An uncapped full-time vacuum port results in a moderate vacuum leak, and will cause the engine to run lean along with the attendant drivability issues caused by running lean. The caps are probably deteriorating due to heat. If you cant find another cap to put on the nipple, you can temporarily cut about an inch off a piece of vacuum hose (maybe you can use a little off the distributor vac hose if it has a little to spare), put that on the nipple, and plug thathose with a screw, drill bit or some such until you can get the right nipple cover.
Hello cucumber1949. My question is, about two years ago I noticed on the Edelbrock 1406 carburetor on my 1983 Chevy truck, the little black rubber/plastic cap on the Full-time vacuum port was cracked. I bought a similar piece from an auto parts store and recapped the nipple. From then on I have checked the cap for cracking. Last year again I found it cracked about half way down, so just tore it in half and put the remaining uncracked cap back on the nipple. Yesterday, found the top of the cap cracked and not reuseable. Until I can get into a far away town with a parts store, can the truck be driven with no cap on the nipple? Or should I at least "instant glue the cap together, put it back on the nipple until getting replaced? How important is the cap to the nipple and the entire carburetor system. Thank you so much for your response. Deb