Yardman mtd 12a559k401

It is useful to have the (what it is) as part of 'make model (what it is)' in questions. It appears that nearly everything they sell is a 24A and there is no manual listed for your model. There is a chipper and a self propelled mower with similar suffixes after the 24A.

My searches suggest there is no decompressor. Pull ropes braking are usually because of friction in the pull cord mechanism and its drive into the crankshaft.

If you can see the cylinder head there should be a cable for the decompressor or linkage that opens the exhaust valve.

https://www.google.com/search?q=Yardman+model+24A-020D701+manual

..

sounds like the motor may be sezing up have it been run with the wrong gas oil mix?

Duncan, Blade #, MTD Part No 742-0578, 27.5" should be replacement bade for Yardman DX70. Check the attached links,instruction and guides, Good luck
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."
GENUINE 27 5 Blade for Yardman DX70 MTD Beatle Lawnflite 503G Ride on Mower
YARDMAN DX70 MODEL 310 SINGLE BLADE MOWING DECK FG424 5900
Yard Man 13A 325 402 Operator Manual

There is probably a switch in the reverse lever that shuts of the mower. It is probably a normally open switch so when you pull the lever to reverse it pushes the switch closed killing the engine. If you unplug the switch or cut the wires going to it, it would never think it is in reverse and you could always mow.

It would be very unusual to be stuck in gear, i think it more likly the brake is stuck or seized on, check the small disc brake on the side of the gearbox, the arm should have plenty of free play, if not adjust the nut on the outside anti clockwise, this should pull the pads away from the disc.

The drain plug will be located in the base plate of the engine at its lowest point. There may be more than one for ease of access, depending on the engine type and design of the mower. Drain oil when the engine is hot and tilt it so as to drain the maximum amount of oil. (Remove the spark plug lead before working on the machine).

you have to wind up the pully before you install the rope.
then when you put the rope on it will wind itself back in the case

i need a diagram showing for a 20hp yardman tractor model number 13AT605G755

If you've never performed this sort of maintenance before, then you should not try it without assistance. The spring is under considerable tension and can snap out suddenly and inflict quite a bit of damage to you.

I recommend that you contact your nearest authorized service center for assistance, or search youtube for a comparable video.

I trust this information proves useful.

If you think the rope pulls too easily the engine may not have enough compression. Consider doing a compression test or a leak-down test on the cylinder.