I'd flown my new Osmose 70 a couple of times this morning (post on its way when I get round to taking some pictures) and hadn't noticed anything unusual. On the third flight of the day (fifth flight ever), it was fuelled and started as normal. There was some vibration when starting but I put it down to my useless cheapo starter which is on it's last legs and only works when it feels like it. I'd promised to let Mike H have a go with the sticks so took off climbed to altitude, handed him the Tx and left him to it.
Within about a minute, however, Mike said "there's something wrong with the engine". And there was - it sounded odd, as though it was nipping up and then freeing off over and over again. Initial thoughts were that it was running lean (the needle had been closed about 3 notches because it was running slightly rich on previous flights) but the O.S. 70 FL is a ringless engine so doesn't want to be too rich, and three notches shouldn't have made that much difference.
Mike handed it back so that I could do a low pass to see what was what, I closed the throttle on the descent and the engine stopped. Fair enough, deadstick landing.
When it was finally recovered from the patch, the reason for this bad behavior became painfully obvious; the engine was loose, and was hanging on by one 3mm bolt, this in spite of being fitted with anti-vibration washers, so I was quite surprised. Another few minutes and we'd have been sweeping up a pile of wreckage.
The bolt holes had been tapped with a 3mm second tap in a nylon engine mount, so they will tend to tighten up towards the bottom of the thread and are usually quite tight. I've never had them come loose before with a 2-stroke, but this is my first four-stroke.
The moral of the story is that anti-vibration washers - apparently - don't always work. I've bolted everything back in place with some slightly longer bolts and locked them with some 3mm nyloc nuts. There doesn't appear to be any engine damage, but we'll see the next time it's started. I might have got away with it...
A.
