My 90 302 has a miss and I have changed Rotor, Cap, Wires and plugs. It still misses on #7 cylinder Could this be a injector and How do I check it to see if it is working properly. Also the plug that I pulled from that Cyl. is slightly gray at the electrode tip. Thanks Don

 

Swap an injector from another cylinder. If the problem moves, its the injector. If not you are dealing with some other headache.

 

Check to make sure the plug has spark while you have it out.
One thing you can also do is crank the motor with the coil wire off and then pull out plug # 7 if its not wet with gas you most likely have a bad injector.
Had one go bad on my 90 had the same symptom's you have.
If you do need one take the old one with you when you go to get one.The darn dealer sold me one for a 351 and I have a 302 !

 

It seems like the injector might be somewhat working at higher RPM'S but below about 3000 it is missing. Is this normal for a injector. I have a couple cans of seafoam in it but it isn't helping.

 

If the coil is dying in the injector, yes, it could be an intermittant problem. Could be that its just not as noticable at higher rpms.