Z24 petrol engine in Nissan Urvan *stock carb*

[datzenmike]

The vacuum ports are openings inside the throat of the primary barrel just above the throttle plate. At idle they are not exposed to intake vacuum below the plate and are at ambient air pressure. As the throttle rises above idle the throttle plate lifts up to and above the ports and some vacuum is applied to them. Naturally at full throttle the vacuum drops near zero so no EGR and vacuum advance at idle and full throttle. Vacuum advance adds beneficial ignition advance under low load conditions when driving. EGR would dilute the idle mixture so much it wouldn't run so it's only active at part throttle driving. These ports are not connected to the intake so if disconnected they don't cause any problems. I would definitely get the vacuum advance working.

 

Ignition timing is set at idle and should be 5 degrees BTDC. Extremely retarded timing can dump a lot of heat into the exhaust ports where the cooling system absorbs it. Timing can't cause rich or lean mixtures.

 

The popping in the exhaust on deceleration could be the anti backfire valve is disconnected. Won't hurt anything, just annoying.

[Sephakrid]

From pictures that I've seen, the port with the 90 degree bend is used for the EGR, while the straight port goes to the vacuum advance. There might be some other components that use them as well, but it's impossible to know for sure without a vacuum diagram. Is there one on your hood?