80Km/h is a very bad speed indeed.
My Cuore on CNG goes upto 120Km/h pretty easily.
But i did have a problem with my cuore's acceleration n top speed. The trouble was not with the CNG but with the car's 'TAPPETS'. TAPPETS dabe hue thay.
I don't think that would be the problem with ur Alto cause ur car is new.
Best of luck for the future.