Marvel Mystery Oil should have no effect on the engine/carb if you poured in the amount suggested on the can. Have used it for 35 plus years to include pouring right down the carb while the engine is running. It won't gum things up even if you used more than recommended.
In your pics, if you have not already adjusted, your float is adjusted too high. I would not be too concerned with float drop. It is the float level that is what is important so that it shuts off the fuel supply when the bowl is full.
Here is the Federal-Mogul guide on the Carter AFB. Note that they recommend a 3/8" float setting - Page 20. Look where they measure it, towards the front of the float. It is probably the equivalent of the Edelbrock AFB setting of 7/16" because the Edelbrock locations seems to measure the float further back for some unknown reason. So check this float setting out in the guide:
http://www.mymopar.com/downloads/carterafbtuning.pdf
I usually blow on the fuel inlet fitting while the top is upside down and the needle should be closed not allowing any air in. Then I flip it right side up and blow into it again to make sure the needle is not sticking even though the float has dropped. If you encounter a problem, you will nip it right then rather than install the top and find out you have a flooding situation.
The guide also states that 6 PSI of fuel pressure at idle is too much. It should be 5.5 PSI and they recommend a pressure regulator - Page 6.
The accelerator Pump Connector Rod should be in the middle hole. The guide shows you how to measure the accelerator pump plunger "stick out" by fully closing the primary throttle plates so they seat closed, install the pump rod in the middle hole, make sure the "S" link is installed correctly and not backwards, then measure as shown. If not correct, you bend the Pump Connector Rod to get the correct measurement - top of Page 21.
Check to see if your carb has a Bowl Vent, which it should have somewhere. Make sure it is working and not stuck or plugged, not sure where it will be located - Page 7.
Running ethanol fuel can mean you need to step up a couple jet sizes as ethanol fuel runs leaner. Keep this in mind. You may be able to open up/back out your idle mixture screws to get the primary's to run a little richer and see if that helps. Initial setting for the idle mixture screws - Page 9 If it does help, you might want to up the jet sizes.
Timing can produce stumbling problems. If you have a high compression, 9.0 or more, and iron heads, you have to know how to set your initial timing, total timing, and vacuum advance. If you back the timing too far off (retard) to eliminate the rattling of detonation, the engine may not be "crisp" when you accelerate from a low speed. Advancing the timing will make the engine run better and respond better, but you might experience detonation - and this can destroy an engine if not addressed or if you allow it to detonate too long. Higher octane gas is the first cure. Adjusting the timing and timing curve is the next thing. So timing
could be an issue in some of this. You need to know what your initial timing is at the crank/balancer, what your mechanical advance is from the weights in the distributor & at what RPM they are "full in", and then how much vacuum advance the distributor has through the vacuum can on the distributor. You also can use "direct" vacuum from the engine or "ported" from a carburetor source. One can work better than the other in some instances.
I would also get a vacuum gauge if you have not already checked the engine's vacuum. A vacuum gauge can be a very valuable tool in engine diagnosing and trouble shooting - to include adjusting your carb/timing.
Do one thing/adjustment at a time. If you do several, you won't know what has worked or what has made things worse.
1. Work with the carb first and get it set-up correctly.
2. Then I would take a look at my engine's vacuum and see if anything is going on.
3. Then tackle your timing & timing curve - and I would include checking to make sure your coil is good as well as you have the correct voltage going to the coil. As pointed out earlier, have seen symptoms that you would have thought to be a carb problem only to find out it was an electrical problem - something as simple as a coil being too weak.
