I bought a house with this toliet but I have no manual. How can I get a manual/

Then you either have to lower the water height so that it doesn't overflow the overflow tube, or you have to replace a seal on the ballcock or replace the ballcock with a modern style.

A vent would cause an odor problem but not a back up problem. There should be a clean out between the house and septic tank, if it is full the blockage is down the line, if its dry the blockage is in the house.

Actually sounds like trash in the toliet water valve.....or possible that if teflon tape was used to seal threads has been drapped accross inlet threads and caused blockage......Best way to check this is to take supply line loose from toliet and slowly turn water on while pointing the supply into a bucket or something to catch the water and see if the pressure is ok......Then check valve to see if grit or blockage is in the toliet valve.........Depends on valve design to whether or not you will be able to clean....may have to replace toliet valve with new one......hope this helps

you may have too much water pressure leading to your house, it should be between 70-80 pounds if its higher you can turn down the valve that shuts off the water flow to the tank, if that doesn't work feel free to contact me

Adjust the float height to allow mor water into the tank before it shuts the valve

It sounds like a vent problem.
On top of each house is a vent stack.
There should be a vent stack above each toilet, and above kitchen sink.
Look for pipes sticking out of roof.
These pipes are open to the sewer line.
When you run a column of water down a drain, the vent stack lets air flow in front of and behind the column of water.
This air flow prevents the gurgling sound.
The gurgling sound is the drain water pushing air in front of it because of a clogged vent.
If you have a two story house, there are vents that run between floors to prevent the air flow from gurgling or bubbling sewer air into each drain as water column moves down pipe.

http://www.bradyinspects.com/plumbing-vents.html
http://chestofbooks.com/home-improvement/construction/plumbing/Standard-Practical/Venting-Part-3.html

Look for bird nest over vent pipe on top of house.
Run a snake down into pipe from roof and see if the is a stoppage.
You many need to call plumber to track down each vent and the cause.
For example if there are quick-vents installed inside a wall and the quick vent is no longer working.

You can also take advantage of fixya phone support.
Fixya expert will walk you through the steps for a price.

They don't have rebuilt parts kits that I am awareof, but you can get replacement parts for them at most any RV service center. They are pretty easy to repair yourself.

Tool Belt Diva

first shorten the chain from flush lever to the flapper valve by one or two links at a time until flushing properly

second the flapper valve might need a wipedown as well as its seal (top edge of tube where water goes down into bowl) the leaking is because the flapper valve is not seating properly.. you may need to replace the flapper itself.. easy to do, and a common part at hardware store.

broken pipe inside the wall