@trojan
i am not expert but can tell u few things to check first.
*air filter ?
*oil change time ?
*replace plugs with original onces and tap a lil the plugs tip and narrow the distance of tip from electrode, it will help to solve the jerks and increase the pick,
make sure there all connections of wires are intact with kit and battary.
now final thing is most imp, make sure that gas flow screw is adjusted proper, like it requires abt 1/4 open approx, ( might wrong in few other cars ) but mostly its not more than that, u can do it urself easily, adjust the optimum ammount of opening, that will help in milleage.
set idle rpm , not to much not very less. like near to 900 and set petrol rpm at 1100 or 1200 , as per ur convenience.
try them or cross check them, cos i am sure most of them u have already checked,
another suggestion, talk with Bilal Sb, LOGAS, here on pakwheels, in cng section, LOGAS SHELF, he will certainly help u out,
Regards