New Charcoal Canister and Idle Change

I replaced my original charcoal canister today, because it smelled really strong of gas after stopping and opening the hood. I installed the new one and replaced the vent line and the line that tees into the pcv. Drove good and gas smell was gone, but it seemed like the idle was higher. I don’t have a tach, so I attached the timing light and the rpms were up about 100. I lowered it back down and let it run for 25 minutes and all seemed good.

Would a clogged/defective canister bog rpms? I’m not understanding why the rpm’s went up with the new canister. If anyone knows a few short words would be appreciated.

Possibly. More fuel into the carb making it run a bit richer?

@Joshua Keith Good suggestion. I checked the vacuum on the idle mixture screws and it was running fat. I dialed them in to a steady 20.5 hg and reset the idle. I drove it around for a bit and all seemed good.

Looks like you got the wrong canister. OE is a 5 port. New appears to be 3 port and one appears to be plugged. Higher idle after replacing a canister, to me, points to a vacuum leak.

@DoubleDingo I don’t have any of the emissions on the truck any longer. I hooked the prior canister back up, because it made the made the tank vent operational with the carb pcv line sucking the fumes through. That’s why I bought a canister for a little a 1972. There is are sucking through the vent tube. I don’t know if there was previously, because I don’t think it worked at all. It smelled like a open gas can.

I kept looking online for examples and figured out what was different. I didn’t understand the purge valve and it was never hooked up previously. The only vent prior was the small built in one on the canister, which didn’t allow for much flow causing buildup of vapor, so the gas smell. With the new one it’s constantly pulling air off the pcv line, which would lean the idle and give the increased rpm. Unless someone can tell me why an inline purge valve hooked up to ported vacuum is needed I’m going to leave it as is, since it seems good after retuning the idle air mixture with a vacuum gauge. I figure as soon as I hit the gas pedal/increase rpms a inline purge valve is going to open and do the same thing regardless.

Maybe the '72 vacuum lines were routed to different ports on the carb? Maybe get a carb for a '72 so it has less ports on it?

Years ago on Mean Green (1965 C20) I ran a '76 carb, it had minimal ports to plug, and all I needed was the vacuum advance hooked up. It ran great. Then my brother wrecked his '85 or '87, I can't recall which year. Anyhow, he said that carb ran great on his truck, so I rebuilt it and slapped it on Mean Green. It got crappy mileage, ran like crap, etc. I told this to Pop and he said, "You are choking the hell out of that carb. All of those ports are meant for certain things, and you have all but one of them plugged off, it can't breathe and operate like it was intended to." So I rebuilt the old '76 carb, and the truck ran like it did before and the mileage went back up 14-16 on the open road.

I say all of that, because you have basically eliminated the need for the other ports, and could go to a simpler carb. Assuming you're running a q-jet. If not, I know nothing about Edelbrock or Holley.

@DoubleDingo I changed to an Edelbrock 1406 last fall and kept having issues until I replaced every vacuum line and port then finally found out the fuel pressure was way too high without a regulator. The QuadraJet was real good to me for 5+ years, but so many ports were capped off and the choke was even zip tied to have it work. The current setup runs good and will light up the tires. I just was puzzled by the rpm change, but realize it was because the purge diaphragm line was never connected previously, so the line teed into the pcv never pulled vacuum. Since the pcv valve opens as soon as the gas pedal moves I don’t see the point in adding the purge valve, since I don’t sit with the truck idling in park for hours where it would cause a lean condition increasing the coolant temperature. I might be all wrong, but I’ll figure out if it’s good to go in the next few days. Thanks, again.

Squirrely Brother said:

@DoubleDingo I changed to an Edelbrock 1406 last fall and kept having issues until I replaced every vacuum line and port then finally found out the fuel pressure was way too high without a regulator. The QuadraJet was real good to me for 5+ years, but so many ports were capped off and the choke was even zip tied to have it work. The current setup runs good and will light up the tires. I just was puzzled by the rpm change, but realize it was because the purge diaphragm line was never connected previously, so the line teed into the pcv never pulled vacuum. Since the pcv valve opens as soon as the gas pedal moves I don’t see the point in adding the purge valve, since I don’t sit with the truck idling in park for hours where it would cause a lean condition increasing the coolant temperature. I might be all wrong, but I’ll figure out if it’s good to go in the next few days. Thanks, again.

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Hopefully others with more knowledge than me will chime in. I am a q-jet fan. I have to be in dire need for me to run anything else. The higher idle is a head-scratcher if it's not a vacuum leak.

I had a really bad gas smell in my engine bay and ended up using this configuration.

@Joshua Keith If I add an inline valve I’d have the same routing. The valve actives as soon as you press the gas, so I didn’t see the point in spending another $38. I adjusted the idle and idle mixture screws to compensate losing between a 1/4 & 1/2 hg of vacuum at idle. Vacuum is at 20.5 hg and it’s running good. I’ve put more miles on it in the last week than I have since July. I forgot how much I like driving it.

@Squirrely Brother
Sorry to bring this up again, just i recently bought the same canister you bought since i cant find oe. Did it work fine? Any updates on it compared tonthe oe one you ran prior?

@Oalejan12 the new canister works great. No gas odor anymore and I don’t have issues with the gas nozzle stopping or spewing when filling up. I teed it into the pcv hose about a foot off the carb, the other spot was used for the tank vent and all has been good to go. I hope you have the same results.

Joshua Keith said:

Possibly. More fuel into the carb making it run a bit richer?

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Huh? You obviously don't understand how the canister functions.

SquareRoot said:

Huh? You obviously don't understand how the canister functions.

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If you have a clogged vapor canister, would it not be possible that when you get a new one and blip the throttle that more fumes would enter the carb then what was happening previously?

Also, the question mark at the end of my statement that you quoted kinda implied that I was just throwing an idea out there. Thanks for helping the guy

SquareRoot said:

Huh? You obviously don't understand how the canister functions.

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My reasoning would be for a malfunctioning canister. So yeah, it wouldn’t really be working the way it was intended..