HEY all, Merry Christmas. I realize this is not a Vette problem but ,I still need some help on my 1978 Chev truck 350. I use this truck in the winter so I can keep my Vette in the garage. A week or so ago I was coming home from work and it acted as if it were running out of gas with a full tank so I replaced the fuel pump no help then I believed I was getting a leak thru the distributor cap , I replaced the cap and rotor.Now I believe I may have a bad lifter or valve spring this is where I need some help.It pops thru the carb at idle but not every cycle more like every 3rd cycle and it stays pretty consistant any suggestions. Please forgive me if I am placing this not being a Vette But I need to get it repaired and I am not in a postion to pay the going mechanic rate. Thanks to all

 

I would idle the truck and listen carefully to each valve cover. You may be able to hear which cylinder/rocker/lifter/valve is bad. Pull the valve cover on that side. Carefully inspect each rocker, see if one has noticably more wiggle than the others. Look to one that looks to be significantly out of alignment with the others. Look at/touch/wiggle each of the pushrods to see if one is bent or if there is catastrophic lifter failure. Inspect the valve springs/retainers. If you can't see anything wrong start the engine (with the valve cover off, you will get some oil spilling, but not too much, and see if each of the rockers moves like the others, the same amount, frequency and duration.

It might also be oil pressure related if it does this only at idle. It could be that the lower oil pressure of idle can't keep one of the lifters from collapsing.

It might also be that your ignition timing is a mess, due to bad vacuum advance diaphragm, bad timing gear, bad advance springs, and occasionally fires VERY early (too advanced) in the cycle. An intermittent vacuum leak could do this. Have you adjusted your ignition timing?

These are, of course, just guesses based on your post. Try these and keep us posted. Good luck

 

Bob has some good tips there...try them..the problem Ive just read is the diagnosis is too vague..if you knew for sure the lifter or rocker area was indeed bad, you could go from there.It can be a carb problem,slipped or worn timing chain,timing out of whack,
bad cam lobe,vacuum problem,etc.Its difficult to say for sure without seeing the truck itself.

Internet trouble shooting means we all gotta take guesses and throw out any ideas.Ive seen the best ideas turn out to be it wasnt even what was wrong to begin with,but good ideas anyways.
Good Luck and hope you get more ideas to check out.I know the shops today are getting way expensive to have them diagnose 50/50 whats wrong with it.
:)

 

say dennis try posting here
http://www.73-87chevytrucks.com/

 

Funny, that is the symptom that you will get when you have a cam lobe starting to go on you. Obviously, it could be a collapsed lifter, broken valve spring, or burnt valve.

The earlier advice is solid. Remove the valve cover and eyeball the valve train.
Purp

 

I suspect a worn cam lobe, most likely an exhaust. You can short out the sparkplug wires one at a time by using an old fashioned test light (one with a bulb in it). Just ground the wire on the test light and slide the probe between the plug boot and the wire until a ground path is made. You should hear a difference on the offending cylinder. Then remove the valve cover, unhook the pink wire at the distributor cap (to disable the ignition) and have someone crank over the engine while you watch the rocker arms for equal movement.
If you find one not moving or not moving much, you have no choice but to replace the cam and lifters.