Not sure if you have solved your problem yet. There is some good advice here for you but to to better help you resolve this we could use a little more information.
You say the motor stalls when you put it down? So that means when its on a starter box or you are holding it in the air it runs ok? When it is in the air and running are the wheels turning at all (like its running in gear) or are the wheels stationary while the motor is running. This should help identify if its a clutch issue or a motor issue.
If when the car is running (while the wheels are off the ground) and the wheels are turning then there is a clutch problem. What is happening is when you put the car on the ground the traction is putting load on the motor via the clutch causing the motor to stall. Think of it like driving a real car - try taking off from stationary in 4th gear with no revs - the car will stall.
If when the car is running (while the wheels are off the ground) and the wheels are NOT turning then the clutch PROBABLY isnt the cause of the motor stalling. This may point to a tune problem.
If you could comment on the above we will try to further identify the problem.
I have known of similar issues caused by the body blocking the carby intake. Runs fine while holding in the air by the body (as body is flexing away from the carby) but once the body is released it goes back to its natural shape and blocked the air flow.
I dont know half of what the people who have already commented know but more info = better answers