Hiya,
Firstly check the glow plug, even if you think its ok change it to see if that helps.
Then if you havent done so already go back to the factory setting on the HSN, and see if you can run ok rich. Then slowly lean it out.
Failing that, check fuel lines and fuel filter to see if theres a problem with your fuel delivery.
Check the fuel it could be stale, try some from a new bottle.
Next take the carby to bits and give it a good clean out.
Just try and eliminate all the possibilities, starting simple and getting more complicated if and when you have to.
Also theres a possibility that theres an air leak somewhere.