heavy duty 391

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!

You have a burnt out exhaust valve.Depending on whether it has sodium filled stems or not, that could be a fairly cheap fix, the rest of the engine shouldn't need anything done to it.
The 391 is a heavier casting than the car engines,but the only things that won't interchange are the distributor and oil pump and drive, they're bigger on the pump drive. The bushing hole in the block can be drilled out so the 391 pump and dist. will fit in a 390.IIRC its a 33/64 bit.

Also the 391 uses a forged steel crank and is very desirable. It has a thicker crank snout than the std FE cranks. As mentioned by R Pope the 391 uses a HD casting and some have very thick cyl walls. I'd say for sure yes it's worth rebuilding.

G.

UPDATE!!!!!!
valve is burned out. I was doing good untill I slipped a ratchet and busted my lip. Now I look like BUBBA on Forest Gump.
What would make 2 of the cyl have a big amount of carbon build up? The intake weighs a ton!

Ouch!!!! Them dang FE head bolts are really tight, huh??? Which cylinders have the carbon buildup???

I think I read somewhere that the 391 was the 427 block casting. That would explain the thick walls and stronger bottom end.

Thank you guys for comming back to me with info. I never had much exposure to ford motors before. I always had small block chevys when I was younger and played around with engines. I do like looking at the fat motors though. I also have a 71 lincoln with the 365 horse factory 460 cid. Its really cool for a big car to get it done. It was the car that was on the T.V. show Jake and the Fat Man. It was sold at a universal studios auction and my uncle picked it up after that and I ended up with it. It has about 77,000 miles on it. It isn't show quality but it is very respectable. You guys have talked me into keeping the spare complete 391 that I picked up with the F600. I also have a spare 71 460 all complete for my lincoln. I like spare parts and my yard shows it.

50 bucks for a ( thats one ) exaust valve. Sodium filled. Would hate to buy a set!!!!!!!!!!

I did not know!!! Thanks. but can I wire brush the carbon off them with out blowing up???

yeah you can do that... you can even have them reangled... but you just cant grind too deep... its best to have a pro thats experienced with those valves do them

Grinding the face of the sodium valves is okay, the hollow stems are a ways away from the head. The heads are solid.

I never gave it a thought that the rocker arms are not adjustable. Can I do a light valve grinding compound to the valves? I am going to take all them out to clean them and put new seals in them. All the seals are cracked into little pieces. I dont have the money to have someone redo the heads for me. I am concerned that if I compound the valves then the rockers wont let the valves close. Should I leave the other valves alone? How do I check to see if the valves will seat properly and not hang open a little? Thanx for the info guys!!!!

Thank you . I will do some reading... Thanks for all the help .

IT RUNS!! CLEANED UP EVERYTHING AND PUT BACK TOGETHER.

ITS ONLY A WORK TRUCK NOT A SWISS WATCH. THANKS FOR THE INFO GUYS!!