So I am a bit confused. Yesterday took the drivers side carb off and cleaned the idle jets. That seems to have sorted the backfire. On todays test run tick over seems high and erratic.  Noticed the drivers side carb wasn't quite shutting off.  Slackened everything and all good. Tightened it up and same again.  These are quite new carbs and linkage so something isn't quite right.  I had an extra spring on it so added that so it pulls everything back and al is good.  Consulted my Dellorto book and can't see what I am doing wrong. Any suggestions or ideas.  Can leave the spring on as its running well but would like to sort it

Are the carbs returning fully to the closed position. On my 34ict I had to alter the spring anchor location to get them to shut as the springs had very little tension on due to the pull angle.

Thats exactly the problem with the drivers side carb.  Not quite going back unless i add a spring.  I tried endless combinations ie moving the arm on the hex bar its self, adjusting the linkage rod .... but even when I think its right several flicks of the carb and it goes back to not quite closing.  So the spring is on and its all working (in fact runs fine and no longer erratic) ... was just hoping I could get rid of the spring.

oo just notice my typo in the title .... Deli !! haha Dell 

there is a small amount of play in the linkage joints so the carbs can vary slightly on closure the spring is the only way or fit a center bell crank linkage with ball joint or bolt up ends.
the Jas idle speeds up after a good run with the hex bar fitted they are not the best designed linkage set up well the one I got supplied is not.