Help diagnosing escaping air on a Stuart 10V!

I have a Stuart 10V which is very free running but it appears to be blowing a lot of air straight through to the exhaust pipe. When running I detect the 'pulse' of air at each end of the stroke but there seems to be air running straight through.

what could be be causing this?

The slide valve is centred, with a little free play and exposing the top and bottom port at each end of the stroke. The piston is a good fit in the cylinder.

Quite possibly the valve not sitting down onto the port face properly and allowing some air under it. Can be caused by the nut being too tight in the valve's slot or the thread on the valve rod or through the nut  not being true and throwing the nut and therefore the valve out of line.

Edited By JasonB on 23/03/2021 19:32:25

As Jason suggested it could be the valve not seating. I suggest you make sure it is free to move on the nut, sitting on the port face, but not sloppy, and apply a good dollop of steam oil, then re-test.

Thanks chaps, I previously suspected the valve not seating properly and checked the valve float. I'll have another look in the morning and see if there is any way it's being held off the port face.

Just a second thought - did you check for cracks between steam ports and exh port? or for erroneous drillings too deep from steam port into exhaust port? as unlikely as they seem these things have happened from time to time. (especially if you have taken over a partly finished project - no idea whether your engine originated that way)

Don't forget the good dollop of heavy steam oil - this often helps first seatings of slide valves. Good luck.

Thanks chaps, now sorted and running very well. It was the valve being a bit too tight on the valve nut so it didn't seat on the port face properly. Now free enough for the air pressure to push it onto the port face.

Obviously not the case here but sometimes beginners put air in the exhaust which has a similar effect, if it runs at all.

Glad it's running Martyn!

Good point Neil, inlet and outlet ports for slide valve steam engines are both useful items but are hardly interchangeable......to paraphrase Col Potter on the old MASH show, talking about something else.