@danishijaz
Please share the actual mileage of the car. Do you have it since factory or are you a second or third owner. Whats the oil change duration. This gives the general idea about engine condition.
There can be 2 to 3 reasons to your problem. But I fear that you are only aware of the services you mechanic performed on your car (like servicing carburetor, changing fuel filter, changing plugs) without the details. You have to confirm if the carburetor was cleaned by thorough dis-assembly. Also share the ignition timing on CNG and petrol. I am sure your mechanic used intuition over a timing gun.
First of all, if you car runs fine on CNG (by "fine" I mean accelerates good, touches 120 easily on highways, idles stable after warmup, does not reduce engine oil, does not heat up), then your valves cant be the problem. And neither do the plug wires seem to be faulty. Your mechanic is screwing up with you.
Faulty hydraulic lifters or burned valves give idle issues and don't allow the car to accelerate that easily.
To trace the source of this problem on car that has been mostly run on CNG is that, the power piston or power valve (the one responsible for richer mix under Wide open throttle positions to prevent pre-ignition) can have play. This is because of excessively driving the car on CNG. The piston operated under dry conditions (no fuel in carb during CNG use) causes wear on the valve and eventually when the driver shifts to petrol, this piston allows unregulated fuel to be delivered and thus causing jerks in low load conditions.
This is what it looks like
The second thing can be the plunger valve. This is responsible to avoid jerk when you press the accelerator each time. One has to check if the port opening in primary is clogged in some way or not. In short, you have to observe its port with the air cleaner assembly removed to check if it throws a stream of petrol when you pull the accelerator cable slightly.
The third issue can be your fuel pump. The FF CNG cultes upto the year 2006 have a directly wired pump without cutout replay. What this does is that the pump keeps on running when ignition switch is ON position even when you select CNG from CNG switch. Only the fuel line solenoid is turned off. This eventually causes the pump to go weak and cause lack of fuel supply under high load conditions.
Even after all these factors, the most important one comes below. First confirm this
When users complain that their Cultus consumes alot of fuel, the mechanic lean the mix and retard the timing to compensate. The car runs okay for low load conditions. As soon as you hit the highway, problem starts unveiling itself. So do confirm this if such a requirement was made by you to your mechanic to improve the mileage.
