Received new bike 250cc Zongshen. Doesn't run to well any ideas. It idles good but when third 1/3 throttle opened it pops and back fires but if at full throttle it disappears and goes good till you back off again.Bike is new and has done it since new ridden only twice. Rang supplier and have done his recommendations to no joy. So far checked valve clearence, adjusted fuel mixtures in and out from original setting, checked plug, checked air box for oil and obstructions, tighted all manifold bolts, Drilled out fuel outlet in tank and changed fuel line to 1/4 hose to get better fuel flow. Still no joy?