I was told by Stephen at one of the BBBs that he cleaned his M67 out by running it dry (we'd tried it on glyc, which accumulated as gunk at the bottom in the torroid).
I thought I'd clean mine up the same way after a lot of NaOH glyc melting and water cleaning duty, it was looking pretty bad.
I set it up in the middle of the garden on a paving slab and got it started. Much to my delight, all the glyc stuck to it started to bubble and melt to run down the sides. I put some kitchen roll down to stop it running onto the grass (which it did anyway).
At this point there was a lot of chuffing out of the chimney and a foul smelling cloud coming off the metalwork. I called the Suzanne to see if she thought it was a bit antisocial to the neighbours (she thought it was fine).
It was at this moment that there was a hefty WHUMP and the whole thing (and a fair bit of lawn) was enveloped in fire. The chimney started belching a black tower of clag and soot showered everywhere.
I was immediately concerned about fire licking over the fuel tank, if that let go then I'm sure there would have been even more fire than there was already - I reached into the flames with an adjustable spanner and managed to turn off the drip tap. The fire came a little more under control, so Suzanne stopped rolling her eyes at me and returned inside.
One the glyc all burnt out, the torroid was all nice and clean, and inverting it and tapping on the ground cleared out a lot of gunk, so I'm happy with the technique - but might clean it a bit more by hand next time,
And there is evidence of scorching on the lawn. But that comes with the territory to an extent
