,it hasn't been run for a awhile and previous to that mild spitback when cold which was only for a short time then running nice idle of around 950rpm till my second tach died 
.Whats has changed here ???..can only think it is blocked idle jets as it has a poor idle and is running on "tip in "throttle only when cold or warm and lots of carb backfire.
Reset and balanced the throats but really found them close.
Carbs were rebuilt before fitting the 911 2.2T motor,,fuel pressure via Carter pump/regulator is 3 psi and constant,timing is correct and valves reset 550kms ago.
Anyone had this issue as this on here ?,would have pulled the jets but raining outside and car ready for storage elsewhere .
TIA.