Steam blowing through

[JohnP]

I've got a problem with my Metro locomotive. At some points steam is blowing through to the exhaust. For those that don't know the design, it's a 5" gauge inside cylinder loco with slide valves operating "back to back" in a common steam chest.

The blow is quite fierce and the effect on the fire is quite impressive.

So far I've found that the blow is not continuous, but comes and goes as you move the loco slowly.

Possible leakage paths that I can see are 1) Across piston - I re-packed the pistons on Tuesday with no effect.
2)Valve so badly adjusted that it oncovers exhaust port to steam - I don't think that's likely
3) The passageways from the ends of the cylinder to the ports could have been drilled too far and into the exhaust, thus allowing steam to go directly up the chimney
4) Damaged port face causing the same effect as (3)
5)Valve not seating (but why wouldn't it blow all the time then?)
Have I missed anything? I know I need to do some more diagnostic work....
Regards,

JohnP

[GeorgeRay]

If this has only just started to appear then there is I think that it is either steam blowing by the piston or a damaged slide valve. There are ways of telling this in full size by setting the engine in various positions to determine what the actual cause is. I will see if I can find my locomans handbook which I am pretty sure has the information.

[GeorgeRay]

John. Further to my last post I have found the relevant section. You test each cylinder in turn by putting the crank of the cylinder under test on the top or bottom quarter, reversing gear in mid positionthen move the reverser from fore to reverse gear. If you get a blow up the chimney in fore gear which stops in the middle and returns in back gear then there is a leak past the piston. A continuous blow could be a defective valve, on a two cylinder loco it could also be that the other piston is letting by. So reset to test the other side. Hope this helps. There are some other simple checks to test for broken valve laps. I see no reason why these full size tests wouldn't work in our sizes.

[JohnP]

Thanks, George, I'll see what happens when I try those tests. I'm borrowing a compressor from work tonight so things don't get so hot when testing.

JohnP