Why does my mazda etude 1.8 sedan 1995 have high fuel consumption on 1995 Mazda Protege

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My mazda etude 1.8 litre sedan 1995 model is very heavy on petrol. It does about 430km on a 50litre tank and I do not know how to fix this problem. I bought it recently and it has had the same consumption since. It has 244000km on the clock. I serviced it recently, changing the spark plugs, oil and air filters but it is still doing 430km on a full tank. The timing belt makes a slight squealing sound and the car sometimes jerks a bit during acceleration when I am driving on the highway. Could this be what is causing the high fuel consumption? What could be causing the jerking and what should I do to get a fuel consumption rate of at least 12km on a litre.
(3) Answers
Check between mass air flow sensor and throttle body for any leaks or cracks in intake hoses. Check accessory belt(s) for proper tensioning.
These BP-engined Etudes had poor fuel consumption upon release, but it can be repaired.
I drove one for 7 years averaging between 11 and 12km/litre.
Timing needs to be adjusted to around 11 degrees BTDC and mixture leaned out - you will need proper equipment to be able to do this(agents). The exhaust system is very restrictive; I fitted 50mm s/s piping(3 x 57mm boxes).
Problem solved. A less restrictive air filter will also help.