Working on a '79 Bronco with 351 and Motorcraft carb. Failed emissions test with distributor vacuum connected. Disconnected and plugged to pass emissions but then ran into driveability problems.

Vacuum diagram under hood (calibration 9-60-R10) shows 3 vacuum ports labeled S, E and V. It appears S is Spark Port, E is EGR Port but I don't understand what V is for.

Here are the vacuum readings:
S - 0" at idle, then increases as throttle opens
E - 2" at idle, then increases as throttle opens
V - 0" always

Vacuum diagram indicates port E is connected to distributor via PVS or vacuum restrictor, depending on coolant temp. In either case, I thought distributor vacuum should be 0" at idle (like condition when vacuum removed for emissions test), however, if the EGR port provides the vacuum, and it's 2" at idle, it would be impossible.

Questions here:
Is it correct that at idle, no vacuum is generated at either the Carburetor Spark Port or the EGR post. (got this from Chiltons)

Is it correct for the EGR port to have 2" vacuum at idle? (Would seem incorrect based on Chiltons research)

Is it correct for the "V" port to not develop any vacuum?

Related topic:

There is a 4 port vacuum switch in this configuration labeled S, E, M and D from base to tip. It seems, when warm, S and E are connected as are M and D.

Do these labels correspond to the carb vacuum port labels or should the lines connect as depicted in the under hood diagram?

The diagram indicates the EGR port should be connected to the port labeled "D"

 

The "spark port" and EGR connection both source from what is called ported vacuum, of which there is none at idle. This is because the source is above the throttle plates. The Chilton's manual is correct.

The EGR valve should not have any vacuum at idle because it needs to be completely shut in this case.

I'm very sorry, but I'm not familiar with what the "V" must stand for. Are these letters on a ported vacuum switch in the diagram, or what?

Regardless, it's likely that after 30+ years, the ported vacuum switches you are probing are inoperable. They rely on a wax ball inside that seldom stands the test of time.

You might be able to pass emissions with simply a single-port vacuum advance connected straight to ported vacuum, and an EGR valve connected to ported vacuum through a 2-port (open/close) ported vacuum switch. There are dozens upon dozens of cryptic vacuum setups over the years, and they changed with great regularity. As such, it's hard to make anything out of these diagrams. It's even harder to find replacement parts.

 

Thanks for response. The S, E and V designations are on the vacuum diagram and refer ports on the carb.

I'll take a better look at the carb and verify, what is designated the E port, obtains it's vacuum above the throttle plates. Could be a different carb than original.