Chainsaw won't stay running

I have a proplus chainsaw (Chinese brand), I took it out the other day after it was sitting up for a few month. It fired up and ran but started to cut out when revved it up. It has gotten worse and now it will fire up but it over revs and cuts out immediately.
 
I have changed the main fuel line as it was perished and cracked. I have checked the tank vent and it is okay. I have cleaned the carb and replaced the diaphragm, needle and spring, and gaskets, but it is still the same. It seems to me like a fuel problem but I cannot figure out where the problem is. It has a walbro carb on it but I don't appear to be able to adjust the H and L needle on the carb. 
 
Any help or suggestions would be greatly appreciated. 
All the best 
Paddy

There isn't any "Limit" covers on the L & H air screws. I thought I couldn't adjust them but I just need a new tool. They are D shape fittings, not the flat head I am used too. I have a tool ordered so I will try and adjust them to see if that helps. 
 
I have noticed that there is is pressure in the fuel tank after it has being trying to run. Is this normal or could it be forcing to much fuel trough the carb into the engine?