I don't think any of the carb suggestions can cause the blow back at the carb. Bad ignition timing, but as reported it's been checked, or sticky valves or over tight valve clearances are to my mind the only way that much cylinder compression can be blown back through the carb.
I assume you are still using the mechanical fuel pump. You could try running the engine with the fuel line to the carb disconnected (use something to catch the pumped fuel) to see if the pump is working properly. The engine will run for a while using the fuel in the float chamber.
Are you still getting the blow back from the carb?
Iain
PS
I had problems with my mechanical pump and changed to a low pressure Huco sited next to the battery. I did fix it on rubber mounts where the mechanical pump was but it suffered from fuel evaporation.