
That should be all you need to convert it to single speed.

You can get that stuff at anyone of the following places:

[link=http://www.allaboutfunhobbies.com/redcat_himotoracing.htm]http://www.allaboutfunhobbies.com/redcat_himotoracing.htm[/link]


[link=http://www.clermontmotorsports.com/servlet/StoreFront]http://www.clermontmotorsports.com/servlet/StoreFront[/link]

[link=http://www.exceedrc.com/]http://www.exceedrc.com/[/link]