Carb help? Distributor?

Jean Clyde Damn Van is giving me fits. Must know I got another van. Anyway, I was heading out to pick up my new to me Challenge car project when under load and over half throttle it started to miss. If I backed off it would get better and go away. Then I got on the interstate. No issue merging up to speed limit. Less then a mile and now its starts missing again, with an occasional backfire. Again, under load (uphill section), with tip in over half throttle. So I back off and get to my exit. Sitting at the light it idles okay. Head up the road and now it's back and not getting better. Got about a mile, mile and a half and its dropping cylinders. Pull over and it dies as I come to a stop. Won't fire back up. Spits and pops, but no go. This all happened following a similar scenario last week after which I replaced the HEI coil and ignition module and dried out the distributor cap. (I was driving in a torrential downpour that day). The most recent happened on an 80 something degree day.

It's a three year (?)  old rebuilt 350 small block. Nothing fancy. Edelbrock carb and manifold. Carb is close to 20 years old by now.

I got it towed home and took a look at it today. I swapped the cap, rotor, and condenser for spares I had on the shelf and replaced all the plugs. The ones in the engine presented as rich and sooty. It started fine and idled so I went for a quick drive up and down the road including a steep incline. No problems. I took it for a longer drive (about 10 miles to the grocery store). Fine on the way there but its mostly down hill. On the way back I gave it a little throttle and it hiccuped and missed. I backed off. I then tried again going up a steep hill and ot started to act up again. I had to take it easy the rest of the way home. Anything over half throttle causes the misfire, spitting and popping. 

Where to next? Fuel pressure? Pull the carb?

Oh, by the way. I had to free up the weights in the distributor as the armature would not snap back quickly when twisting the rotor. On another distributor I have it snaps back instantly. 

Thoughts?

Re the weights in the distributor: see if you can see why it is hanging up (dirt, rust, solid grease).

Have you checked timing?

After 20 years carb might benefit from rebuild in a can. 

Does it sit much?

Mechanical fuel pump or electric?

I am wondering if you don't have an electric pump failing, or a mechanical pump and enough restriction on the fuel tank side that it is going into vapor lock or something.

Rule #1 is that it is always the ignition, though!  Mechanical advance that does not return easily gives me a bad feeling, sufficiently advanced detonation feels like a misfire.

Have you checked the fuel filter??

I had a 350 do that, due to a lobe on the cam going flat (flat tappet cam), even though it ran pretty good. Take off the valve covers and run it, you can see if one or more rockers aren't moving as much as the others.  

Hopefully yours is just ignition or something simplier.

Does it have vacuum advance?

I was thinking about the fuel filter tonight.

I managed to get the weights free and lubed before the test drives today.

I do need to recheck the timing.

And yes, the age of the carb could be an issue.

So I will check the filter, check the timing, and order a rebuild kit for the carb.

In reply to Pete. (l33t FS) :

original electric pump with aftermarket adjustable fuel pressure regulator

Checked and the fuel pressure was at 12 psi. Turned it down to 6. Much better. Now there is a hint of stumble with throttle tip in but I can push past it. I ordered an Edlebrock rebuild kit and a fuel filter. Still need to check the timing. Getting closer.