Hello Everyone.
The problem has been found, there is a huge amount of very strange goo in the silencers. It isn't the usual sort of rubbish one gets, but some very odd bubbly looking stuff. This has been restricting flow no doubt. Now how did it get there? Well my theory is that the engines were not put away properly for the winter.
My friend's usual technique is a bit idiosyncratic. He first puts in a good amount of air tool oil and works this around the engine. he then puts a small amount of fuel in the tank and lets the engine run until fuel is exhausted. He then works in another good dose of air tool oil and that is it. Quite why he does it this way. I have not got a clue. He is over 80 and has always done it this way, with excellent results.
My theory is that he put in something else rather than air tool oil. This has reacted with the normal castor goo in the silencer and the witches brew bubbled up, giving a restriction in the silencer. Once it is cleaned out, then the engine run returned to normal. If I am correct, then I don't have a clue what it was that he put in the engines. There is a fair selections of bottles and cans in his workshop and it could have been any one of them, in a senior moment. I am surprised that the engines would restart easily after the two minute run, but then there was no rush to get airborne so maybe they cooled sufficiently to restart? Still a few queries as to the why and how, but the problem has been nailed.
Oh yes I checked engines and bearings and there appeared to be no damage to anything, so he looks to have got away with it!
Thanks for all the helpful suggestions, I was somewhat reluctant to post this one because it was a "how long is a piece of string question" and wasn't sufficiently defined! Oh by the way The glow plugs are new Enya 3s, the original plugs were tried and they were not very good. They dropped substantial revs when the glow lead was removed and a trial flight with one of them produced a bit of a faltering engine run. So maybe the liquid X didn't do a lot for the plug. Probably a case of a poisoned element too? Strange that the "two minute" flights appeared to be normal. The plug problem wasn't noticed at the time and that is strange too. Maybe we were not as attentive as we should have been.
Regards,
Andrew.