Welcome to AF.
We need more info.
What engine?
How many miles on tune up?
What kind of plugs?
Replace plugs and wires or switch them from another cylinder.
Check for good compression and spark to that cylinder.
Any vacuum leaks?
Is the plug wet or dry on the end?
How is the dist cap and rotor?
Check fuel pressure and camshaft retard setting.
Check for good injector pulse to that cylinder.
And make sure the injector is pluged in good and making cnnection.
If the injector is bad you do not replace the unit on that year they can replaced seperate.
Let us know what you find.