I just replaced my holly 4160 with another 4160,600 cfm carb.I am trying to tune the carb,setting the idle adjust screws, How much vacuum should I expect, I am getting 18"at the front main vac port on the carb. I have a very rough idol and no power at all. Almost sounds like a miss. It has also backfired a couple times, all I did was swap carbs, could all this be due to a vac leak.

 

the camshaft will dictate how much vacuum you have- some really mild cams will pull 22" of vacuum, some race cams will be lucky to pull 5".

are all spark plug wires in good shape and hooked up to the proper places? it's really easy to mix up the #5 and #7 wires and will cause anything from a weird random misfire to flames popping out the carb..

sounds like you've got a fairly mild cam, so check to make sure all the vacuum ports are plugged- there is a port on the back side of some Holley carbs under the fuel bowl that is easily overlooked. make sure the mounting gasket isn't torn or that you didn't get a wire or something between the carb and intake when bolting the carb down- i've done that before and chased what turned out to be a vacuum leak for about an hour because of it. make sure the baseplate isn't warped and that all 4 mounting studs or bolts are equally tight.

baseline the carb- click off the fast idle and back the idle speed screw off until it's no longer touching, then screw it back in until about a turn past where it touches. turn the mixture screws all the way in until they gently bottom out, then back them out 1.5 turns. unhook the vacuum advance and plug the hose..hook up a timing light and start it- turn in the idle speed screw until you get a stable idle, and set your initial timing to 16-18 degrees. hook up the vacuum advance to a port with full manifold vacuum and back off the idle speed screw to lower the idle, then set the mixture screws for max vacuum..

 

That back vac port you mention is labeled full manifold vac. My old carb did not have that port. I put a tee in the 3/8" line going to the manifold and connected to that port as well, or should I have just put a cap on that carb port. I did nothing with the timeing during the swap, and I have checked all plug wire's just to make sure I didn't pull one accidently. Guess I will pull carb again and replace gasket just to make sure, and I will check the spacer.