If it does it immediatly when you let off the throttle or when your running at high rpms, your probably running lean on the high side.
Also, if when you let off the throttle it tends to stick with the high rpm's for a second, then drop down to idle, your lean on the high.
If you pinch the fuel line while it's idling about 2 inches from the carb, it should start to stall around 3-5 seconds. If it's more than this, it's to rich on the low, if it's less, it's to lean on the low.
Also, on the low, if you let it sit and idle for a 30 seconds to a minute, then peg the throttle and it blubs on you, then opens up, your to rich on the bottom.
If this continues after checking the above, replace your fuel lines and seal the carb neck where it goes into the block with high temp sensor safe silicone.