toyota HJ75 Petrol - flat spot

Could be quite a few things , you say you have had the carbie done , but it could be quite easily the excellerator pump not delivering the right amount of fuel when you put the foot down , it could be a spark plug breaking down under load , even a plug high tension wire breaking down . Sometimes you can pick it up immediatley , sometimes it takes time :)

I had this and it was the diaphragm that opens up the secondary at about 1800 rpms. It costs about $30 for the part from memory.
It hangs off the front side of the carb. It doesnt get done in a rebuild unless you ask them.
Oh,and if its petrol ,its an FJ75;)

If you have a old style points spark system, try replacing the condenser. The condenser can give an impression of a carburetter flat spot. I have also had radio interference supression capacitors cause problems.

The second barrel sounds as though it may be the problem. Taking the air cleaner off and listening to the sound of the airflow will also tell you a lot. As the carburetter starts to starve, the sound if the hiss changes.

God, I love fuel injection ! no more cold choke and no more flat spots ! ! ! ! !

Dunedigger