About twenty years ago, I had an onze normale which had crud in the drain plug. I poked it out using a wire coat hanger and a little bad language. When it was out, I ran it with a hose in the radiator and the drain plug out, for about ten minutes. Then I put some radiator flush in it, and went about 15km- the flush was dark brown and there was a lot of bits of everything in it. Afterwards, I refilled the thing with water and antifreeze, set up the timing properly, and there was no problem.
Tim Walker