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I don't know Roundhouse equipment but let me blabber a moment on a couple of your points.

1. Preheating the water in the microwave really should not be very necessary. It may help in reducing the time to get a head of steam up, but I doubt very much if it helps all that much... may depend on how long the water sat in the boiler before you got the fire started.

7. The safety should NOT lift at 5PSI... no way... maybe show some steam wisps around it, but not "lift".

14. Seems to me that you have a small fire for some reason. Should not have lost so much pressure just moving a few feet. Do you have any major steam leaks? I suspect you would be seeing steam all over if so at the ambient temperature you mention (55).

18. If you got the valve stem completely out of the housing without a huge fire in the smokebox and coming out the chimney then there has got to be something wrong. I assume the fuel valve is like others I have seen and it should be somewhere around 5 to 10 turns to unscrew it completely and "full open" should be no more than 1/2 to 1-1/2 turns.

Is the line kinked someplace? Or is there something in the line that is inhibiting the flow (dirt? flux? got soldered nearly shut someplace?)

On the other hand... Did you have any water around the fuel tank? Is it possible on this loco? 55-deg is right at the lower limit of getting butane to vaporize well and when it does it will cool the tank such that it won't vaporise any more and that could also cause too little fuel to the burner. Maybe try pouring LUKE WARM water (NOT HOT!!!!!) on the tank and see if the burner surges. If so then that was probably the major problem and maybe not a clogged line.
 

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Ha ha, ya gots two "Charles M" helping you... well, one goes by that as his on-line name and I go by Semper Vaporo, but we both are a "Charles M"

Anyway, I wonder if a few runs with mineral water would deposit enough scale to affect steaming qualities this much. Does the engine have a sight glass to show water level? If so, does it show scale on the glass? Can you shine a light into the boiler some place (safety opening, fill port, etc.) to see if the flue pipe has any scale on it? Maybe shine a light in one port while you peer into another.

The safeties on my Aster engines weep while the engine warms up and then seal better, so I would not worry too much about "some" wisps as that is probably not a great drain on availalbe steam and I doubt it is the problem as you have described it.

If the gas tank is on the roof of the cab then it is supposed to be kept warm by conduction of heat from the boiler through the chassis and such, and by convection from the boiler heating the air and that carrying it to the roof of the cab.

But at 55-deg (F) ambient temperature, you might not be getting enough evaporation of butane to get the fire going enough for either conduction or convection to produce enough butane to maintain the fire. Was it also windy? What was the humidity, if you know? When the humidity is high it can drain heat away quickly in a slight breeze.

I had a fellow at my track once with a new Ruby he had built and we could not get it going well. It just acted like it was out of fuel. It was "cool" (I don't know what the temperature was) and a bit breezy. We eventually took the fill valve out and looked into the tank and it had some liquid in it! We thought it was water. Upon turning Ruby upsidedown it poured out and immediately vaporized... oh! it was butane! That was the problem, not enough heat to vaporize it in the cold tank... no vapor, no fire. We heated the tank just by putting a thumb on it and got a fire started and then there was enough heat conducting back to the tank to keep the fire going, but it was sluggish and didn't work well... I think it was just because it was not warm enough that day.
 
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