if your lsn is to rich and the idle is set to high then when you make a run with it the engine will rev up for a bit depending on how bad it is and then as it sits there and loads up the idle will drop. What you will need to do to get the idle right is, with the truck on the starter box or on the pit wall get the truck up to temp of coarse, then rev the engine up high enough its cleared out but dont over rev it, if the idle goes high for a few seconds then drops then you need to lean out the lsn, keep doing this one hour at a time until you get a steady idle make sure you dont get it to lean, the idle will be a little high now so slow the idle down to where it should be and your set.