The source of the engine misfire has now been located and I have to confess the discovery was more by accident that deep found engineering knowledge… Each cylinder has 2 valves, one to let the mixture of fule/air in and the other to let the burned gases out. The adjustment of those valves is fairly critical if you want the engine to perform at its optimum but it will quite happily run below optimum if those adjustments are not ideal.
It transpires that 2 of the 4 exhaust values had decided to slowly un-adjust themselves over time so although I was pretty confident I had adjusted them correctly over a week or two they slowly opened up the clearances which is why I was struggling to get the engine to perform like I thought it should.
The exhaust valve clearances should have been 4 thousands of an inch, but over time these opened up to over 25 thou and then stopped. This didnt stop the engine running but it was not happy.
To adjust the valve settings you loosen a clamp bolt and then turn up/down and adjusting screw which sets the clearance. When I tried to tighten the offending clamp bolts, they both snapped. Oh how I laughed. This of course left both of those adjusters with zero clamping ability and a dead engine until fixed. Resolution necessitated removal of the shafts and rockers from the engine, a full strip down of those components and then a gentle oh so gentle tickle with a tiny grinding disk on the ends of the broken bolts in the vague hope that the resultant machined slot would accept a screwdriver to un-do them. Luckily, with patience (and a fair amount of time) the snapped bolts were removed.
Funnily enough, bolts should not snap, what I found was the thread they were screwed into was damaged so they were locking up partly but not quite clamping the adjuster screw. At that point I decided to remove all the clamping bolts and replace them with new ones as a just-in-case. All I then needed to do was adjust the ‘tappets’ to the required 4 thou clearance on the exhaust and 3 thou on the inlet.
Except… after 80 years the rockers that push the valves open get worn so inserting a measuring feeler gauge gives you a false reading because the feeler gauge spans the groove worn by the valves. I have an engineering solution for that which requires a Heath-Robinson contraption of steel plates and a highly sensitive dial gauge that measures movement. So if I put the dial gauge on the rocker, I can measure the actual vertical movement regardless of the wear !
Those pesky valves have not un-adjusted themselves since and engine having received some love is quieter and now happily revs beyond 2000rpm. All I need to do now is sort the carburetor settings which are too rich – the car is leaving a hazy blue smoke trail sometimes.
Of course – all this doesnt mean the aging engine is good and will last some years before needing a rebuild, but it does mean…. we are back on the road!