Sound like it is running too rich. or there is an exhaust leak under the cowling reducing the O2 available for combustion
maybe the rubber plug over the pilot jet came out in a carb or maybe the fuel pump is bad leaking fuel into #3 crankcase.
but then I found on my C40TLRX someone had drilled out the main jets before I got my hands on it
took me a long time to figure that one out.
like you I could remove the cowling and silencer , and as long as I ran agaist the wind it ran OK but with the wind it had trouble at high RPM
does it repeat the part where it runs well while cold?
if so maybe your prime start system is not turning off extra fuel.
have you tried turning the red lever to off?
