Could be a few things, but sounds like an air intake leak
Cracks on the vacuum take off caps, Replace them as a matter of course as they are only about a tenner for 4
Cracks starting to go all the way through on the intake manifolds,
To much cut off the the diaphragm springs when doing the Ivans modification causing the diaphragms to float.
Carbs not fully fitted on the rubbers when carbs replaced.
Sticking throttle shaft on the carbs.
Dodgy TPS
Incorrectly set pilot screws after Ivanising.
Carbs not balanced properly
As a start point check for intake leaks try dropping the idle speed, balance the carbs and reset the TPS
Cracks on the vacuum take off caps, Replace them as a matter of course as they are only about a tenner for 4
Cracks starting to go all the way through on the intake manifolds,
To much cut off the the diaphragm springs when doing the Ivans modification causing the diaphragms to float.
Carbs not fully fitted on the rubbers when carbs replaced.
Sticking throttle shaft on the carbs.
Dodgy TPS
Incorrectly set pilot screws after Ivanising.
Carbs not balanced properly
As a start point check for intake leaks try dropping the idle speed, balance the carbs and reset the TPS