Even on my CG I faced this issue as I am using a Chinese CG Euro II replica.. it was doing 106kmh eapy pieasy on its stock china made plug I changed it to NGK stock CG plug (for no reason bas awein he) .. and I noticed bike is stugglinf around 95kmh...
mera shak tuning/tappets weghara per gya without realiseg that plug might have issue the I consulted mechnic he also chaecked plug its was fine and color was good...
I finally convinced my self that its a china CG its the max it can do.. but recently (and luckyily) the plug went bad and bike started missing I bought another same brand NGK plug and now bike zooms cross 108kmh in no time..
then just to make sure I pulled out plug again closely compared both.. both are exactly same but the results are totally different...
itni lambi story sunanay ka maksad sirf rahnumayee hai bike sab theek hotay hain bas ye choti moti chawalien jo parts manufacturer marty hain uski waja say bike sahi bhagta nahi...