All or nothing running
Hello all,
I have just finished reconditioning an A4 loco, so have replaced all the seals, given it a thorough strip down, removed old gunge and added fresh oil. It now runs, but will not run slowly. When starting, it needs lots of steam through the cylinders to warm them up, regulator well open and often requires a help to get the wheels turning (on a rolling road). Once it catches, it races away at high speed. When slowing down, the wheels stop suddenly and nearly always in the same position. I have spent some time carefully adjusting the timing and have run several tanks of water through now, hoping it would settle down and play nicely.
Listening closely to the steam from the chimney when it is hot and just starting to run, I can rotate the wheels slowly (using my trusty cocktail stick) and I can hear a point where the steam stops coming out of the chimney. I can rotate the wheels further and the steam returns to passing through the chimney. I can find this position twice per wheel rotation, 180 deg apart. This seems to me like the rotary valve is in the closed position and sealing happily and the steam is held by the pistons. However, my question is this - why only twice per wheel revolution ? It suggests that one cylinder/piston is not doing anything and the other is doing all the work. Does this sound right ?
If it were a normal engine, I would consider honing the cylinder bore and fitting oversize rings. What's the equivalent here ? By the way, it does have brand new O rings in. Any suggestions ?
Thanks,
David
Another update as I have been experimenting.
The problem, as I see it, is how to get oversize piston rings to see if they seal better. I have raided the kitchen and tried tin foil under the rings. I cut a couple of thin (1.25mm wide) strips of tin foil and carefully wrapped it round the groove in the piston where the O-ring sits. I wrapped it round twice - very fiddly ! Then refit the O-ring on top of it and refit the piston in the loco.
It does help. I have now had the same A4 running round the layout pulling half a dozen 12-wheel Pullmans. It's still missing that low down torque and so needs push starting and it also struggles with inclines / sharp curves, both of which may layout suffers from. But it's an improvement.
The loco is still weak on one cylinder - so maybe I should try out thicker gauge foil or wrapping round more loops.
At least it's progress !
By the way, Eric, I did try to measure the piston diameters where the O-ring sits. It's right on the limit of my Vernier reading skills, but there maybe 0.02mm difference between the pistons. I don't know what the manufacturing tolerances should be, but it seems like a small change in the quality of the piston seal creates a huge difference in performance.
Cheers all,
David