I have a 1987 Ford F150.. Straight 6.. Manual Transmission & 4WD.. when trying to turn the key to start it turns over but almost like there is no power. We've tried a known good battery and also boosting off a running car and still when we turn it over it sounds like there is not enough power. We've replaced the starter as well.

 

it could be that there is no fuel pressure. it might be bad fuel pump relay. Can you hear the pump running? or no ignition spark, - it could be a bad coil. try also looking at dirty connection - try to clean everything. it would be best to check all your ford f150 parts and see if there's other unsual things that you notice. give us an update afterwards. good luck..