ah think i found the prob , this has been a strange one coz it didnt give a problem all the time , some days it was ok , i narrowed it down to one barrell after swopping carbs over to confirm that it was fuel problem . even tho the choke mechanism is wired shut and i dnt use it , the pin that holds one of the quadrants in had worked loose allowing the plunger that is the choke enrichment to get jammed up , this would happen when the carb spat back ,causing that barrell only to be on choke .two screws undone take off choke mechanism tap retaining pin in job done . has took me weeks to get to the bottom of this