[steve25]

Take some mechanic’s wire or cable ties and lock out the secondary’s on your Q-jet just like what the levers and rods do when the choke is on and the motor is cold.

Next take the car out for a drive bringing it up to 5000 rpm several times thru the gears ,

If no issues take place then you can rule out timing and spark issues and then focus on things like the carbs secondary air valve diaphragm being leaky, or what’s more likely a fuel delivery problem keeping the carb from getting the volume it needs.

Checking that diaphragm is easy. Just unhook the hose from it. Push the lever in and then use you finger to block the hole in its feed tube.
The lever should stay in all the way for as long as your finger is on it if the diaphragm is good.

You may now have a fuel volume issue because the now bigger 800 cfm carb is making more power and needs more fuel volume to do that then the Eddy carb did.
This issue could be as simple as dry rotted rubber hose jumpers that are cracked and allowing air to be sucked in instead of fuel, and yes this can many times take place without seeing a drop of fuel leaking.
It’s far easier for the fuel pump at high flow rates to suck in much lighter air then the heavy fuel in the tank!

The next fuel issue could be that the fuel sock / filter at the end of the pickup tube in the tank is clogged, or it could just plain be that the steel fuel line has a pinched off spot in , it could be that flaps of internal rust in these fuel lines are making for a restriction , or it could plain and simple be that the motor with the bigger carb is just making more hp then the sock size fuel lines can support!