I own a MTD Yard Machine riding mower that tries to turn over very sluggishly but won't turn over completely. I have changed spark plugs, replaced choke cable, purchased new battery that required I add the acid myself and then charged battery, cleaned carburetor. Gas evident in carburetor. Was working previously if jump started with old battery. Now it just won't turn over - very slow,sluggish. Please Help! Thank You!

MTD Yard Machine - 13A0695G062 16.5 HP Lt 42" Twin Blade Deck

Briggs And Stratton Motor - Model 42A707, Type: 1237 -01, Code 9508305B

18HP Twin II

Hi,
It could be the pinion on the starter, but let me tell a story:
A bit less than two decades ago I was a service manager in an upscale suburb. Average income was significantly more than mine since most inhabitants were business, technical or medical professionals. A woman came in with her husband's mower using her neighbor's truck and having children in tow while he was at work. Her cmplaint was the same as yours. She said that her husband had just replaced the battery, and everything worked few the first time. She further elaborated that the yard needed mowing and wanted her husband to do it when he got home. I looked the mower over, and quickly saw the problem. The battery was not filled correctly, the fluid was well below the top of the plates. I informed her of the situation, and told her that the plates would have been oxidized if any charge was put on the battery in that condition. She reiterated that her husband had just bought and filled the battery which he had done while she watched. I had the same model battery in a dry-charged state in my shop, so I proceeded to service it. I filled each cell successively to the full mark. Byt the time I finished the final cell, I had to go back to the first cell and refill it because the absorbant material between the plates had soaked up the electrolyte to the extent that the cells appeared less than half full. I refilled each cell; then made a third pass to top them off to full. When I told her that was how it was done, she looked at the bottle I used to fill the battery and remarked that her husband had a lot more left in the bottle he used and had not done it that way. She asked to purchase the new battery; I said it had to be charged first.
With that she told me to charge it and inquired about delivery (she had to get her neighbor's truck back.) I said I would have a driver in the afternoon. She replied, "make me a bill." I did, and she started to round up the kids, load them up and leave when she could hold no longer and broke out laughing. A bit puzzled since this whole episode cost two batteries, labor and delivery, I asked what was so funny. She simply looked at me and said, "My husband is a chemical engineer and could not fill the battery right!"
Now I do not say that this is your circumstance, and someone taught me after all; but if this happens to sound a bit familiar..
Peace,
Clarke