Update:
I haven't had time to check the valve clearances on this mower but did try something Saturday.
So Saturday while I was mowing my yard, in the meantime I hooked a battery charger to the mower I was having issues with just to see if it would start.
On a side note: I mentioned previously I can't seem to get more than a couple of years out of these lawnmower batteries (on any mower I've owned since I've lived here). I can get almost 10 years out of a car battery with a 2 year warranty as long as I keep it topped off with DI water once a year and hook it to a trickle charger if the car sits for more than a month. My guess is because my yard is so bumpy, the constant bouncing must be damaging the batteries somehow? Or they are just cheap batteries. When I owned a Snapper, the battery on that mower lasted one year. So after that I just used to recoil starter for about 10 years until I sold the mower.
Anyway, the battery on this MTD must have a bad cell in it because after a day or so, it tests around 11 volts. But after a few hours of charging, it has enough juice to turn the engine over.
So after a few hours of charging, I hit the starter and it fired right up without starting fluid.
I think the problem maybe the battery itself. The battery may have enough balls to spin the engine over, just not fast enough. But because starter fluid is not volatile, makes it easy to start on a slower spinning engine. Now as to why the same battery will start my B&S 21 HP Husqvarna just fine, I don't know.