@mustansir44
Congratulations on your marriage; Welcome to Masoom club now 
So your Khyber also went in your Baraat too. Masha Allah.
Was waiting for these updates since long.
Average is too low and this dramatic sudden decrease is leading towards the main culprit; Carburettor.
Go to Abdul Rehman Carburetor specialist at Tariq Road and get Carburetor fully overhauled.
Rusty tank, poor fuel lines are secondary but of course, they needs to be in optimal condition too.
For alternator issue, you can check with Abdul Malik or similar name for Alternator shop at 13-D traffic signal near Practical center for 2nd opinion after Maani.