Starving of Fuel

I'm at wit's end. My 69 G10 has a 230 with a Rochester Monojet. It runs really well at low speeds with the manual choke wide open, but stumbles and hesitates when accelerating and at higher speeds. At highway speeds, if I pull the choke to about half way, it starts to run really well again. I assume this eliminates bad gas, timing and compression as culprits. I replaced the distributor advance and hoses, PCV hose, and intake/exhaust manifold gasket to address possible vacuum leaks. I had the carb rebuilt. Dicking with the mixture makes it better (or worse) but doesn't really completely fix it. I've run out of things to rebuild or replace. I must be missing something. Any brilliant ideas out there? Thanks!!

timing a engine with a vacuum gauge
http://automotivemileposts.com/garage/v2n8.html

Since it runs better at high speed with the choke pulled in some which is giving you a richer mixture (more gas less air) I would look for things that are giving you a lean condition.

Air leak somewhere in the intake manifold or carb base (are you pulling a good vacuum at idle).

Carburation issue jets to small, float level set wrong, etc.

Fuel getting to the carburetor, clogged fuel filter, fuel pump, fuel line / gas tank blockage (fuel filter sock in the tank)

Is this a fairly new problem or has it been running rough for a while?

Do you have this gasket upside down,,,,,
Spray the outside of the carb and intake  with carb cleaner to see if RPM changes
When you step on gas pedal, is throttle plate open all the way?

Thanks for your thoughts guys. That's one reason why I enjoy this site so much.

The van has always run rough and stumbled since I bought it. Systematic tuning and replacement of parts has made some difference, but never solved it.

One more bit of information. Someone removed the smog pump and pinched the connections at the head. It's possible one or more of these would be the source of vacuum leak? I'm wondering if there are proper plugs available to replace the pinched ones, or how have others dealt with smog pump removal. I'll see if I can post a picture (my only complaint about the site - not a simple copy and paste).

Is this link connected to the power valve piston on carb?

I'm not familiar with power valve piston. Here is a photo I took before I had the carb rebuilt. Not sure if it shows what you're talking about.

Its below where the gas line comes in ,,,there is a cam that should have 2 rods connected to it,,,,,Those plugs go to the exhaust manifold

If you see any gasket sticking out, its installed wrong....

Like Donivan said, the best way to find a vacuum leak is when running in idle spray carb cleaner all around the intake manifold and base of the card and listen for the RPMs to increase, if they do you have found your leak.

another way to find a vacuum leak is to blow smoke through it. with engine off, remove any line connected to manifold, hold a rag over the carb and blow smoke into the hose. I had an intake manifold gasket leak from the bottom and could not get to that area with starting fluid and from above made no noticeable difference. the leak was easy to see with the stream of smoke blowing out. I would not rule out the carb either! I would check for vacuum leaks first, then install a carb that is known to be good.

Okay, the weather has cooled down enough to get back to work.

I sprayed around the carb and manifold with carb cleaner at idle as suggested. I did not detect any change in rpm.

The carb to manifold gasket is installed correctly.

I thought a fuel pump either works, or it doesn't. Is it possible for it to lose pressure at higher rpms? I see plenty of gas flowing through the transparent fuel filter.

Here is a photo of the power valve piston linkage. Everything moves in concert when stepping on the gas pedal, and the throttle plate opens wide.

Although I would have thought a rebuild would catch any internal carb problems, I'm kind of thinking my next step is to simply replace the carb, if I can find one. On Rock Auto, all they have available is Return and Rebuild, which I've already done.

How about unscrew the top, check the float level and check if the metering rod is 0.070 after making sure its not on fast idle and  backing off  idle screw,,,,

What about the rest of the engine,,,,,
,,,,IS....
compression 130
Points 0.019
Dwell 31-34
timing 4 btdc
made sure when #1 piston is all the way up, timing notch is at 0,
meaning rubber mounted pulley aint slipped
vacuum advance is 23  degrees
centrifugal advance is 32 degrees
that you dont have fuel filter in carb put in backwards,,,,,,,

Thanks for the advice! I converted it to HEI a couple of years ago. Thought I would just replace the in-line fuel filter at the same time as checking to make sure it was not in backwards (it wasn't). The filter had a fair bit of sediment and crap in it. After replacing it, the engine runs pretty well at all speeds as long as the choke is half on.

I think the next job, which perhaps should have been the first job, is to drop the gas tank and get it properly cleaned.