Thanks for the replies fellas. There are no markings on the carb at all except for "choke". I suspect it is off an ancient motorcycle or similar. Following your advice, I will make an aliminium key and install it. Unfortunately the starter gear on the MIC starter motor has sheared itself flat and I won't be tinkering with this again until I source a replacement. It was listed as a replacement for B&S STR5983, but the pitch seemed wrong from the start. Perhaps you know the specs for the correct gear?
http://brisbane.gumtree.com.au/c-Ca...tratton-Ride-On-Mower-W0QQAdIdZ272864308
