I have a losi xxxnt that has fast top speed and acceleration after a few feet, but is kind of slow off the start. I was told to tighten the slipper clutch a turn or two. Is this likely to speed it up off the line, and also, are there other things I can try. I can't believe given the top speed that this couldn't pull the front wheels off the ground. Any advice would be appreciated.

Tightening the slipper (assuming the diff is already properly tightened) WILL help with "take off" speed...

Keep in mind that Nitro vehicles are almost the total opposite of electrics... An electric truck makes it's maximum torque right off the bottom (easy wheelies)... A nitro truck has to get up to some optimum RPM before it develops good power (tough to wheelie)... In order to get wheelies, you need a trick clutch and some trick motor tuning... Also keep in mind that if the engine idles for more than a few seconds, it will "load up" with fuel, and will take a bit to "clean out" and make good power...

Lack of low end acceleration could also be attributed to an improper low end needle setting but like Tom says, after idling for a short time a nitro motor "tends" to slightly load up and will need to clear out before running off the bottom at an optimum level. Try the slipper clutch adjustment first (making sure that your diff is properly tightened) and if that doesn't work out for you it might need some low end adjustment.