thanks to all the input from everyone including offcourse cris and bangkok bob..........
i have finally tracked down the culprit of this saga that had been going on every 200 klms or so
to my 72 shovel...pictured.

its called the condenser, also shown as number 15 on the picture above......but wait there's more.

the real problem was the harley original condenser i replaced 3 times over the past year or so...
every time i put one in, it would go after a couple of hundred klms.....

someone then suggested i should try to put the bosch equivalent.......now over 350 klms
and touch wood.....no problems.

i was nearly going mad and i was going to replace the original carb and also piss the points and go electronic

im so glad its ironed out without doing any more and hense keeping it original.

thanks again folks.....

gay Met
I had the same and converted to a cv carburettor from Killer Carburettors they rebuild and retune to shovelhead requiremnts my bike a 77flh now runs like a dream with no starting issues blowback and coughing eliminated I had to by a cv twin cable throttle set as my wason a single cable that was available on ebay as well all up it cost me about 600 incuding freight

gay Met
I had the same and converted to a cv carburettor from Killer Carburettors they rebuild and retune to shovelhead requiremnts my bike a 77flh now runs like a dream with no starting issues blowback and coughing eliminated I had to by a cv twin cable throttle set as my wason a single cable that was available on ebay as well all up it cost me about 600 incuding freight