Stew,
I've done this many years ago but will have to do it again! Not to put to blunt a point on it, it's a right pain in the arse with inside cylinders and valve gear!
Do you have LBSC's notes from model engineer. I borrowed all the copies I needed from our club a while back and am looking at them now ... it's all coming back to me!
As he says, for this engine the eccentrics lag the cranks by 90 degrees. This is because of that motion shaft with the rockers. They reverse the motion of the eccentric rods so when the rod is travelling forward to front of engine, the valve is travelling backwards.
So, first thing to do is set the eccentrics as near as you can to the diagram by eye:
Set the crank so you have the right hand crank at forward dead centre.
The eccentric sheave at the far right of the crank (no 1) is for forward motion for the right hand cylinder set that to it's extreme upward position.
The next sheave (no 2) is for the reverse motion of the right hand cylinder, so set this 180 degrees away from the first sheave. That is in it's extreme downward position.
The next sheave (no 3) is for the reverse motion of the left hand cylinder. Set this to its extreme forward position … towards front of loco.
The last (far left, no 4) sheave is for the forward motion of the left hand cylinder, set this to the extreme rear position, 180 degrees apart from the previous sheave.
This has achieved the starting point.
If this is actually possible, tighten the setscrews in sheaves 4, 3 and 2 (all but far right one) just enough to prevent accidental movement and put on the straps.
Nip no 1 set screw so you can still move the sheave but it will stay where you place it.
Take off the steam chest cover but put 4 of the screws back in to maintain steam chest’s correct location.
With the right hand crank at forward dead centre, pull up the half of the eccentric strap attached to its rod tight up against the sheave and have a look at the valve.
Make sure the reverser is in full forward position and a hairline crack should be appearing between the edge of the valve and the front inlet port on the right hand cylinder.
Whilst holding the eccentric strap tight up against the sheave still with one hand, apply light pressure to the front face of the valve (i.e pushing it towards rear of loco) to see if the hairline crack appears.
If it isn’t there, you need to wind the sheave on very slightly (clockwise if looking from the right hand frame of the engine) and reapeat the above test until it does.
If the crack is already too large, wind the sheave back very slighly (anti clockwise if looking from the right hand frame) and repeat the test until the crack is a hairline one.
Once you have achieved this hairline crack with the crank at forwad dead centre, lock up the setscrew.
Now hold the half eccentric strap against the sheave and turn the wheels over in forward direction until the crank is on back dead centre (piston has moved to extreme rear of cylinder).
Have a look at the valve whilst still holding the eccentric strap against its sheave. It should have moved forwards.
Again, apply light finger pressure to the valve, this time to the rear of the valve pushing towards the front of the loco.
If all is well, the same hair line crack should be appearing on the rear inlet port now.
If it does, bobs your uncle, tighten the set screw up as much as well as you can and fit the other half of the eccentric strap.
If the back port doesn’t show a hair line crack after having successfully adjusted the front one, it means the valve is too long. It can be shortened, but you need to take an equal amount off both ends of the valve to ensure the exhaust cavity remains bang in the middle of the valve. It will only be a minute amount you need to take off.
If you do need to do this, it will enlargen the gap at the front port, but you don’t need to re-time it, you just alter the position of the valve slightly on its spindle. Move it forward by the amount you skimmed off that end.
Now check the rear inlet port with the crank at back dead centre and the strap up against it’s sheave again. If the hair line crack is present at forward and back dead centre, you’ve cracked it. The timing is correct, the strap can be tightened and the other half of the strap replaced.
Now you need to put the reverser to the full reverse position to set the right hand cylinder for backward running!
This is done in the same way,
set the right hand crank to forward dead centre
remove the back half of eccentric strap no 2
Pull the strap and rod up to the sheave and look at the valve whilst applying light finger pressure pushing towards rear of loco.
Check for the hairline crack, if it’s not there rotate it slightly anticlockwise this time if looking from right hand frame (wheels will be spinning anti clockwise when in reverse).
Keep repeating until get the hairline crack between valve and front inlet port.
Once achieved, nip the setscrew up, hold the no 2 strap against sheave and rotate the wheels anticlockwise until crank is at back dead centre.
Repeat procedure, have a look at valve whilst keeping strap hard against sheave and applying light finger pressure on valve pushing towards front of loco.
If hairline crack is there, you’re in business. If not, you need to adjust the length of the valve as before.
Once the hairline crack is achieved at both forward dead centre and back dead centre lock up the set screw and refit the other half of strap!
That’s 1 cylinder down, all you need to do is repeat for the other side!
Remember that eccentric no 3 is the reversing eccentric for the left hand cylinder so treat that the same as number 2 and treat no 4 eccentric the same as the first one.
If you achieve all this the engine will be timed perfectly.
The main problems I had as I’ve said before was locking the eccentrics positively, and it’s all so fiddley to get into.
I hope this is of some use when you give it another go. You’ll get there, it’s just a very frustrating procedure and it doesn’t help that it’s so difficult to get at everything.
Nick
