Yes you are all quite right. The lorry was still in use up until the 1970's, therfore the engine must have been adjusted to take leaded petrol into consideration. Now we have lost leaded it needs readjusting again (or a supply of leaded petrol). Going back to the manual, it states (and please excuse me if i have given too much information, but it makes interesting reading):
Diagnosing troubles of automobile engines.
To manage successfully an internal combustion engine, especially one that works under a variety of conditions, often very severe, as the automobile engine – it is first of all necessary for the operator to make good use of his reasoning facilities.
The symptoms of derangement, when taken singly, are often such as may be caused by any one of several possible defects. In nearly every case the defect, whatever it may be, will produce several symptoms, a careful study of which will lead to the elimination of causes that do not tally with all the symptoms as for instance causes affecting all cylinders when only one or two are misbehaving, or vica versa. When the user has reached this point, generally a short further investigation of the points at which trouble of that particular sort is most likely to occur will lead him to the discovery of the true cause. The cause of loss of power, due to such faults as a lose battery connection, a sticking inlet valve, or a bit of dirt in the carburettor, will at once be recognised in its true character by the experienced operator. The only way to attain final proficiency in these things is by extended experience of the particular engine in hand; but on the other hand there is no excuse for the aimless groping of many inexperienced users, who will often send needlessly for a tow, or will pull an engine to pieces in their search for some simple fault that might be located by intelligent diagnosis. (You heard it here first – I am “an aimless inexperienced groper”).
Reasons for failure of engine to develop full power.
A failure of the engine to develop its full power should not be confused with frictional resistance in the transmission system or dragging brakes. The chief reasons for this trouble in the engine itself are:
1). Back pressure on account of choked muffler. Opening the muffler cut out or relief valve will at once indicate whether or not this is a cause of the trouble.
2). Lubrication insufficient, especially in the cylinder
3). Overheating.
4). Compression leaks at the valves, around the piston, or through a porous cylinder wall or a loose cylinder plug.
5). Particles of carbon or other dirt under a valve.
6). Valves not properly timed.
7). Mixture too rich or too lean.
8). Ignition spark weak on account of weak battery.
9). Water entering cylinder from water jacket.
10). Timer slipped on shaft causing late spark.
As it works fine with leaded but not unleaded we can narrow it down to just items 6 or 7. As we have tried the adjusting the mixture the problem looks to be a timing issue. Any thoughts or other good ideas?
Tim (too)