I just bought new denso spark plug for my GS 150cc and narrowed down the gap from 1mm to 0.8mm in order to boost the performance , i didn't notice any improvement though. Now i'm experiencing sudden shutdowns of engine as i release the throttle, and backfire problem is even worst now. Any help would be appreciated

The spark plug i removed was from NGK with standard gap of 1mm and it was working like a charm, the reason why i removed is because it has done 10,000KM.