Brother regarding the starting problems of your Mehran, i would suggest to get the accelrator settings on CNG. However, you may inspect the joints on CNG lines by your self through the surf mixed water. Drop the water on the joints and check the leakage if bubbles come out. Secondly, if bubbles come out then shut off the main valve near cng cylinder to save CNG.
Secondly Please visit the CNG mechanic who is a certified mechanic can resolve your issues if persists.