The secondary air valve choke/pull off, whatever you want to call it, can cause some strange problems. Especially as these carbs age & the rubber diaphragm goes hard from age & heat.
Usual symptom is like stutter when the secondaries start to open. What happens is the AV releases too slowly, AV acts like a choke & the mixture goes very rich.
Fix is very simple; buy a new AV & if at all in doubt, replace it.