My Alto VXR was behaving bad when starting on CNG. When started it gave no problem. The CNG mechanic told me that; The kit is leaking from inside and "overs" and the gas accumulates in carburetor and air filter etc. So car does not start because of this over gas. He fixed the kit by replacing a faulty valve from inside and the problem was solved.
But I wanted to discuss this thing in technical terms with pakwheels guys. Alto has a 1000 cc engine which means only two rotations of crankshaft are enough to take in one liter of air/gas from carburetor/air-filter and expel out of exhaust manifold. So any accumulated CNG around carburetor should be "cleaned off" and new air should be entering into carburetor and the engine should start. Or at-least after three or four or five rotations engine should start. But it never started. So the reason might not be that the CNG accumulates but something else. What do you say? I think many CNG experts are present here on Pakwheels so their opinion can surely satisfy me.