by Old Skool Mon May 25, 2015 9:08 am
THAT for sure sounds like a lifter, however IF you had to ask about hydraulic lifters that puts your question into a different category,, LOL
ALL the big sixes in ours came with hydraulic lifters unless the engine was built as a racing motor and someone installed a solid camshaft.... LOL..... Some heads with non adjustable rocker studs but the 240 not one of them. The big sixes with a oiling issue in the rockers as the rocker end can wear out and the upper end is light on oiling pressure to begin with let alone an old motor with a lot of miles on it.
WARNING!!!!! DO NOT MIX LIFTERS UP IF YOU TAKE ANY OUT !!!!!!! you WILL flatten the camshaft out. Lifters and cam lobes wear into each other with each one wearing in differently than any other. If you mix them up you will almost positively wipe the camshaft out!!!!
Try adjusting them firstly as it may just be out of adjustment. The book calls for 1 1/2 turns which I dont use and have been adjusting valves since 16 LOL,,, I adjust them at 3/4 turn as it puts less pressure on the lifter and if a person happens to over rev then the valve has less distance to stay open and hit a valve......
Anyways, the correct way to adjust them is to bring the motor up to operating temp so that everything has expanded to were it should be, however as yours is so bad I would find the one that is bad and look at it to see if some thing is drastically wrong like a bad valve, broken spring, etc, etc,,, bent push rod could do it easily also. Anyways,, if you cannot see anything obvious then start it up and find the one(s) that are making the racket and tighten the rocker nut until it JUST starts to go away,, then let the motor get to operating temp.
Start with the first rocker as you will be doing ALL of them to make sure they are all adjusted correctly. Parts wear and so others may be out of adjustment also??
Loosen each one as you go until you JUST start to hear it make noise and then SLOWLY tighten it down the 3/4 turn. Like I said start with the 1st one and do all of them. IF this doesn't solve the issue THEN shut the motor off and take the rocker off of the one that is not adjusting correctly.
Again,, inspect the valve and try to move it sideways to see if there is a bad valve guide, so on. If not THEN I would remove the side cover and pull THAT lifter and inspect it. It may have dirt inside the plunger and not letting it work. Put in a new one (it will not have a worn surface and you dont have any choice anyways) put in the new one and then adjust the valve temporarily.
I usually try to soak the new lifter in oil in order to get the air out by standing it up in the oil and pushing the plunger up and down with something to expel the air while submersed.
To adjust it temporarily before the final adjustment after the engine has warmed up,, I turn the motor over while watching the rocker arms for that cylinder. Which ever valve you are working on you need to watch the other valve for that cylinder close (meaning that rocker comes all the way up and then goes down),,, this tells you that the valve you need to adjust (temporarily) is getting ready to go through its cycle and when you see it open the valve and then come down you can adjust it.
I roll the motor over while watching the opposite valve open and close and continue rolling the motor over until I see the one I am working on open and then close. THAT is when I stop turning the motor over and tighten the valve about 1/2 turn.
Start the motor, let it heat up and then do the final adjustment like in the begging of this reply,,
Old Skool
Someone could have floated a valve or who knows what? Never have seen all of them go at once unless it was a small block Chevy. I personally am not sure I would want to put that much NEW wear and tear onto the OLD camshaft lobes. Those lobes have had enough abuse and IMO not worth the bet?? To me, its like doing a valve job on an engine and THEN it blows the old rings out. 
IMO 
