340 idles at 1500 won't stay running in gear

"Right now it will start up and idle but i have to feather it up to about 1500 and it will stay at that with my foot off pedal running. If i put it in gear it dies almost immediately.
I was able to get it to idle then in gear and drive a little down my street but then i could feel it start dying. This was a couple days after it initially went down and it has progressively gotten worse. I've also noticed that i have like a black soot built up on the outside of the manifold on the drivers side down by the valve cover.
I think it's a bad intake manifold gasket, tried the spray test but didn't notice any thing. I've also checked the vac line to the pcv and at one point blocked off all of the vac ports on the carb ."

Are you running a vacuum advance?
In order for the high idle to occur with a vacuum leak, the carb will need to pull fuel to go with that air, else it would stall. So either the throttle blades are far enough open to either activate the Vcan, or the transfers. Or the PV is kicking in. Or the centrifugal advance in the dizzy is off the stops, kicking in advance.
So first of all, a vacuum leak that is so small as to be hard to find, just becomes an idle-air bypass. To be sure it needs to be fixed, but it can be tuned around.
Secondly, if the Float valves were leaking,at idle it would likely flood the engine to a stall.
So the first thing to do is isolate the problem to see whether it's a fuel issue or a timing issue or a bit of both.
For starters I believe your primary throttle blades are too far open, and I think your PV is acting up, and possibly the idle timing is late.
The easiest thing is to defeat the vacuum advance system,and check the idle timing, and crank some in, say about 20* for now.
Then close the primary throttle blades,some. If you need to keep the idle speed up, use the secondary cracking screw.
As for me, I would take the carb off, flip it upside down and set the transfer port sync.This means the Tports should appear square to slightly rectangular under the throttle blades with the idle speed-screw touching the curb-idle stop. I would also pull the PV out and temporarily install a PV plug.Finally I would reset the mixture screws to 1.5 turns out from lightly seated.And then,reinstall the carb.That's what I would do.
Using this method the secondary cracking screw becomes the idle speed screw, and thus on a big-cammed engine is able to compensate for a small intake leak.And the PV, obviously, can no longer confound the issue.
So now, get it running and warmed up.Bring the idle speed into range say 1000rpm,with the secondary cracking screw. Adjust the mixture screws as may be required.Blip the throttle. Does the engine return to the previously set idle speed?
If yes we can continue.
If no, put a timing lite on it.Shut the engine off and restart it, to get back to the base idle speed. Now let's see what the flyweights are doing in the dizzy. So what's the base idle timing?Rev it up 200 more rpm. What's the timing? Let it return to idle. What's the timing? What you are looking for, is to see the timing return to base more or less following the idle speed, or not moving at all. I like to rework my dizzy to not begin advancing until well after the idle speed is left behind. Since most street engines (that I work on,at least) idle from 700 to 900, this means the idle timing will remain at base until around 1200.You cannot tune the carb until the timing remains stable. Do what you gotta do to stabilize the idle timing.
-All right, let's say the idle timing is stabilized.Begin to bring the idle speed down, still and always with the secondary cracking screw. If it will not come down to a reasonable speed, without threatening to stall, we need to check the valve lash, the compression, and/or do a leakdown test.
But first;remember that up to this point,I have assumed that the vacuum leak is minor. Maybe we should prove that first.Put a vacuum/fuel pump pressure gauge on the dipstick tube (the kind that reads vacuum and low pressure), plug all other entrance-ways into the crankcase (such as removing the PCV,and breathers). Then start it up and let it idle, while watching the gauge. Pressure should slowly build up, beginning almost right away.Don't panic, it's not an air compressor, it does'nt build that fast! If that's the case shut it off before exceeding 3 to 4 or so psi, or the pressure may start to blow out somewhere that you don't want it to. I have never had a problem at 4 psi(your results may vary) But if the gauge starts to read vacuum, it's off with the intake, cuz the gaskets have got to be sucking from the valley.After replacing the gaskets I like to repeat the test, to prove I did a good job. Then replace all the parts where they belong.
But, before you pull that intake, there is one thing you can try. Loosen all the intake bolts on the leaking side. Then begin to tighten them from the center out, in a circular pattern. Torque them in at least 3 steps. Repeat the CC vacuum test.
-OK so lets continue; now with a stabilized idle timing and a non leaking intake. At this point you will want to revisit the mixture screws.
-So once again,can the idle be brought down. If no,now we go for the previously mentioned tests. I like to do the LD test first,cuz results cannot hardly be argued with, and I don't have to take stuff apart.
-But let's say; YES!, the idle speed can now be brought down. Ok, see how low she will go. We are looking for a nice stable idle, of somewhere in the range of 700 to 850, in gear.The exact number is personal preference. Keep in mind, we need oil pressure so don't go too low.So find out what the engine likes.
-Finally we will revisit the idle timing,re-install the PV, set the maximum power timing, set the rate of advance,and tune the Vcan if you got one.
But for now, you got homework!