Your Coil may well be breaking down , unit begins to miss and falter when ran at high rpm after initial warm up of 10-20 minutes . Once cooled down it runs fine again for short periods .Other than that your high speed jet may have backed off restricting fuel at High rpm . Have you replaced or inspected the diaphragm or inlet needle valve under the float or the prefilter screen which can cause same issues , since I would assume you inspected or replaced the fuel filter when you replaced the lines ..

I don't believe that I have taken the carb apart yet. I hadn't heard about needle valve variable issues. What would I be looking for? I suppose the only solution for a possible coil is a coil replacement. There isn't any kind of test??

Actually, I have a couple of odd options that I haven't looked into yet. This particular unit is quite old but while acquiring the extra expand-it tools from an e-bay seller of returned/unboxed/opened etc. stuff from Home Depot etc. who usually sells several items together for cheap with very low shipping too; I have an extra power head or two that I could either see if works as is or canabalize. The newer ones appear way less rugged however. They may have different parts.

A small engine shop might be able to test your coil. In the 70s we had a coil/condenser tester. It would energize a
coil to fire across a simulated gap or fire your plug. The machine would also test a condenser and
had a list of values.

Try cleaning out the jets 1st and fuel filter within the tank . Then if you have a spare coil switch it out if the cleaning does not solve your issues.

Thanks ewalk, my general blind approach is take carb apart and wash/clean/blow out jets and hope for the best. If the newer engines are really the same, I will indeed have a spare coil to play with. Loger, we do have a huge engine repair place with good ol' boys that work on small engines along with real tractors.