Just in case other Forum members have had this issue, I will share the aftermath.
After I followed Chuck's advice, I was able to delete the speed limit, I could speed up the engine and avoided having it stall over my three back to back switch tracks.
Then, I tested the engine with a heavy load (11 heavy post war operating cars plus caboose). After running fast for a few feet, the engine reverted back to its original slow and steady speed. It's as if the engine refused to speed up with a heavy load.
I was able to delete the speed limit again, following Chuck's advice on about the 10th attempt. The engine ran fast on my inner loop without pulling any train cars.
I have only had this engine for 1 day. It looks great and performs great in all respects except for this speed limit issue on my conventional layout using the MTH Z4000 transformer. It seems that with my MTH transformer, if pulling a heavy load, the engine won't accelerate once the throttle reaches 11 volts.
Installing TMCC/Legacy Lite might be a much better way to run this Lionel Legacy engine.
However, I have many more MTH Proto 1 engines and a few MTH Proto 2 & 3 engines.
Does anyone know how well these MTH engines run when using TMCC/Legacy Lite?
After installing TMCC Legacy Lite, can I shut it off when running my MTH engines using my MTH Z4000 transformer?
Arnold
