Originally Posted by DoubleVision
My 92 Chevy Silverado has a warmed up 4.3 liter V6. Last year I removed the TBI and the swirl port heads and switched to 1998 Vortec heads, a Marine Vortec intake, quadrajet carb, bypass style fuel pressure regulator and a vacuum and mechanical advance HEI. The vapour line that runs from the carbon canister used to run to the vacuum port on the front bottom of my TBI's throttle body. Since I switched to a carb I've left the vapour line off until I finished up the rest of the mods I'm doing under the hood. However some days ago I decided to experiment. I pulled the smurf condom off the lower front vacuum port on my Quadrajet, this source pulls full time manifold vacuum. When I connected it, the idle speed went up by at least 3 to 400 rpm. Then I disconnected it and left the vacuum port open and the idle dropped to around 800 RPM. I put the smurf condom back on the port and the idle returned to it's normal 700 RPM. When the TBI was still on the truck the vapour line was connected to a manifold source as TBI's didn't have ported vacuum and it didn't effect the idle speed at all. Why does it with the carb? So then I was thinking I could just connect the vapour line to a ported source elsewhere on the Q-jet and it be okay since it would only pull vacuum off idle.
However I was also thinking on some TBI engines they used some kind of device that connected to the vapour line and to the TBI's manifold vacuum source. I'm guessing this was some kind of purge valve that purged the vaccuum canister in timed cycles. However my truck didn't have one on it when I bought it in 2006. So can I just connect the vapour line to a ported vacuum source or do I have to get a purge valve? Any help is appreciated.
No, you just can't connect the line to ported vacuum on the carb, these systems use a purge valve that admits fuel vapor only under certain conditions, idle not being one of them. Older systems would have a vacuum operated valve, your OBDI, TBI has a computer controlled valve that got cycled by the computer from data about throttle position, manifold vacuum, and RPM from sensors.