Belts slipping and breaking

The bronco is a variable speed mower from what I remember. There is a belt that goes from the crankshaft to a variable speed pulley and another belt from the variable to the transmission. If your 1st belt is too long it will never get up to speed. Do you have to push the pedal down further now than before to get it to move. ? If so your main belt is too long.

ya it can be freed up with some W-D40 if you pah the cluch down then try to get the belt off i can get you a new pulley let me know

Once a belt has slipped to the point of "smoking" it is time to replace it. It will have a worn spot on it that will cause a vibration throughout your entire tractor when you are driving in addition to being weak at the burned point

Remove the shifter knob, 2 screws and wire for safety switch from the center plate on the tractor. This will give you access to the variable speed pulley and limited access to the transmission pulley. Reach in under the seat area toward the transmission and "feel" for the pulley, slip the new belt on there first but check to make sure you are inside any belt guards. Next you want to get the belt on the top groove of the variable speed pulley (you may need to remove the belt guide or POSSIBLY bend it just far enough to get the belt on then bend it back in place. Lastly, you need to get the back side of the belt against the flat surface on the tensioner pulley in between the transmission and variable speed pulleys. This is a very stout spring and takes some effort to do but I lift up on the edge of the belt with one hand just enough to clear it and pull the tensioner pulley over with the other hand.
You can Google " MTD 13al675g062 " without the quote marks and it will give you a pretty clear picture of it

The bolt should be tightened to a hundred lbs. make sure the center pulley is free to move up and down, and you ahve the right belts. Avoud belts from Lowes or Home Depot od Wally world, They NEVER fit right

mtd 12/38 blads not stoping ? theres a ajustment rod on right side under the mower turn clockwise to losen belt .

The front belt should be a 954-0280A, rear 954-0370. When you go to higher gears, the front belt should be getting tighter as the vari-speed pulley is moving further back. Remove the shift plate, put the transmission in neutral. Start the engine, put speed selector in highest gear and slowly let out on the clutch. You should be able to see what the problem is. It will do everything the same as if were in gear, except move. Make sure you have the correct belts.

the tension on rear end you can jack it up in the rear & spin the right tire to get it back on & make sure the brake isn't on

Take off the plate around the shift lever between your legs and take a look at the belts. Most likely they will be in poor condition. You can even leave the trans in neutral, start the engine and move the speed selector through the gears and observe the belts in action. They do the same in neutral as in gear when you let out the clutch with the engine running. You should be able to see what the problem is, It will either be one or both of the belts or possibly the needle bearings in the vari-speed pulley itself,

Get your model and serial numbers from under the seat. Then go to the MTD website where you can download the owners manual which should show the belt diagram.