Stocks of Chateau de Nitray Brut in the cellars at Millar Mansions having become sadly depleted, a quick pre-Brexit dash across the channel to Athée-sur-Cher was called for. Unfortunately, René never designed a cargo version of the RF5, so the trip was made by car. After loading up at the Chateau, I was delighted to be invited to visit Rene, who is in remarkable health and spirit, despite the difficult times brought about by the virus. Together with my good friend and former RF3 pilot, Brian Morgan, we were entertained, as usual, with some of René’s inexhaustible fund of entertaining stories, lubricated with a glass from the nearby establishment. This was followed by a walk round the grounds of La Harbuterie to view the magnificent display of cyclamen, rivalling that of my long time Fournier friend Eric des Gayets.
Since, on my return, I now have to quarantine for 14 days, I have some time to jot down some musings. Not much Fournier flying in the last week or two, due partly to the downturn in the weather and partly to investigations into a mysterious loss of compression in the club Dimona. With only 25 hours on a new exchange Limbach L2000 engine the compressions were perfect at 10 hours. Now a differential compression check has revealed a significant to major loss of compression on all cylinders, varying from a not very inspiring 70/80 to a truly dreadful 35/80! Following Stu Hoy’s advice I removed the inlet manifolds and rockers and tested again. The howling gale coming out of the inlet ports was not encouraging and a technical tap on the valve stem heads made no difference.
Removing the cylinder heads on a Dimona is not exactly an easy task. Unlike the superbly accessible RF5 engine (thank you René!) removing the heads on the Dimona is practically impossible without taking the engine out. So after 5 hours of cursing and worse it then took 20 minutes to remove the heads. There is no really obvious problem, just all inlets and one exhaust valve not seating properly. Looking gloomily at the combustion chambers, one bright spark says “Oh look, there are cracks between the No 4 inlet and exhaust”. Hmmm, bigger gloom and a dose of Adrox, which didn’t really look convincing to me. However, when I showed them to the eagle eyed Amanda at home she immediately said “They’re not cracks, they’re ridges!” And she was right. Comparing with its fellow chamber on the other head, one can see identical lines in exactly the same place – casting marks from the mound. Phew, That’s a relief! No expensive head replacement required, just a trip to the local cylinder head specialist for seat recutting and valve face grinding, I hope…..
But why should this happen within 15 hours running? There are darker crescents on the seats, which look like leading up. Somewhat against my liking, the club uses Avgas in the Dimona and that is known to cause loss of compression eventually, though not after 15 hours! That, combined with serious overheating, could be a possibility. Stu tells me that trying to climb at full power in coarse pitch rapidly cooks the heads, but there have been no reports of the CHT (on No4 only) exceeding or even approaching the limit. It’s a bit of a mystery. Answers on a postcard please……