fouled plugs?

How easy is it to foul plugs?I just got my new motor running with an old carb and it is running rough.The plugs are jet black (not from oil).It doesnt want to idle no mater where the timing is at or the idle mixture.Around 1000rpm it starts to stall out.It will not idle lower than 1000 and when I put it in gear it tries to die.It deffinatly has a miss to it.I have tried different carbs but they are all about the same.I ran the cranking compression and they are all close to 175-180 but it hasnt really been broking in good yet.The plug wires are ones I allready had but I thought they were good.But I have some new ones on order,everything else is new,cap,rotor,plugs,spring advance kit with the middle springs installed (HEI).They vacuum at 1500rpm is about 12 or lower and it jumps around some,I think because of the miss but I'm not sure.If i rev it up to 2500 or higher I can feel it shaking and missing some.It levels out and runs alittle smother after a second or two but still misses.I have reran the valve lash and sprayed carb cleaner at the base of carb and intake but I cant find any vacuum leaks but thats what it acts like is a vacuum leak. I AM STUMPED. ANY HELP?

I'll try the new wires and see what happens.Can the plugs foul very easy?Why does the vacuum bounce some at idle but seems steady at 2500rpm?I checked the vacuum again and it was at 5 at 1000rpm.What am I doing wrong????The vacuum was 20 at 2500 or higher.

double-check the float level.

Float level is fine,I think DennyW is right.My new wires should be here wednesday,I'll find out then.I noticed that it coughs back through the carb some when I try to crank it.I think the spark is jumping to another cylinder causing this.I just hope this hasnt hurt my bearing on this new motor.I installed new plugs and they went black after about 3 minutes of running.Three different carbs,same problem,so I dont think its the carb.It has to be ignition.

Is the firing order correct? 18436572

Chevrolet and Chrysler 1 8 4 3 6 5 7 2 . Driver's side from front to rear; 1 3 5 7. Passenger side; 2 4 6 8.

The angle of the intake manifold might not be parallel with the angle of the heads. It is possible in this case to suck gases from the crankcase into the intake ports and you'd never find it by spraying ether on the outside of the motor because the gasket is pinched tight on the outside, but is not being pinched at all on the inside of the cylinder head/intake manifold interface.

Here's how I set up the intake manifold/cylinder head interface to prevent vacuum leaks from the crankcase to the head ports....
Measure the thickness of a new intake manifold gasket. Get flat washers or shims that will measure that thickness. With the manifold off and the mating surface on the cylinder heads de-greased, put a dab of RTV on them and stick them on each corner bolt hole on the cylinder head. Let the RTV set up. Stuff paper towels into the ports to keep debris out. Make up 16 pea-sized balls of modeling clay. De-grease the intake manifold at the ports. Place the balls of clay on the top and bottom of each port, squishing them down well so they will stay in place. You want them to be thicker than the shims/washers that are RTV'd to the heads. With your fingers, coat a little oil on the heads where the clay will meet the head to keep it from sticking to the head. Now carefully place the manifold into place on the heads and use bolts on the four corners to just snug the manifold down until you feel resistance against the shims/washers. Remove the manifold carefully and mesure the thickness of the clay at all 16 positions with the depth function end of your 6" dial caliper. You'll know pretty quickly if the manifold/head interface is square. Record the measurements on the manifold with a permanent marker like a Sharpie. The widest measurement will be the standard to which you will want your machinist to cut the other positions on the manifold to make it square with the heads, thusly sealing the motor up.

I got the new wires on and it runs a ton better but it still has a little miss and the plugs are still black.I couldnt believe the difference the plug wires made.Now I just need to fine tune it.Tech might be right about the internal leak.That is something that I need to check.I know the carb is not running right but its all I can afford right now.It is a 750dp witha fresh kit installed.I have tried it and a 750 vac secondary and an edelbrock 650 all run about the same.I dont know why its running rich.I need to readjust the secondary float level on the dp.It wont adjust any lower with the needle and seat so I must have missed setting it when I installed the kit.The float level is about 1/2 way up the window,so it could be getting some extra fuel there.The front is fine.And this cam has the 4/7 swap so the firing order is 18736542 and it is right.Thanks for all your help especially you DennyW the wires made a world of difference.

Hey camaro77 I'd make sure of that vaccum leak still. And let me know what you think of the 4 7 firing order swap. I'v been throwing that around for awhile

I had that problem and it was a combo of plugs and cheap fuel.

Bob