[nassi]

Is it possible that the fuel is vapourising? Try insulating the fuel line. When you switch off, the heat soaks through the spacer plate and boils the fuel in the bowl.  
I suffered this at one point when the timing was retarded to much.
Just a couple of ideas, others will be able to help more.

[ron]

If you're spinning and flooding,the mixture isn't getting lit,so your ignition system is a fault. If you have the GTR40 is the bypass circuit fitted(or working).A lot of people don't bother reconnecting this.Hope this helps. Ron