I did a little road trip last week and noticed that once the car gets below 1/4 tank of fuel the idle becomes funny and at mid throttle it sputters and boo's. By funny idle I mean it's either between 600-750rpm and really choppy with possibly a little misfire or at around 1600rpm.
I am fairly certain that it's not an ignition issue as all the ignition components from coil to plugs and everything connected to the distributor and all distributor components have been changed and both valve and ignition timing have been checked. Compression also shows a healthy 10 BAR in all cylinders (warm) with a few decimals variation.
This issue has not happened before and previously I have gone well below 1/4 tank deep into the red on multiple occasions.

Any ideas what could cause this. I know I have a carb with a bad reputation but it can't be this bad as to go out of tune within 5000 km of a tune adjustment.

1984 242DL
B19K with Solex Cisac Z4 and M45

 

I would concentrate on the problem at idle since the car does not have to be moving for the problem to occur. I don't know if your model has an electric fuel pump in the tank in addition to the mechanical pump on the engine. I would remove the tank pickup cover in the trunk and then remove the pickup from the tank. You will need to fabricate a tool to open it. Replace any rubber hose and the filter in the pickup. Also look into the tank with an LED light. You should be able to see the tank bottom clearly and it should look shiny. Don't switch any electric devices on or off near the tank opening.

After that inspect the fuel lines running from the tank to the engine. Even small holes can make it hard for the engine pump to draw fuel. You could also connect a fuel pressure gauge at the carb inlet. I don't know the pressure spec but you should be able to look it up in a shop manual or tuneup manual.