SU Carbs- Idle Woes

["Allen W." <AJWILLIA@UA1VM.UA.EDU>]

Have HIF4 Carbs on my 74-MGB. Has rebuilt engine with 8.8:1 pistons and
upgraded cam. Runs great all the time and idles great most of the time.
Can set idle @800 rpm and it will stay there for some random period of time.
Then it will drop to <500 rpm and nearly quit running. This usually occurs
after warmup. Floats and needles seem OK(carb rebuilt < a year ago). Can't
find any vacuum leaks. I need some suggestions. Thanks to all.