Lawn chief mower throws belt

It would be helpful if you could provide some pictures so I could see what it is doing. Generally the belt will be thrown off if it is badly worn and hence it is loose, so a replacement might fix the problem. You should also make sure that it runs correctly around all the pulleys as it is easy to get one wrong, finally make sure that all the grass has been cleaned from the tensioner and around the pulley guides, and that the pulleys are not badly worn. If they are badly worn, the belt will sit in the bottom of the pulley instead of on the sides, and will throw off really easily. A damaged belt will also cause it to be thrown off.

check for split pulley

Lawn Chief is now owned by MTD but does not show up in their list.
Try this link, you may find some help.
Operator Manuals

Double check the routing diagram. Check to make sure your idler pulleys are in good condition. Bearings are good. Lastly the tension spring may be worn. The belt should fit very snug. No more than 1 1/2" of deflection when you push on the belt with your thumb

You removed the engine??? You probably just needed to remove the blade. The only spring I can think of would be a tensioner spring for the belt pulley, which is up front if it's a front wheel drive mower.

How to replace the drive belt on a Toro LX466 Lawn Tractor

1. Jack up the front of the mower-place a couple of blocks under the front wheels (retaining wall blocks from your flower bed work great). Block the back wheels so the mower won't roll off the front blocks.

1. Remove the battery and battery compartment from under the seat. You can now see the cooling fan blade under the seat. The belt runs from the pulley directly under the fan to the front of the tractor.
   

2. Lower the mower deck to its lowest position. Remove the deck hanger rod from the slots in the front of the mower deck.
   

3. Remove the metal rod guard below the engine pulley. It's held in place on one side with a bolt, and the other side with a cotter pin.
   

4. Take a large pair of channel lock pliers (about 18 in long work great) and clamp them down directly on the engine pulley, but not so tightly you distort the pulley.
   

5. Remove the bolt from the pulley. It's a right-handed bolt, so turn it counter-clockwise to remove.
   

6. Slide the pulley down the shaft until you can work the belt off. If the belt is too tight, then remove the spring from the idler pulley under the tractor. It's hooked on a bolt near the side of the tractor. You can remove it by grabbing the end of the spring with a pair of needle-nose vise-grips and slipping the end off the bolt. Then remove the old belt from the tractor.
   

7. Take a new belt (preferably Kevlar) and start at the back of the tractor (with the pulley under the cooling fan) and thread it through to the front of the tractor. The flat side of the belt rests against the idler pulley with the flat sides.
   

8. Work the belt over the engine pulley and slide it back on the shaft. Tighten the bolt on the engine pulley hand tight. Then check the belt to make sure it hasn't slipped out of the groove on the cooling fan pulley.
   

9. Clamp the engine pulley with the channel lock pliers and tighten the bolt.
   

10. Make sure the belt is routed correctly on the idler pulleys. If you had to remove the spring on the idler pulley, replace it. You can do this by grabbing it with the needle-nose vise-grips and placing it on the bolt-make sure it's above the washer. Then remove the vise-grips and gently pry it over with a small screwdriver until it slips into place.
    

11. Replace the metal guard below the engine pulley. Hook the mower deck back onto the deck hanger. Replace the battery compartment and battery. Unblock the mower.
    

im no expert but had a similar problem with an old craftsman mower. the belt keeper which is an L shaped piece of hardware located at the back of the deck next to the idler pulley spindal. Also could be the deck is cockeyed and needs to be leveled or the wrong v-belt.

check ur drive belt [ runs from the engine upper pulley under frame to the rear trans axle thru a series of pulleys and spring loaded adjusters] the belt has probably worn down or has streched to where it slips under a medium load

Hi. The belt must be trying to move a pulley that is either frozen, partially frozen or excessively loose. remove the belt and check the action of each pulley. If you have a frozen one, use some WD-40 to unfreeze it and if you have an excessively loose one replace the entire pulley assembly as it is worn out.

1) Remove the cover on the top of the belt and
remove the belt from the transmission pulley

2) Lean the mower back on its handle and secure it
with whatever you have that will keep it there while
you work on it, any heavy object will do

3) Remove the blade bolt, the blade, the blade
adapter and the plastic belt guard from the bottom
You should now be able to remove the belt

4) While you have the belt off, closely inspect the
pulley on the engine shaft, they sometimes get
grass built up in the groove and this will cause
the belt to run off and also wear prematurely.

Hope this helps you out

Chuck