Wur Vacuum operation

[aricaandjustin]

I have a 77 CIS system....many owners and many attempts to get it running right. I have tried a few Wurs now to get the thing running right. Just to veryify a few things:
1. the vacuum line off the top of the wur has no vacuum and it just goes to the air intake...it doesnt matter if its even hooked up.

2. the bottom line is from the Vacuum control unit and it always has a strong vacuum

If this is true then the diaphram in the wur is always being pulled downward onto the plug which is where it would sit normally wihout a vacuum because of the springs. Knowing that spring tension must be decreased to richen the mixture for WOT, how does this system work?

Also, rather than find out what is bleading off my system pressure when the engine sits (and results in hard warm starts) can I just put a check valve in the return line?

[kodioneill]

Has strong vacuum at idle = manifold vacuum. when you crack the throttle the vacuum decreases proportionately with throttle position this lowers the control pressure for enrichment during acceleration.

[914efi]

Your 1 and 2 are correct.

If you are losing too much fuel pressure during shutdown could either be the accumulator or the check valve in the fuel pump. Adding another check valve won't help, you need it between the pump and the accumulator.

Vac pulls on a diaphragm that acts on the control pin in the wur, less vac (throttle open wider) means less pressure on the pin which allows more fuel pressure to bleed past valve in wur, lowering control pressure.

Control pressure is what pushes against the air plate upward motion in the fuel dist, less of this downward force allows the air plate to rise more from a given airflow which richens the mixture.