Hi Dan,
Have you checked the compression with a compression tester?
I've found the carby insulator gaskets sometimes look fine but are still worn out. The other problem with the Chonda motors is that sometimes the linkages need to be bent around slightly to actually activate the choke fully.
Try replacing the gaskets and triple checking the butterfly is closing fully.
Also - I apologise if this sounds like a stupid question but are you using fresh fuel?
Pete.
