There is a friction control wheel that juts out at the "back" of the throttle base unit, back being the side that is facing the user. If you open the throttle assembly, that wheel is connected to an angled piece of metal that sits on top of the throttle (top being what the top would be when the throttle is upright --- you have to turn the unit upside down to open it). This metal bit would have a black curved plastic piece on it and the pressure applied by the metal lever on that plastic would control the friction... with me so far?
A quick mod would be to wrap that plastic bit with a bit of tape to "fatten" it up a bit and thus increase the friction/pressure exerted on the throttle.
Hope that helps!