what causes this??

[hilljob26]

well,my dad put a new holley on his truck,he messed with it alittle bit,seems to run great,but now when he shuts it off,it deisels a little,how can he fix that,any ideas,its a new holley 600 that does not have adjustable floats,its called a next generation needle/seat,just doesn't look right without that nut and screw on the top,but seems to work okay,just this little problem.....mike