75gmck25
Full Access Member
- Joined
- Oct 13, 2016
- Posts
- 2,174
- Reaction score
- 2,047
- Location
- Northern Virginia
- First Name
- Bruce
- Truck Year
- 1975
- Truck Model
- K25 Camper Special TH350 NP203
- Engine Size
- 5.7
My ‘75 truck originally had a dedicated purge line port on the Quadrajet, and I assume it worked. However, I now have a Quadrajet on there from an ‘85 C30. I first just T’d the purge line into the PCV line, but that means I have introduced more air all the time.
Now, for my FYI for those who are still reading.
I recently bought a separate vacuum-operated GM purge valve, and assumed I could connect it to ported vacuum (purge valve is normally closed). It would then not purge at idle, and only purge when I open the throttle and vacuum is applied.
However, when I tried to test it I found my carburetor does not have any port that provides reliable ported vacuum. The small, lower passenger side port has weaker/uneven vacuum, but it never goes to zero. After a little research I determined this is an issue with some Quadrajets. I pulled this carburetor from an ‘85 Chevy C30, so this port may have been used as a vacuum bleed for a specific smog device (EGR, etc.).
Now, for my FYI for those who are still reading.
I recently bought a separate vacuum-operated GM purge valve, and assumed I could connect it to ported vacuum (purge valve is normally closed). It would then not purge at idle, and only purge when I open the throttle and vacuum is applied.
However, when I tried to test it I found my carburetor does not have any port that provides reliable ported vacuum. The small, lower passenger side port has weaker/uneven vacuum, but it never goes to zero. After a little research I determined this is an issue with some Quadrajets. I pulled this carburetor from an ‘85 Chevy C30, so this port may have been used as a vacuum bleed for a specific smog device (EGR, etc.).
