I don't know the specs of the engine, I didn't build it and the last owner didn't know.Start here you need to make sure the engine is not sucking extra air.
Spray around intake and carb. Find leak.
Then you can start checking carb(which is way to small imho), timing and ignition.
Also what are specs of engine?
Cam
Pistons
Compression ratio
Heads
curious - what carb would you suggest?