Garden Railway Forum

Can anyone offer any advice on setting up the valve timing on a Roundhouse 0-4-0 chassis

I have built a new chassis from a Roundhouse kit and just can't seem to get the timing right for an even 4 beats. I have set it up as per the Roundhouse instructions. ie the return cranks are correctly set at 90deg, the valve ports open correctly as the wheels are rotated. The chassis runs on a very low pressure of compressed air, but I just can't get the exhaust beats even. At the lowest pressure, it sticks with the left hans cranks horizontal, either pointing forward or backwards. I have played around with all the adjustments, just can't get it even. Any suggestions as to the best method to achieve an even beat?

Hi,

I find this method works well:

steveh99 wrote: ↑Sat Mar 02, 2019 5:34 pm... it sticks with the left hand cranks horizontal, either pointing forward or backwards.

That might not be a timing issue (otherwise it would be always forwards or always backwards) and suggests that the left coupling rod is tight on the crank pins. Is there any mechanical resistance when you turn the wheels by hand?

Thanks Tony

No the chassis runs really freely, there are no tight spots anywhere. I'll see if I can post a video of what it does. I can get various patterns of exhaust beat by fractionally moving the return cranks (I have not pinned them yet) or by the slide valve settings, but nothing so far has produced 4 equal ones. I have not had this much trouble before. Despite the off beat, it runs really freely on a very low air pressure of around 5 psi

one other difference I have noted is that not only does the chassis run very freely, it does not build up any compression when pushed along the track. If I push other Roundhouse locos, compression builds in the system such that when I let go, they roll backwards a short distance.

That might be down to how new the cylinders are. A kit chassis doesn't benefit from the wearing-in period that factory engines do, and the valves won't seat perfectly until the engine has been run for a while. The valves on your older engines have worn the port faces smooth and are sitting in a nice, suction-y oil film. The new ones lift off more easily and won't hold "compression" from being pushed along. This is my guess, anyway.

Is it possible one of your expansion-links is a little bent? I ask because I accidentally bent one by pressing on it while trying to fit something else, and I had the same time as you- couldn't get the timing perfect even though the chassis seemed to run well enough. I measured the valve travel and one valve was travelling a different distance forward than it was back- actually hitting the edge of the valve-chest at one point. It was then that I noticed the slightly bent expansion link- almost imperceptibly different than the other one. I was finally able to get the timing perfect. Although frankly, more often than not you'll notice full-size engines have a CHUFF-chuff-chuff-chuff-CHUFF-chuff-chuff-chuff - at least the two-cylinder ones. When I was a kid I thought that was how they were supposed to sound.

Haven't had a chance to try further, but have drawn out the sequence of events for crank, slide valve and piston for one wheel revolution which helps clarify where everything is and which way it is travelling for any given wheel position.

That's a help but it would be better if it showed the valve positions at just past back and front dead centre rather than AT back and front dead centre. So, instead of showing the three and nine o'clock positions for example, for the right-hand side show four and ten o'clock for forwards and two and eight o'clock for backwards, and the corresponding opposites for the left-hand side. The crucial thing is: When does the valve open the port? And it should do that just past the dead centre positions.

Thanks Tony

The drawing helped me with what was going on, rather than specific valve opening timing, but I'll do that when I get the chance.

Your issue sounds similar to the issue I had with my Fowler kit. It's possible that your return cranks aren't clamping down onto the crankpins properly. Tightening my pinch bolts, the cranks seemed solid, but after a few revolutions on air, with the motion lubricated, they were already moving out of position. I was breaking pinch screws from over-tightening, and the cranks still moved.

In the end, I fitted larger stainless screws/nuts, and opened out the slot to allow the crank to flex a little more and grip the crankpin more tightly. At that point, I was able to run the thing long enough to start evening out the exhaust beat. It's still not perfect, and you will always have to compromise between forward and reverse timing.

Might be worth checking out, anyway.

I would suggest getting them "close enough", then running the chassis in on air like Roundhouse does. I found the rods and valve motion needed a few min in each direction on air to "loosen up" a bit. I would stop every few min and add some light machine oil to the air feed line(we are not on steam so a light machine oil did fine). After several min running at a medium speed in each direction, I then reset the valve timing and kept tinkering till I had 4 square beats in forward and really close to it in reverse. I never did get them in both direction, there is just to much "slop" in the expansion link set up that RH uses for me to get it spot on in both directions. Even watching factory built models, one direction is more "square" than the other. A chuff pipe along with a slomo really brings this out.