Move this post if need be. But I need help.

I have a 1986 (call it an 85) Nissan 720. Everything has been done as far as tune up goes. Valves are set. Plugs. Wires. Cap. Rotor. Carb rebuild. New O2 sensor. All vacuum lines good. EGR works. BPT works. MC solenoid works. Throttle switch set correctly.

I went from running a Weber for awhile back to stock because I have to pass smog (california - yes I know, don't care. Born and raised, can't change it). It runs awesome when Vac switch A and B are disconnected (tricks the ECC system to think it is WOT all the time, basically open loop) but failed miserably on emissions. It runs OK with EGR vac line disabled but failed NO miserably.

Takeoff is rough with EGR connected. Is it possible, because the BPT controls the vac signal to the EGR, that of the catalytic converter is plugged enough to run OK but have enough back-pressure, that it can open the EGR too soon resulting in rough takeoff?? Or is it possible the BPT is worn out to it's too sensitive??

That's the opposite of the problems most people get from a BPT, but if it runs well with the EGR disconnected then it has to be getting exhaust flow at idle and low RPM where the EGR should be closed. The fact that it runs well sans the EGR also vindicates the EGR itself, it can't be sticking open. The bottom line is, both of your suggestions are possible. Most of the BPT's use a belleville spring internally to calibrate the stiffness of the diaphragm, so they can get "weak" after years of operation. Obviously a partial exhaust blockage would have the same effect. You can rule out the latter by checking engine vacuum at idle and then at 2~3K RPM for about 10~15 seconds, then rapidly dropping the throttle. If the vacuum drops precipitously at the higher speed and then doesn't snap back immediately when the throttle is dropped, you probably have some sort of exhaust restriction.

Your idea of the engine vacuum makes sense why I let off the throttle and feel a jolt in the drive drivetrain. And why it's jerky also letting off then putting throttle back in. I need to check the cat. It's original, 156,000 miles, and I drive faster than any 720 owner should drive daily on the way to work nearly 30 miles away. I was consuming like a quart maybe 1500 miles and I've daily it for a year and a half.

Thank you in advance to anyone who can help.

I no longer believe the issue is EGR related, however with everything else blocked off besides EGR it runs OK but amazing without EGR active. I've capped vacuum lines off in an effort to figure out what is going on. Please bear with me, it's a lot, and again thank you to anyone that can help.

The TVV is bad. When warmed up, it still allows air in from the air cleaner. I have the 3-port design, so the EGR line and the vacuum advance both go there. The FSM isn't entirely clear on testing as it mentions only drawing vacuum in from the distributor advance port and not the EGR port. On mine, it seems air flows through one to the other when I block off the top port that goes to the air cleaner

---- air cleaner
--- EGR
--- DISTRIBUTOR ADVANCE

When messing around under the hood yesterday, I pinched a line in the EGR series of lines that goes to the vapor canister. I then heard some noise like a vacuum leak near the vapor canister and I noticed it was the larger line (both of these lines come off the hard lines in the front of the engine). I then pinched that one and it stopped. The larger line comes off the curved piece on the intake manifold, shared with the line to the air cleaner and the AB valve. So it appears the valve on top of the vapor canister may also be bad, because when I blocked just the vapor canister off it ran better.

If anyone can clarify the TVV that would help me out greatly. In my mind, it shouldn't be sharing vacuum signal like that between EGR and vacuum advance.

If memory serves (and it may not exactly) the 3-port TVV's were a single valve with two output ports, whose purpose was to cut off vacuum to both the EGR system and the vacuum advance until the engine warmed up. So you should basically have no vacuum on the output ports with the car cold and full manifold vacuum when warm. I seem to recall that getting the inlet location mixed up with one of the outlets could cause headaches because one of those two would have vacuum all the time and the other would get it normally with temperature. The bottom line is, it shouldn't be passing any vacuum until the engine warms up. If it does that, it's working properly.

That part I understand. What I mean is, if I plug the one that goes to the air cleaner, which is the port that shuts off when warm. I can use the vacuum gauge on either of the other ports and it passes to the other. As is they are shared, which wouldn't make sense because both ports get their vacuum signal from different ports on the carburetor.

That sounds like something isn't routed correctly. Been a long time since I worked on one so I could be mistaken, but my recollection is that both should get straight manifold vacuum with the only porting being the TVV itself. The EGR then gets controlled downstream by the BPT etc.

They're routed correctly. One is from the front corner port on carb base, the other on the lower side carb port. Disconnected, bare TVV, if I connect vacuum hand pump to bottom port, plug top that goes to air cleaner, the suction goes right out the middle port and vice-versa. Like the thing is hollow.

I'm pretty sure it should be sealed to both top ports when the valve is cold, so that definitely doesn't sound right.