1988 F150, approx 150,000 mi. seldom driven yard work truck. Had been running reasonably well until recently. Cat was removed years ago. EGR was disabled [sealed at port] also years ago.

Starts instantly but barely idles when cold. Somewhat better when hot. Vac about 9 in. at idle. Have used two cans carb cleaner looking for Vac leak. No results. With careful pumping of throttle, can get up to 2000 rpm, sometimes 3000 rpm. Vac drops to zero above 2000 rpm. Goes to 20+, momentarily, on deceleration. Severe back firing over 2000. No power. No codes other than the obvious, EGR. Have not, as yet, checked compression. Ignition looks good on all 8 but sounds like it is running on less than 8. Very rough with severe vibration.

Have no clue where to go from here.

Thanks for any help.

 

I would say the timing chain is probably loose,or has jumped time.