So today I have replaced the coil and cleaned out the carb which was, as suggested, full of rubber bits. It is running much better now. I need to re-adjust the fuel mixture as it will not stay running without being on full choke. Then I noticed it was dripping fuel from the float chamber again. This is a Stromberg CD150 with the mixture adjusted from below. So I pulled the carb off again, for the umpteenth time, and discovered that I had yet again split the float chamber O ring. I am not convinced that the previous one I fitted was the correct size. It did not grip the bushing screw as I would have expected it to, but it did not leak to begin with. Hunting around the interweb, I can not find a definitive answer to what the part number is or, the size of the O ring. I'm assuming it is an imperial size? Can anyone tell me what size this O ring is so I can get in a small supply of spares?