Did I mention that my Else was not free frim ailmemts?
Fired up, she would run on blocks. Once on the track, she would stall, hissing under a big plume of steam.
I rounded up the usual suspects:
Centering of valves,
quartering of cranks.
No dice! Finally, I felt I could trace it to the starboard cylinder. I decided to bite the bullet I had so far managed to dodge: Taking off the rear cylinder cap to have a look at the piston. The catch is the innermost bolt which is quite impossible to reach with a spanner. If you succeed, you can turn it but a tad at a time.
Eventually I could pull the plug, no, the piston out:
The piston stayed behind, and I had to ease it out, Sushi way.
So this is what happened: Running light, the piston would stick so-so to its rod. With load, it would slip off and leave the ports connected with all the steam escaping.
You can make out traces of solder.
This is not unheard of in various circumstances: a smokebox fire melting the solder, condensate breaking the joint, or excessive use as a push-along toy lacking proper lubrication.