1968 G20 stutter

Hey all,
I've benefited a ton from this place already, so thanks in advance.  
I have a problem that's driving me nuts, here's the deal:
My '68 250 has a stutter.  It idles great, runs smoothly when I really stick my foot in it, but when I back off the throttle a tad, like you do when you get up to highway speed, it starts to stutter and backfire like crazy.  If I stick my foot in it again, beyond that point, it accelerates smoothly again.
So far I have: replaced fuel filters, air filter, new spark plugs, rebuilt my rochester monojet, vacuum timed the distributor (with electronic ignition), vacuum tuned the carb, pulled the intake/exhaust headers and replaced the gasket. Also ran a tank of premium gas with some Heat in it, to account for water in the gas. No difference at all.
Here's one thing -- it was doing it a little bit months ago, and then i left it sitting with gas in the carb for a month while i visited family.  when i came back, it got really bad.  Now when i rebuilt the carb, the kit I got did not have a replacement float, so i re-used the old plastic one.  I've read that they can get sodden, so I ordered a brass one which i haven't installed yet.  Could that have anything to do with this?
What about a coil issue?  
Its driving me nuts....
Thanks!

So when you pump the gas pedal with the engine off, do you see the accelerator pump squirting down inside the carb,,,,,,when the engine is idling, does it run better if you spray some carb cleaner down into the carb,,,,,,what electronic ignition do you have,,,,,,does it have an external coil,,,,,,,,unplug the vacuum advance and drive it,,,,,does it act better,,,, look at the centrifugal springs and weights,,,,does the rotor spring back when you twist it by hand?

Hey Donivan!
I do get the accelerator pump squirting when i pump the pedal. Not sure about the type of electronic ignition, it was there when I got it and all my past vehicles have had points. I believe the advance is working, there is correct vacuum when revved past idle, but I will definitely try driving it with the vacuum advance disconnected. It also idles smoothly, so while I will also try the carb cleaner trick, it seems like its a fuel delivery issue?
Thanks!

,,,run the tests,,,see if anything shows up,,,,you say it runs bad at a certain speed,,,,and backfires out the tailpipe?   Fuel problems usually don't pick a certain speed or recover really fast,,,now spark is always there,,,,,if the breaker plate or advance units are sticking or worn out,,,that shoots the spark out at the wrong time like when the exhaust valves are open,,,,,so disconnect the vacuum advance hose,,,that stops the breaker plate in the distributor from moving,,,we need clues,,,,,is the throttle shaft in that carb loose,,,,,,put a clear fuel filter in the fuel line,,,see if the flow changes when the engine acts up,,,,,,you could shut off the engine and take the top off the carb to see if the fuel level is high or low down in the bowl,,,,,,,you don't have an automatic choke flapper closing up on you going down the road,,,,,,

what type of dist are you running, is it a point type converted to electronic ign, or
is it an hei? if it is a point type might check sideways play in the dist shaft.
I had a 250 a while back act similar to what you are describing, that one had
serious valve seat deformation in the exaust valves because of the unleaded fuel
cyl head needed to be rebuilt with hardened seats, when we took off the valve
cover and the rocker arms all the exaust valves were taller than the intake valves
by almost 1/4 of an inch.

I had a similar problem with the wikivan.  I replaced everything in the fuel system and it was still doing it.  I thought it was the fuel pump going bad.  I replaced the fuel pump, but it still had problems.  

Turns out it was rusty crud and sediment in the gas tank that was clogging up the inlet screen.  These vans can get rust in the fuel tank from condensation over the years, and it can clog the pickup tube in the tank, causing fuel starvation.  The best way to fix this is drain the tank, remove it from the van and clean it out.  Pull out the sending unit and inspect the inlet.  Make sure the pickup screen is still in tact, and the inlet tube is clean and not clogged.  If the pickup screen is gone or damaged or has holes in it, you can get a new one for a corvair or base model chevelle (with 6 cyl or 283, so it will have 5/16" fuel line) and I think the pickup screens are the same.  Or you can make one yourself with some screen material from a screen door or the braided stainless steel material off a braided fuel line kit.  

A temporary fix, or to positively determine that there actually is a clog in the tank,  is to detach the rubber feed line just aft of the fuel pump.  Remove the gas tank cap and then 'back blow' the fuel line.  You can do this with compressed air, or just use your mouth and lungs…  but take deep breaths or soon you'll be seeing stars!  Blow until you hear bubbling in the tank…  you want to blow all that crud backwards out of the pickup tube.

I've experienced this same problem with both my 65 and my 64. Both times it was the same problem, crud in the tank clogging the pickup tube and inlet screen.

lube the advance plate in the dizzy!! also check the vacumn can on the dizzy with a vac tester- see if the can is bad or leaking??

Hey guys --
Thanks for all the advice so far, here's where I'm at. Was banging my head against a wall, replaced float in the rebuilt monojet, timed and tuned some more, nothing. Hooked up my vacuum gauge to the vacuum advance, saw that the vacuum moved the advance, it held steady vacuum, and returned when i released the vacuum. Was banging my head against a wall driving it today, suddenly remembered your advice (Donivan's first): drive it with vacuum advance disconnected. So at a light i reached down and popped it off and......
It runs great. A little less power maybe, but no stutter at all anywhere in the rpm range. What gives, if it looked like it was working when I used the gauge? I will lube the advance plate and try again.
Any other thoughts?

REPLACE IT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

So,,,,,,when you test the vacuum coming out of the carb with a vacuum gauge,,,,,is it 0 at idle,,,,,like 17 at high RPM, then drop back to 0 at idle? and do have one of those World Famous Skippy White HEI distributors, a Petronix or a stock GM one?

Replace it?!  Really?   The whole van?!  Seems kinda harsh...
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j/k...  yeah, agreed, a new vacuum advance is coming.  Not sure what the actual vacuum coming from the carb measures at, dumb move not to check but will as soon as I can.  The distributor is a no-name red HEI, looks like this one here (stock photo)." />

Just wanted to close this little chapter, i know when i'm searching through these archives to troubleshoot a gremlin, its nice to know what the eventual story was...

Long story short, the whole distributor grenaded.  I had taken it all apart, cleaned everything, and re-assembled with light lube.  Ran great for a couple hundred miles, then the same problem, and then kapow.

New distributor, everything is peachy.  Thanks for all the help.