rob roy tank engine

[jackdunn]

Hi
I am building a Rob Roy tank engine .
I have just put air through the cylinders and find that the air is coming straight out the exhaust. Can anyone give me any advice as to what's wrong with the engine. many thanks .

[chris vine]

Hi Jack,

I think this is a slide valve engine?
Check that the valve is sitting on its port face and is not held or sticking a bit above it. If the buckle or nut is a bit of a tight fit on the valve it is easy for them to hang a bit above the face and then the result is as you describe.

Chris.

[AndrewP]

Welcome Jack, I'm building a Rob Roy too but am not quite at that stage yet.
I assume you have a complete chassis with cylinders and all valve gear mounted?
First thoughts are valve not seating or valve timing, if you turn the wheels is there any position in which the air does not come straight out of the exhaust?
How much air? - running my Juliet chassis on air for the first time I needed 40psi and all that a 6cfm compressor could give just to move it.

Cheers, Andy

[baggo]

Hi Jack,

As Chris and Andy have suggested, it's probably one or both valves not seating on the port face, possibly due to the drive nuts being too tight in the valves. The problem is not helped by the valves being on their sides rather than on top of the cylinders where their own weight helps to seat them.

I recently made a new set of slide valves for my brother's Rob Roy and he had exactly the same problem - the valves would not seat properly. It turns out I had made them to an old set of drawings with different dimensions to his and the slots were too deep for the valve nuts on his loco. This resulted in a large gap between the bottom of the valve nut and the bottom of the valve slot and the valves could lift too far away from the port face. The gap between the valve and the port face was so big that it proved impossible to build up enough pressure in the steam chest to reseat the valve.

My brother cured it by making some new, deeper valve nuts that allowed the valve to only lift about 10 thou. The loco now runs happily on about 7psi of air!

John

[jackdunn]

Hi All

I have cured the problem with the valve shuttle. The engine now runs on air at about 10psi with on leaks. thank you all for your help in this matter.

Jack