Hmm...

Definately sounds like an air leak is causing the problem, if you have ruled out a clogged HSN assembly in the carb. If you've already checked the lines and tank for leaks then the next step would be to seal up the engine. There is little point in actually checking the engine for air-leaks, as it is time consuming and requires the engine to be removed from the chassis. Instead, just use tepmerature safe sealent to seal the base of the carb to the crankcase (fine bead around the top of the block and then re-fit the carb to seal), the backplate to the block (remove, seal, re-install) and the exhaust manifold to the block (same deal again).

If you take your time and seal it properly you will eliminate and air leaks, and reduce the chance of air leaks occuring in the future.

If you do this and the engine is still not working properly take a look at the HSN assembly. If there is ANY gunk/dirt etc here it will screw up the flow of fuel to the engine, and give you no end of tuning problems. Just unscrew the HSN completely until it comes out and give it a blast of compressed air/WD-40 etc.