First check all the simple stuff.
Make sure the valve clearance is right and the air filter is clean.
Spray some brake cleaner around the throttle body to see if there are any vacuum leaks. The engine rpm fluctuate if there is.

Check the spark plug this will give a good indication on how the engine is running black is to rich.

Also check the electrical components resistance. Injector, tps etc... the correct values are in the workshop manual and Haynes manual.

In my case I had a ground problem on the injector cable under the tank.
Remove the tank and check the cable for wear and tear maybe it is making contact on the frame.

If this is the case the injector will always give a full blast of fuel resulting in bad running, black smoke, fuel smell from Exhaust and off course bad mileage.

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