Try swapping with a known good carb. If problem still happens then problem is in the motor. If it fixs the problem, then it is the carb. When you broke down the carb, was the bowl full? if so, you could have trash in the idle paassageways. I had a problem like that and I took off the bowl. I had to take pipe cleaner to them and carb cleaner. I got a lot of varnish from old gas out it. I took the jets and cleaned them with cutting torch tip cleaner(wire type). Put it back togather and has been running good ever since. You can try this, but I am no carb expert like Superman is.