Mystery (Day 2977)

Off to town this morning to order some bits for the lawnmower, pick up some stuff for a job, and to meet up with R to discuss where he wanted his outside tap fitted. I was left to get on with the job and once I was finished I decided to take a drive round to the bike shop to see if there was any news on what is wrong with my bike.
As I arrived, David was just removing the seat which allows access to the bolts to remove the petrol tank. With the tank out of the way he started on a few tests. First off, the spark plug was producing a spark, so that isn't the problem. A compression test was next on the list and it showed very low compression on the affected cylinder. A look down the spark plug hole and at the end of the plug suggested there was no catastrophic damage to the engine. The next test was to plug compressed air into the cylinder and see if we could find where it was leaking to. That involved removing the radiator to allow access to the bolts holding the exhaust on so the exhaust port of the affected cylinder could be checked. We could both hear air escaping, but weren't really sure where from. Removing the cylinder head was the next step, but to do so, the bank of carburetors had to be removed. Which was an utter pain to do. Once the carbs and the camshafts were out of the way, off came the head. Various tests were done to find where the compression leak was, and nothing looked like a definite source. The next most plausible suggestion was that a piston ring had broken, so the cylinders had to come off. The block came off fairly easily and the cylinder bore looks in perfect condition. As do all of the piston rings.
We had both been hoping for a real "smoking gun" indicator of what is wrong, but there is nothing. The valves may not be sealing perfectly, but they look ok. There is a slight difference in the look of the valve seats on an unaffected cylinder, but it really doesn't look as drastic as we would expect.
My brief stop in to see how it was going turned into half a day standing watching (and occasionally helping with) my bike being pulled apart. The good news is that there isn't any catastrophic engine damage, but we don't really have a diagnosis. To put the bike back together will take longer and the gaskets it now need are a couple of hundred quid. David is going to lap the valves in to see if he can get them to seat better, then the bike will be re-built. Hopefully the mystery will be solved then.
I had hoped to get along to the horses with my beautiful wife this afternoon, but horses of a different kind took up my attention instead.