Stihl bg86c running problem

[Tgw]

Hi im looking for some info to see if this is a common issue with the stihl blower 2017 bg86c-e 

 

Problem i have is when its warmed up and i use it at 1/2 to 3/4 throttle it sounds like its misfiring , (revs up and down very fast ) very annoying to use the blower at that speed and typical that is the speed that is best to use on the blower aswell , so far no matter what i adjust i cant tune it to run perfect all the way from idle to full , 

 

Starts , idles spot on , and revs spot on at full throttle and 1/4 throttle revs ok ,just the 1/2 to 3/4 is the problem 

 

Carb adjustment is per manual 

its a uk model with no spark arrestor

everything is clean (air filter,spark plug ) 

spark plug is the original bosch from factory (only done a few tank of fuel,  its not a tool i use much ) 

Plug gap is set per manual

mixture is a 97 ron super unleaded fuel with stihl hp mineral oil @50:1 (i use this in my stihl strimmer with no problems at all) 

 

anyone else had this problem ?