The chain keeps falling off the largest three sprockets on the cassette, when pressure (e.g. during a climb) is applied.

This is usually the setting of the rear shift.
Check that the rear d?railleur is properly aligned when it is set to the middle sprocket in the cassette.
The chain must be fed exactly to the middle of the sprocket teeth

This varies from model to model.
Check that with the largest rear sprocket, and the largest front sprocket that the derailleur lower arm is not pointing straight forward,and smallest front and rear sprockets selected that the the derailleur is not all of the way back, and the chain is loose (Hanging slackly)
The new chain will come with links to spare, and you will have to cut it.

needs a gear adjustment hopefully. it might have a stretched chain along with worn cassette teeth. if you have ridden alot of miles in the smaller gears in the rear you might have worn them out. you would have to replace chain and rear sprockets/cassette

assuming you have the chain adjuster in the bar adjuster hole. This and the bar nuts will normally keep the bar in place and the chain adjusted. However if your drive sprocket on your saw is worn (deep grooves where the chain rides on the sprocket) it will cause the chain to try and climb over one tooth pulling the bar toward the saw loosening the chain. This can also happen if you put a worn chain on a new sprocket. In the end this overtight or loose condition will lead to failure of your sprocket nose bar. One other way can be the chain, the bar and the sprocket are not all the same pitch.

Hi Josiah:
According with the repair guide...4.0L Engine

1. Before servicing the vehicle, refer to the Precautions section.
   
2. Drain the engine oil.
   
3. Remove or disconnect the following:
   Negative battery cable Engine from the vehicle Oil pan Engine front cover Cylinder heads
4. Lock the jackshaft tensioner by installing a pin.
   Jackshaft sprocket and chain assembly Left front cassette retaining bolt Cassette chain and tensioner assembly Rear jackshaft plug from the engine Right rear cassette retaining bolt and spacer Right rear cassette chain and tensioner Timing chain (s)
   
   

Fig. Remove the jackshaft sprocket-4.0L SOHC Engine 2000-2001
Zooming link for Remove the jackshaft sprocket-4.0L SOHC Engine 2000-2001

To install:

1. Install or connect the following:
   Timing chain(s) Right rear cassette chain, tensioner and sprocket Jackshaft sprocket and chain on the engine and remove the tensioner pin
   
2. Torque the jackshaft sprocket bolt in 2 stages:
   1. 32-35 ft. lbs. (43-47 Nm).
      
   2. Turn an additional 65 degrees.
      
3. Install or connect the following:
   Cylinder heads Front cover Oil pan Engine to the vehicle Negative battery cable
4. Fill the engine with clean oil.
   
5. Start the vehicle, check for leaks and repair if necessary.

1. Before servicing the vehicle, refer to the Precautions section.
   
2. With the vehicle in neutral, position it on a hoist.
   
3. Remove the intake manifold.
   
4. Remove the fuel supply manifold.
   
5. Remove the accessory drive belt.
   
6. Remove the thermostat housing.
   
7. Remove the roller followers.
   NOTE You must retime the LH and RH camshafts when either camshaft is disturbed. Turn the crankshaft clockwise to position the number one cylinder at top dead center (TDC).
   
   NOTE The special tool must be installed on the damper and should contact the engine block to position the engine at TDC.
   
8. Install the special tool.
   NOTE The right-hand camshaft sprocket bolt is a left-hand threaded bolt.
   NOTE If necessary, use camshaft gear torque adapter to loosen the camshaft sprocket bolt.

Hope this helps; also keep in mind that your feedback is important and I`ll appreciate your time and consideration if you leave some testimonial comment about this answer.

Thank you for using FixYa, have a nice day.

Turn one of the two screws on the top so that the mech. (derailer) moves toward the frame a fraction of an inch. Keep adjusting this until it doesn't fall off.

Sprockets are all connected and turn together as a unit. The chain could be skipping over the teeth on individual sprockets.

I suspect severe chain and sprocket wear.

http://sheldonbrown.com/chains.html

ok there is three sprockets one on the transmission a guide in the middle and onle on your back axle. take a wrench and loosen up the middle sprocket. slide it all the way back untill the chain is snug or tight. tighten up your bolt and it should notice it drives faster and thats how you keep it from falling off. hope it works for you

OK, for a beginner in the world of 18 speed bikes, put the chain on the big sprocket up front, it should be about 46 teeth, make sure it is center and doesn't make noise, if it does, adjust it just a hair either way, now, the rear, put the chain on the big sprocket in the rear, make sure the front didn't start making noise, if it did, adjust it again. You are now ready to take off, in low gear, as you start out, it is easy to pedal, soon you will be peddling air, shift into second gear in the rear only, don't touch the front. Just a little push with the thumb will make it shift, now were going faster, easy peddling, shift into 3rd, then 4th, etc, coming to a stop, down shift the back sprocket back into low gear (big sprocket) and be ready to take off again. Soon you will be shifting that thing like a Ferrari! The front sprocket, will be used for moderate terrain, wheelies, and mountain climbing, that will come later, learn how to shift the rear first. As each gear climaxes, hit the shifter, it will get smoother as you go, and keep the chain quiet, no climbing on the next sprocket or falling out of gear. Hope this helps.

Does the motor continue to run when chain stops? Electric saws usually don't have a clutch, so I would look for badly-worn chain sprocket, or a possible sheared key in the sprocket hub, or a loose mounting nut. Remove the chain and bar, try to turn the sprocket by hand (wear good gloves). There may be some type of overload clutch built into the sprocket.