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I have not run into this issue before. For my 1940 Super Eight. Had to let it sit for around three months before tried to start the car. Ran fine for a couple miles, got some gas. Then started to run very rough, but nursed it back home. Had to let it set for a few more weeks. Now cannot get it started. Today got a friend that owns a local garage to assist me as I thought I was missing something obvious. There seems to be spark everywhere and to the plugs. Tried a new set of plugs, just in case that was an issue. Still nothing. Tried some starting fluid, and got just one backfire through the carburetor. Checked compression on one cylinder nothing using a compression checker and the finger test. Then checked three more cylinders, no compression. My friend states that when cranking there is even sound so sounds like all cylinders not compression. He put some oil down one cylinder to see if it makes a difference, did not. Blew some air down a few cylinders and comes out the carburetor. He says letting it sit and some bad fuel has stuck the valves open?
Before I pull the head any thoughts? Thanks very much, as always

Stuck valves is what it sounds like to me too so best to remove the head to properly deal with it. As an alternative, maybe pulling the plugs and filling the cylinders with diesel oil or even regular oil and letting it soak for a couple weeks, keeping them replenished (most will just drain through the stuck valves unfortunately) and keep the plugs loosely in the holes to keep out contaminants. Just be careful as when I had that happed, the valves had gotten so built-up that the engine actually stuck with too much pressure against the cam causing one valve to get bent during removal. The material on the valves was as hard as a rock!

I could try letting them soak? Do I drain the oil out first if I use diesel? But if no other answers probably best to just pull the head? Thanks

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This is exactly the test and diagnosis I would have done. Unfortunately I don't have much advice pertaining to unsticking the valves except to not use more than month old gas in it next time. If you don't drive too much, just have a couple gallons in the tank and get a couple more on each trip. (Probably obvious advice )

Removing the head is the best thing to do but if you want to try soaking first and if you use diesel oil, no need to drain the crankcase at this time but you will need to before trying to start it. A little diesel oil won’t hurt but the debris that was loosened and in it definitely will!

Whenever my cars are going to be parked for an extended time (which is all the time) I always put a fuel preserver into the tank. When I went to drain the tank in my 1954 Patrician that had been parked for decades, I was surprised to discover that the petrol still smelled as it should. Of course, it wasn’t very burnable but it hadn’t turned to into something else either.

Will get some diesel oil, won't hurt? When you say filling, it that to where you can see it looking down the spark plug hole? Filling it up to the hole? Then keeping it topped off. Thanks

Yes, to the top but it won’t stay filled with stuck-open valves. Most will just drain back into the crankcase before it has much of a chance to do anything useful. Probably taking the head off is your best bet now that I think of it.

Unless there is a lot of luck on your side I don't think you will be able to do much with freeing the valves by trying to fill the cylinder with diesel oil. The spark plug is over a valve so not much will get to the stem that way. Even if you fill the cylinder, if the valve is closed it will not let any pass and if stuck open, unless there is some kind of air disturbance to make a mist and force diesel to reach far enough to coat the stem and drip down into the guide, oil will just drain into the intake or exhaust manifold. All in all it sounds like more of a mess without much if any that will get onto the valve stem and guides.

Some have had the valve issues which seems to be caused after starting and running the engine on old stale fuel. When trying to start it again after a few days of rest the valves are found to be stuck. Whatever the cause, speculation is an additive must deteriorate with age and after being burned must deposit on the stem where it cools and seems to work like expensive glue. A few engines have been so bad it took disassembly and then brute force to the point of breaking a guide or bending the valve to free it.

I'd pop off a the valve side covers and crank the motor over to watch the valves. Then you'll pretty quickly if and what valves are involved.

Probably stuck valves. Do like Kev said and remove the valve cover to see which ones are stuck. I've had success with putting Marvel Mystery Oil in the cylinders and letting it soak for several days. At the same time I sprayed the valve stems from the side with penetrating oil. Over the course of several days I turned the engine by hand to work the valves up and down. Over a few days the valves started to move slowly. Eventually they moved enough to feel comfortable to start the engine.

Once the engine warmed up the valves freed on their own and the engine ran smoothly. I did this with two cars and they have been running fine since. Stale gas is your enemy.