widow giving me fits

Close your needle down all the way. Pressurize either the top or bottom filler pipe with a syringe while holding your thumb over the opposing one. First, listen for air leaks. If the venturi o-ring is leaking you will quickly and audibly hear it at the rear screen. You will probably find the screws leaking as well. I place a drop of oil on them and check each one individually. The tank to backplate can leak as well. Here is where a piece of dental floss carefully placed in the v-groove overlapped slightly about a 1/4"  solves that leak. You will also find a leak at tank to crankcase so putting some light machine oil on that joint will also reveal bubbles if it is leaking. In addition, the needle valve will show leaks during this test where the entire assembly is pressed into the backplate. This test is entirely irrelevant to the reed valve, just the above. If the screws are leaking, strip off a piece of Q-tip swab and roll it between your fingers into a string. Wrap this around the upper portion of the screw in the direction that will tighten the cotton onto the screw opposed to making it want to come off. This almost always fixes a leaky screw and makes it service removable and reusable. Ken

I suppose a good analogy would be trying to  drink through a straw with a hole in it. In addition Phil, check the pickup where it attaches to the barb. If it's remotely loose this is another problem area. Not so much when the fuel level is high but as soon as the fuel level lowers it begins sucking air. The pickup tube needs to fit tightly and older ones swell.  Ken

Use copper wire and tie it onto the fuel barb.

I actually prefer the plastic backplates. They're lighter, they seal better and they're less prone to cracking at the screw bosses when they're new. I had several metal backplates that were faulty or had casting issues resulting in pinholes not to mention the entire casted area of the screw cracking out. The metal certainly delivers more punishment in regards to ground pounding, so keep it out of the ground. The barb on the plastic backplates is slightly larger than the metal. I've tried to remove the tubing on metal backplates  only to have the entire barb snap off. Each one has their own share of problems. If your screws are leaking, it's an issue. If you have leaks in all the areas described it's an issue. If the engine isn't needling properly I would replace the backplate. The needle seat might be damaged inside or a broken point of a needle is left internally. The best solution is to use a piece of aluminum tubing bent into a horseshoe joined with a small piece of tubing to the fuel barb. The stock tubing is horrible and 9 out of 10 times it doesn't stay put.

Many other factors can determine if your engine is even in good enough condition to run up  properly. Do you know the history of this engine?  If the case itself has been sitting for a long time it could be  varnished  with gooey castor this will also make your engine run go south as it heats up acting  like a brake on the engine when you try to lean it up. Dismantling the engine and cleaning the crank and case thoroughly is in order. This stuff will not break down and remove itself it must be done manually. Is this the correct piston for that cylinder? Swapping piston to cylinders can also tend to go south if they're not fit properly. The engine begins to sag in short order.

Your statement in regards to running these when your 12 is very similar to some of us as well. The difference however is that we also didn't know what we know now later on after gaining 40 years experience.
Some of the symptoms you describe in regards to your engine run follow a pattern which is similar to bad fuel. In order to diagnose I would take all simple factors out of the picture. GLow plugs can do the same thing even if they appear to be glowing properly. Issues like you describe are common to this engine. In order to figure out why, all factors need to be eliminated. When that doesn't work , wrap the shop rag around it and fire it off and set it free. Ken

I certainly won't contradict Rusty's video in regards to the capillary action of the spring inside of the pickup tube. However, my best runner to date has a piece of K&S aluminum pickup tube with no spring present and never has seen an issue. I'm just stating my experience but in the event I use the plastic or silicone tubing, I first burn the spring over a flame to rid of contaminants and I install the spring into the pickup tube. Ken