Just general info, Im not familiar with the regina system.

most cars have some means to measure airflow at idle, my 240 and 74 being bosche use an air mass meter, My Porsche 944 has an older "barn door" arrangement where a flap lifts and it has a rheostat which changes in continuity. Its a fairly similar Bosche system, but 85 , a bit earlier technology. Regina may be different again.

Point is the idle is usually very sensitive to any source of air which can enter the engine without being measured and accounted for, so any slight leaks may affect idle a lot. at wide open throttle the air fuel mix is probably less disturbed, proportionally.

I spoke ot a Porsche guy recently who described how he cobbled together his own smoke machine , he injected smoke, or something like it into the hoses , perhaps pressurizing them slightly while watching with a bright light for escaping smoke. he said he did find some leakage that he was not aware of , and fixed it which made his car idle a lot better.

maybe others have made up these "smoke injectors" or maybe they can be bought, not sure, others may know more on that. .