I like that whistle.
I had a similar experience (the herky jerky operation) with my only K-Line engine, a Daylight 4-6-4. I got fed up with it and had it converted to DSC. My converter dude told me the jerky motion was due to a bent plunger piston inside. I guess it had something to do with the smoke chuff timing. Anyway he had to fix that as well when converting my engine. SJC may have had the same defect with his engine.
.....
Dennis
