Have an 85 chevy 4x4. I just had the 305 engine rebuilt with a new intake and carb. It was working fine for a couple days and one morning I went to start er up and the engine just cranked and cranked, wouldnt turn over. So I left it there for a day, went back the next day and tried it,and it started right up. Worked good for a a day, then the next day I went to start it up and same thing, it just cranked but wouldnt fire up.....so I left it there for the majority of the day, went back, and tried to start it and it started right up......can you think of why it's doing this? especially sense i had the engine rebuilt.

Your carb is needing to be rebuilt the seals and float and valve seat may be damage. The float opens when the fuel level in the carb is low. If the valve seat is damaged or the float is not working. Gas can keep pouring into the carb resulting in extra fuel. Till the engine stops and fuel pump pressure stops pushing fuel it will keep pouring fuel and will not stop till rebuilt correcting the problems. Buy rebuild kit and rebuild it or try junkyard and replace it.

In 85 Ford used a carb spacer that also had the EGR attached.

It could be that the gasket has broken and it has a large vacuum leak. Also the EGR could be stuck open. Look for other vacuum leaks like the PCV and brake booster vacuum systems. A 460 engine will run with huge vacuum leaks especially when the choke is on but the extra air will lean out the mixture and could cause damage.
Enclosed is a video about setting idle on a Holley carb which you may not need but others looking at this post may like to see.

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You might try to richen that carb intake a bit. You might want to spray some intake cleaner in the intake as well and make sure to get some of the gunk out of there. I would add a fuel system cleaner in the tank as well but a rebuild will net the best results.

Fuel pumps are real common to go out on those vehicles

Check all vacuum hoses for cracks and disconnections. Also check pcv and egr

take too shop, that rebuilt the tranny. see if they cant do a better job. they may tell you the problem. lol.

The vacuum comes from the intake manifold and ported vacuum comes from the carb above the throttle plate. You might have a leak at the carb gasket or intake manifold gasket. Spray some car cleaner or WD40 around the gaskets and vacuum lines while the engine is running and when you hear the idle speed up, you've found the leak. You will need to install the rebuilt carb first, and you should tighten any loose intake manifold bolts. Don't overtighten them. Also check the EGR valve because it could cause the engine to stall at idle. A good tune up might be needed too. I've seen some of those engines run real rough when the ignition timing was off.

If it has a vortech V8 It has to have the right fuel PSI or it won't start. Also pick up in distributor could be causeing problem. Cross over style distributor cap also can cause problem. Out of oil? Fuel pressure regulator where found inside the intake manifold with injector cluster, if fuel psi regulator goes bad could be leaking fuel in intake causing oil to get deluted and burn.

well it could be that you need new spark plugs definatly new piston rings hmmmm was it hard to start before you put the new carb on?