If you're talking about the rubber boot covering terminal, has nothing to do with starting. But when you resat the boot, you may have accidentally tightened the terminal connection. I'd recommend you pull off the boot and check the connections for tightness and corrosion.
You really need to measuse the batterys CCA rating. A bad battery can still show near 12 vdc before starting, and then drop dramatically during starting.
Really sounds like your battery is near its end. Try pulling the caps off (usually under a large sticker on top, even maintenance free...
In my case, I was never able to fix it. The frame took the slight warp to the left. Tried repeatedly bouncing up and down on the right, nothing. Steel too thin on frame. Sorry, no help.
Blades out of balance? Spindles worn? Deck hanging hardware bent or worn?
Tractor frame leans to one side (I had this happen to a tractor i owned, tractor leaned to the left about a qtr inch because i always mounted the unit from the left side. Body weight all on one leg put too much stress on...
Getting back to the OP's question, (and I'm the first to admit that I know next to nothing about it), the purpose of zinc in oil is to work as an anti oxidant. (It helps prevent the oil from breaking down and extends its useful life).
You may be able to find the parts you need for your mower on a site like this (could always post your needs and see if anyone responds)
https://www.gumtree.com/lawnmowers-trimmers/scotland/mtd
The deck is raised and lowered (from transport position to mowing position) via a lever on the right fender (next to the seat). And its a ***** to use, very hard to unlock. Lowering go quick, raising requires a good jerk to get the momentum needed. I liked an old tractor i had, forget the brand...
That's complete nonsense. You don't understand how these tractors work. Both Regency and Broadmoor have manual lift levers to rase and lower the deck off the ground.
The electric lift on the Broadmoor only adjusts the height of the blades inside the deck. On the Regency its done by manual...
Several areas.
First is the tires are too small, makes the chassis too close to the ground. This make it impossible to get the mowing deck off the ground completely. When going from driveway to grass, it gouges.
Also I part my tractor in a shed and use 2x8's as a ramp. This makes it d.r.a.g...
All this talk about carburetors, but what about the op's real problem? I don't know the answer, but found it interesting that everyone skipped over his real problem.
Yup, all good mate, but its the sneaky, kangaroos ya gotta watch out for. Little buggers sneak up on ya and ... pow.
Deadly.
Hey, I seen Crocodile Dundee, three times. I know all about those roos.
Grin