Can you still have sticking valves without any evidence of bent pushrods?  And can this be diagnosed with the rocker arms and shafts removed?   I removed the rocker arm and shaft assembly from each head and checked each pushrod individually.   All looked good to the naked eye by rolling each one on a level surface.   As background, I purchased this 56 Thunderbird with a 312 about a year ago.   The prior owner had health problems and the car had been sitting for about five years.  The engine had been rebuilt in the past and can't have many miles on it.   The gas tank was represented as being "cleaned and flushed" but was actually disgusting with a large amount of sediment on the bottom.  I did not try to start it after taking possession until the gas tank was replaced and other fixes taken care of first.   I was finally ready for a test start up.   It caught fire immediately but ran very rough and died shortly.  I tried it a couple more times without success.  It ran before I took delivery but I suspect the damage had been done due to the bad gas tank and varnish deposits in the valve guides.    Thoughts and recommendations appreciated!

I will suggest performing a cranking compression test on the engine.  Be sure to do that with the throttle in the wide-open position.  That test by itself will rule out a majority of the mechanical issues that can be happening with the engine.  Or on the other side of the coin, point you to what is actually going on.

Lorena, Texas (South of Waco)

Thanks Ted.   I was thinking that might be the next logical step.

Willie, be sure to pull the coil wire out of the cap before cranking it with the throttle wide open.

John - "The Hoosier Hurricane"

At risk of scope of work expansion and this turning into a saga I had to ask myself why I had a mix of 7/16 and 1/2 inch adjusting nuts on the passenger side?    But thanks to this forum and John the HH this was quickly answered.   And it gets better.   Valve covers are aluminum but valve cover studs are not 4.25”.   So the studs were not bottomed in the heads and had extra washers on top of the rocker stands to fit the aluminum valve covers (aarrrghhh!).   At a minimum I will double check the rocker shafts and arms on this side.  Stay tuned.  

From past experience pushrods will not cause sticking valves. with them out i would check to see if lifters might be sticking or possible a bent valve causing it to stick, visually 1 valve may look a little shorter or slightly different angle. While apart if you have a compression tester, remove the valve core, remove the oil fill cap, put something on intake and exhaust to let you know if air is flowing through it, make sure nothing is in the way of when the crankshaft turning and normally leakdown test are at 100psi, id recommend at least 50 psi to each cylinder, listen for where the air is coming from, with rockers removed no air should come out of the exhaust or carb, it will automatically put that cylinder to bottom dead center, if crank doesnt move you either got really lucky or have a massive leak or resistance for that cylinder to move. only the out the crankcase oil fill cap. can also compare how much is going into crankcase by sound but it will only indicate an obvious fault. If nothing, lifters look good, cam lobes look good. i would put it together and start at the basics. was fuel lines blown out and carb rebuilt after fuel tank replaced. hows your timing, did you replace your points cap, coil wires plugs etc. age can kill all of those also. good luck let us know what you find
Seth