engine ideling way to fast

[wibilly]

my truck wont idel down both primarys and secondarys are closing all the way idle screw is backed all the way out to where it does not touch the throtel at all. it seem to run smother with idel mixture screws backed out excessivly this also gives me the highest vacume reading when tuned to run the best it idels at about 1500 rpms to get it to idel down i have to make it run ruff by retarding the timing and leaning the idel mixture both causing a drop in manifold vacume the carb worked perfect on the 350 i removed it from it is on a 305 now with a newer styel intake with the newer base gasket does this sound like a base gasket problem to you guys, it dosen't seem to gain any r's when i spray around it with carb spray