First check your limit and steering switches make sure both drive motors are turning when going straight next l would check your 1a contractor check to see if at full speed it is pulling in if not check your coil and contact bars and tips than l would plug a hand set in and check all tune up numbers if the 1A still will not come in l would suspect a bad control card

Are you saying you can't travel at all or you can't increase the speed limiter?
Using an SCR controller you can't really do much with the speed limiter unfortunately.
If it isn't letting you travel then somewhere in the travel circuit there is an issue. For an RC3000 the order is Brake switches, battery retainer switches (if installed), forward reverse switches, scr panel. If one of the switches is open, then no voltage gets passed to the next.
Also the pots wont do anything unless either the forward or reverse switch is closed.