This is normal dear, jab engine heat up hota hai to acceleration thori see increase ho jati hai, if you want to get rid of this just go to the mechanic when engine is hot ask him to tweak the race or you can do it by yourself. CNG kit mai gas increase decrease karne k liye ek knob hoti hai round shape mai anticlock wise karne se acceleration decrease hogi and clockwise karne se increase...
Same way there is another knob under airfilter cover for adjusting the race... Anyways its not a problem ...