Two-pipe steam system troubleshooting

It is very possible there are no main vents on the mains. There must be venting, either on the mains or the returns, depending on the type of two pipe system you have. Post some pictures of your boiler and the piping and any unusual contraptions that might be on the piping or near the boiler.

In the mean time, make sure the pigtail that your Pressuretrol is mounted on isn't clogged. If it is clogged, it will let the system pressure build higher than the radiator vents can handle and they will open. Also, make sure all the two pipe radiators are pitched towards the return side of the radiator and all the one pipe radiators are pitched back towards the supply valve. I am assuming the two pipe radiators don't have vents on them, right? If they do, post a couple pictures of the radiators as well so we can see what you have. Most two pipe systems have traps on the return side, at the floor. Those traps, if working keep steam out of the returns. Very important.

Looks like a two-pipe air vent system. Keep the pressure down to ounces, using a vaporstat, and a low pressure gauge, graduated in ounces, alongside the standard, but useless required 0-30 psi gauge.
Find a place in the supplies after the last radiator, where a main vent, such as a bigmouth can be tapped in, as the radiator vents cannot be able to allow the air to escape without burning extra fuel to force it out.
Finally, adjust the inlet valves in small increments, to limit the amount of steam coming into the radiator. Open the outlet valves all the way. The radiator should just have enough steam in it to keep the outlet valve only condensate warm, and not steam hot when it is very cold outside, and the boiler is working hard. Caution everyone not to play with those valves on pain of death!
Finally, get a burner expert to check the operation of the burner, using a combustion analyser. Depending on its size, it may be possible to downfire it to match the radiators’ capacity, known as EDR.—NBC

I see in the photos that you have two returns which are tied together below the water line -- as they should be -- and then feed into the Gifford loop (which is fine where it is). Ideally they would each have a main vent, as well as one on the steam main(s) -- but I don't see a particularly simple place to install one. Are there any handy dandy unions back along the return piping somewhere?

I see 2 dry returns dropping down, but is there also a wet return coming up from the floor?
To the right from the front of the boiler, there is a pipe coming up from the floor. There is a metal plate on the floor.
Is that also a return?

Does each radiator return drop down into the wet return by itself?

OMG!!! We found a steamer that was piped correctly!!

You may benefit by trying to add a main vent on each of the two risers going to the upstairs rads, but perhaps not, maybe just aggressive radiator vents like a Gorton C or D on each rad to expedite steam delivery to the upstairs.

Main vents are critical for efficient operation of the system and should be installed on the supply main, after the last radiator takeoff as others have said.

Not too familiar with two pipe systems with air vents on the rads, but it may be possible that there are orifices in the supply or return connections to the rads that already limit the amount of steam going to each one. Try doing as Nicholas Bonham-Carter suggested and turning the supply handles down to limit the amount of steam. If it doesn't really make a difference until you actually turn the radiator off, you likely have orifices installed and can just open the valves all the way and let the orifices do the limiting.

FYI unfortunately, the Megasteam (while being one of the best steam boilers out there for any fuel) is not convertible to gas as per Burnham at this time and would void the warranty. (If this has changed someone please let me know!!).

A common problem with old steam systems with buried returns is that they have a tendency to leak after 100 years or so of being full of water. You can check this by turning off the water supply to your boiler (after verifying that the low water cut off is in good working order and functioning properly) and monitoring the water level over a period of a few days. If you are losing water and it is not showing up anywhere, it is likely that you have some leaks in that buried wet return.

Still can't believe your boiler is piped correctly, it's a rare occurrence, you should feel lucky

If your upstairs radiators are still staying cold there’s a few things I can think of. First is if your returns on those radiators go into a dry return you more than likely have orifices in your supply valves that are plugged, hand wheels are broken inside causing them to stay closed or your vents are plugged. If your vents are screaming then you need main vents on your dry return but make sure they are after your last radiator. If you don’t have orifices and it’s dry return then you need to get rid of your radiator vents and install thermostatic valves on the return of each radiator with a main vent at end of the dry return. Now if those are all dropping into wet returns separately then more than likely air vents are plugged or hand wheels are broken inside and in this case you’d want your main vents on your supply side after your last radiator. Your radiator vents will take care of the return for you. Please Note if you don’t have thermostatic valves on your radiators upstairs your radiators have to each go into a wet return alone they can’t be hooked together in any of the dry space or you won’t have any pressure differential. If you do have thermostatic valves on the manifolded radiators and it’s wet return try putting main vents after the last radiator on the return side up as high as possible from the boiler water line and above the steam supply main. That will help get the air out of the returns. All I’ve got for now my friend