I have a ford f600 cab over with a 391 engine. It has bad compression in one cyl. Like 40 pounds. all the other have 110 to 130 lbs. I use the chassis to run a pto and a mounted pulp loader. It doesn't have enough power left to even run the pulp loader. Any load on the motor and it bogs down and quits. I know that the motor is tired but it has enough to get down the road at highway speed so I dont understand why it quits with loader operation.I think the bad cyl is caused by a burnt exaust valve. I put that cyl at TDC and add compressed air and it blows right out the exaust. What is the difference between the 391 and the 390 that is in light duty cars and trucks? I am gonna tare the top end apart and check valves and see if the cam has any lobes left. Is a 391 worth rebuilding or are they junk? I would like any info about 391 that anybody has to offer. I love to read on stuff that I have and work on. THANKS!