Setting the valve timing?

[Terry Chapman 2Participant @terrychapman2]

Hi,I ran my 3/4" Holden for the first time on air and it seems to not want to run in reverse? What order should I set the eccentrics and valve timing as it seems Im chasing my tail lol?

Many thanks

[Terry Chapman 2Participant @terrychapman2]

[Nigel Graham 2Participant @nigelgraham2]

Eccentrics – plural. So, is this a 2-cylinder engine with single-eccentric valve gear on each, or Stephenson's Link Motion?

If the latter, the engine should be symmetrical, with the eccentrics at equal both sides of their associated crank. In mid-gear at both dead-centres the expansion-link should stand symmetrically about the die-block. If it does not, that suggests one or both eccentrics is out of position.

There may be an effect from how the link is suspended. On a locomotive it is normally held centrally for equal running in both directions. On a traction-engine, as the scale you quote suggests, it is usually held at one end and often the eccentric-rod pins are on the projected slot centre-lines (called a "Launch link" and that might give some loss of valve-timing in reverse. It should still run of course – but a road vehicle is not normally needed to go far and fast backwards.

It should not really matter whether you set it in forward or reverse gear first, but as a starting-point on any valve-gear the valve should move equally about the centre of the port pattern.

We will assume symmetrical port-face and valve of course.

Condition 1: At valve mid-travel in mid-gear the valves should move by no more than [ 2( lap + lead) ] only – some engines are designed without lead. The laps should overlap their ports by just the lap (-lead) itself. It's not easy to measure, but assuming correct dimensions the symmetry here is usually set by the valve-rod adjustment.

Condition 2: On rotating the engine in full gear, the maximum port opening should be equal on both ports: you can measure it with feeler-gauges, on a slide-valve at least. This is the usual way to test the valve's symmetry. Slight out of symmetry won't necessarily stop the engine unless it causes excessive lead, but will give it something of a limp.

If Condition 1 is met but not 2 -,

OR

If 1) and 2) are met in one direction but not the other beyond the slight effect of an end-suspended expansion-link –

then something is out of adjustment on the motion; and assuming no rogue dimension error (possibly on the drawing – I have known this the case) the most likely culprit is an eccentric out of angle.

I'm not entirely certain of the effect of suspending the link at the end, but think it would simply shorten the valve-travel slightly and possibly a bit unevenly from the influence of the lifting-links on the drive from the eccentrics. It is though common practice on traction-engines so can't be all that bad.

'

To summarise:

The valve travel should be symmetrical about the ports in forward, mid and reverse gear.

The expansion-link, eccentric-rods and die block, and the crank, should form a symmetrical pattern at both dead-centres, in mid-gear. If not then if all other things are correct, no dimension errors anywhere, the most likely fault is a miss-set eccentric.

The expansion-link movement into gear is normally equal for both forwards and reverse – but the most likely effect if not is a reduced cut-off from the shorter side, and possibly mid-gear not being where the reversing-lever thinks it is. Probable cause: an error in the reach-rod, lifting-link / weigh-shaft assembly or reverser quadrant placing the link incorrectly.

As I say I'm not sure of the effect of a Launch link with short lifting-links; but it should not distort the valve-events significantly, and then preferably on the reverse side.

'

One effect to note:

With Stephenson's valve-gear the lead will change, usually increasing, as the engine is notched up. The lead is a very slight, early opening to admission as the crank approaches dead-centre, and its value is set by the design, not adjustment.

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