I also stripped the exhaust down, de-coked the header pipe, plugged one end of the centre box and similar with the silencer and filled them both with white spirit. I left it to soak for a long while in the hope it would liquify all the oily sludge, it did. Runs a whole lot better. While I had it all apart I filed down the sharp corner that the engine number is stamped on - it pokes a hole through the underside of the middle box if this isn't done.