Irregular chug

Running gear not properly quartered and/or valves not properly set "square". Remember however, that those big three foot gauge Mikes really work their butts off, and setting valves may be a bit farther down the maintenance "to-do" list than other running gear and boiler work.

If the running gear wasn't properly quartered, wouldn't that result in the chugs coming in pairs---2 close together, then a gap, and then 2 close together?

It's highly unlikely that any of the drivers would be out of quarter.  If any were, the engine would have worse troubles than out-of-time exhaust.  Take a look sometime at how drivers are mounted on the axle, or in this case, the cranks.  Not only are they pressed on there with a force of more than a hundred tons, there is a big hefty key driven into a slot in the axle that matches a slot in the wheel - or in the case of the narrow-gauge engines, their cranks.

It's probably just in need of a good valve setting. 

EdKing

Very true Ed. But, sometimes turning the crankpins "in-place", i.e. NOT on a quartering machine, can get things a bit out of time also.

You mean like increasing the stroke of a gasoline engine by turning down the crankshaft cranks sort of, how you say it, "eccentrically"? 

If you turn one crankpin down, don't you have to turn all of them?

And are we talking about enough out of square that you can tell it in the exhaust?

I had a Motive Power engineering type from a down east railroad tell me years ago that the Southern Valve Gear could never be made to be perfectly square.  I fired and cut coal enough on Southern 630 and 722 after Bill Purdie had done the valves, and if they were out of time I couldn't tell it, and I've got a pretty good ear.  So maybe theoretically they couldn't be made perfectly square, but they were sure as heck square enough.

EdKing  

I assume crank pins are turned down because they wear eccentrically.  But wouldn't shims then have to be used to keep from having a sloppy fit?

Not having been around steam locos that clsoe, I can't comprehend how valves are adjusted.

I don't do real locomotives, but I mess with quarter and crankpin distance on models.  Just get a teeny bit off in either direction and you will have serious binding or lock- up.  On a real locomotive a lock-up could be catastrophic.  I vote with Ed - it is valve timing, and sometimes a leaking packing gland will make you think the locomotive is out of timing.

On O scale models I work to a thousandth of an inch in quarter and throw.  Anything looser is asking for trouble.

opinion.

So I guess you could say that the valve timing on 630 and 722 was Purdie good, right?  Sorry Ed, couldn't help myself.

That's OK, FH - valve setting was just one of the things that Bill Purdie was good at; I believe that the OGR-webmeister could shed some light on it, too.

An anecdote - Purdie and his assistant were "trailing" (setting) the valves on the 4501 one fine day, and as was their custom, they were doing it with the reverse lever hooked up to about 50% cutoff instead of down in the corner as was customary elsewhere.  Owner Merriman arrived on the scene and wanted to know what the h**l they thought they were doing.  Purdie explained to him that if they set the valves with it down in the corner they might not be exactly square when the engine was hooked up, but if they got her square when it was hooked up she'd be square down in the corner.

"Oh."

Oh yes.

EdKing

I wish I knew exactly what you guys are talking about---how it's done, etc.

When setting valves on a steam locomotive, it is absolutely critical that the crankpins be precisely 90 degrees apart from one another. When crankpins are turned on a wheel lathe with a quartering attachment, that relationship is perfectly established.

When turning crankpins one at a time with an Underwood portable pin turning device, you cannot maintain that 90-degree quartering relationship between the pins. The Underwood machine turns one pin at a time. It knows nothing about the position of the pin on the other side of the engine. Consequently, there is no guarantee that the pin you are turning will remain EXACTLY 90 degrees from its mate on the other side of the engine. If the pin is out of round, the machine will cut a bit more material from one portion of the pin's surface than in other areas. The end result is that the crankpin quartering now might be 89.5 degrees, or 90.3 degrees, or anything other than a perfect 90 degrees. That's not enough to make the machinery bind or run hot, but it is enough to effect valve timing.

If the crankpin quartering is off by even a fraction of a degree, the engine will still run OK, but the valves will be out of time, with unevenly spaced exhaust beats.

We used the Underwood machine on the 765 several times back in the 80's and 90's. By the time we put her away for the big overhaul in 1993, she was BADLY out of time and out of square. Bill Purdie helped us set the valves once, and proved to be a master at what he was doing. The engine ran better after he worked on it, but the work was a band-aid to compensate for lots of wear elsewhere in the running gear.

Today the 765 is absolutely "square" and sounds like a well-timed steam locomotive is supposed to sound...

765 - A well tuned machine.  Hats off to the FWHS & all the volunteers who run and maintain her!