How do I tighten the motor mounts on a John Deere LT133 mower

sound like you need to tighten idler pulley

1998-2001 ...is the LT133

Follow fuel line from carb. to pump

1/2 x 144 1/2

try this

john deere service web site

Visit the on-line store, know as reliable-store, they have the repair manuals. I'm sure you will get the solution of your problem.
Best Manuals

If your JD has a float type carb, the problem lies in there as the float is bad or the needle stuck in the open position allowing gas into the crankcase via the cylinder. rebuild the carb and check the float to make sure there is no holein it causing it to sink. Needle and seat should be replaced and new one adjusted. Also you may want to change out the oil with the gas in it as it will destroy the bottom end of your motor if left mixed in.

Your local john deer dealer will have these diagrams.

Go to this website, upper right hand corner and do a parts look up. Search for your model and that good stuff they ask of you and then choose mower housing should be there, browse the site it's really cool and has a lot of info.
http://www.jackssmallengines.com
I did a sample search for generic riding mower deck and found great diagram of belt routing. Give it a try you will find same results Good Luck! Don't forget the thumbs!