Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

That can happen if your D-valve is a bit gummed up with old steam oil. I would suggest first trying some light degumming type solvent/lubricant/penetrant such as Break Free (PCL or equivalent) shot down the exhaust port to loosen it up a bit. Heck, even WD 40 would probably do the trick if that is in fact what the problem actually is that you are experiencing. If that doesn't work after a little while, then you are going to need to open up the steam chest to see what's going on in there.

Nice engine BTW, hope that helps and is actually all the problem that you are having.