I have been through what you are going through now
It can be exasperating for sure. Here is what I did.
I reread the Best of H.L. Chauvin about a dozen times in conjunction with the Blue Ribbon Service Manual, GSS - 1012 Pages 4-7. Both are on the Manual Server.
It is probably something real simple such as the cleaning the tube that runs from the breather pipe to the block. Or, making sure that the main jet is clean including the little orifice on the end. Use a small wire to make sure that it is not plugged and clean with Carb cleaner. Make sure that you are not sucking air. Make sure your plugs are correct (Champion D-21's) and that the gap is correct. Also make sure the gap is correct on the points.
If I forgot anything I am sure someone will pop up to help.
I hope this is of some help to you. But as a little consolation - I knew absolutely nothing about Cubs until I bought mine, and I am in now way shape or form a mechanic, but I have been able to get Ellie running like a top. I am sure you will as well.
Just read the manuals and Chauvin. That should be the ticket.