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my original evap canister has 2 nipples on top but the replacement i bought has 3, a single and 2 stacked. how should i address this. maybe a Y connector? thanks
I can't tell from your avatar what year you have other than '73-'77, so maybe provide that in case it makes a difference. I don't know if the later canisters changed, but you may not have had the original in the car. That said, on the 3 port canister there is a fuel tank line, a large PCV line, and a smaller "signal" line going to a carb port. Do you know how your two-port connected things?
76-77 same thing originally 2 hoses tank and carb.
The 3 port would also include a purge line and your carburetor if original dose not support that.
Well maybe it would support a purge line "Only" if you cut and splice in a "T" into the line from carb to canister.
The purge requires a ported vacuum source same as what you have now.
Last edited by bmotojoe; Feb 6, 2023 at 10:55 AM.
Reason: Well Maybe It would support...
76-77 same thing originally 2 hoses tank and carb.
The 3 port would also include a purge line and your carburetor if original dose not support that.
Correct!
Here are some photos from original Evap Canisters in my collection (except for the last one). The three port with the purge valve were used on the 70 thru 73 Corvettes. The two port canisters started to be used in 74 thru 77 for sure.
The earlier C3 canisters thru 73 are no longer available but I'm pretty sure the 74 style is still sold by the vendors. Much later on there are completely different style canisters used, one example is shown here in the last photo.
Hope this helps.
Tooch
70-73 Style on the left. 74 & later on right. 70-73 style top close up with purge valve 74 & later Style top view Much later style, well into the 80's ?
perhaps i should have started with this... picture 1 is my old, picture 2 is the replacement. is that 3rd nipple with the plastic over it just a vent? i just noticed it is labeled ATR
What I have never understood about this change from the vacuum signal to the canister to the later model without is........what signal is coming from the engine to the canister to tell it to purge....or does it just purge ALL the time. Maybe they later decided that purging all the time was okay. Maybe they tapped off the carb vacuum on the ported versus manifold side???? But in any case, ....the more complicated purge that included a switched vaccum signal was cancelled for one that had fewer lines. My 77 is like the one in the second picture.....it has three ports....but only two are used. One line comes from the fuel tank...the other goes to the base of the carb.....NO vacuum switches of any kind.
What I have never understood about this change from the vacuum signal to the canister to the later model without is........what signal is coming from the engine to the canister to tell it to purge....or does it just purge ALL the time. Maybe they later decided that purging all the time was okay. Maybe they tapped off the carb vacuum on the ported versus manifold side???? But in any case, ....the more complicated purge that included a switched vaccum signal was cancelled for one that had fewer lines. My 77 is like the one in the second picture.....it has three ports....but only two are used. One line comes from the fuel tank...the other goes to the base of the carb.....NO vacuum switches of any kind.
If you dig through a vacuum diagram, I think you will find that ultimately ported vacuum is the signal. No signal at idle (if the carb is adjusted correctly) so you don't have a massive vacuum leak.
If your vac can has a purge port that isn't connected to anything, it may never vent all. This would make for a great Mity-Vac experiment.
If you dig through a vacuum diagram, I think you will find that ultimately ported vacuum is the signal. No signal at idle (if the carb is adjusted correctly) so you don't have a massive vacuum leak.
If your vac can has a purge port that isn't connected to anything, it may never vent all. This would make for a great Mity-Vac experiment.
My car is a 77. There is no signal line coming from any kind of vacuum switch to the evap canister. There is a three fitting type canister that is correct for 77, and pictured above in post #6, bottom picture. One line comes from the fuel tank, the second line....the PURGE line goes from canister to the carb....and I agree it goes to the ported vacuum port. The third fitting on the canister appears to be a cap,...but could be some sort of vent...not sure about that. What I know for sure is that on a 77, there is no vacuum switch ON the canister itself, nor anywhere else. That was my point.....these later evap canisters were simpler....no switching....just ported vacuum into the canister, and thats it. I would assume the OP could choose to opt for this same, simpler system.
WIth a forum search,...you will find lots of threads on this subject. Many people in past have simply chose to NOT run any vacuum lines between the canister and the carb. I get very tempted to do that, because its one more vacuum leak source that could potentially change how the engine runs. The people who have done this appear to have no negative results...not even fume smells in the garage. Main thing is you MUST continue to have some sort of vent for the fuel tank....the factory EVAP sucks and burns those fumes into the engine....but it also becomes a potential vacuum leak. On the ported side.....its apparently not a problem. On the manifold port ....its a vacuum leak by definition. If you simply leave the canister installed, with the line from the tank to the canister.....and let the other port open to free air.....seems like it would continue to be a vent for the tank. The charcoal absorption of the fumes would still work.....so I wonder what the downside would be??
Last edited by Shovels and Vettes; Feb 6, 2023 at 03:55 PM.
Your Dorman 911-261 has the required ports you need.
Purge port to carburetor.
I would definitely remove the bottom cap and reuse that cap on your original as it has the provision to drain.
Here is an interesting post from 2017 from a forum member rebuilding a canister just like ours...
You could always e-mail the Dorman Support team with your ATR question. http://techsupport@dormanproducts.com https://www.corvetteforum.com/forums...r-rebuild.html
My car is a 77. There is no signal line coming from any kind of vacuum switch to the evap canister. There is a three fitting type canister that is correct for 77, and pictured above in post #6, bottom picture. One line comes from the fuel tank, the second line....the PURGE line goes from canister to the carb....and I agree it goes to the ported vacuum port. The third fitting on the canister appears to be a cap,...but could be some sort of vent...not sure about that. What I know for sure is that on a 77, there is no vacuum switch ON the canister itself, nor anywhere else. That was my point.....these later evap canisters were simpler....no switching....just ported vacuum into the canister, and thats it. I would assume the OP could choose to opt for this same, simpler system.
WIth a forum search,...you will find lots of threads on this subject. Many people in past have simply chose to NOT run any vacuum lines between the canister and the carb. I get very tempted to do that, because its one more vacuum leak source that could potentially change how the engine runs. The people who have done this appear to have no negative results...not even fume smells in the garage. Main thing is you MUST continue to have some sort of vent for the fuel tank....the factory EVAP sucks and burns those fumes into the engine....but it also becomes a potential vacuum leak. On the ported side.....its apparently not a problem. On the manifold port ....its a vacuum leak by definition. If you simply leave the canister installed, with the line from the tank to the canister.....and let the other port open to free air.....seems like it would continue to be a vent for the tank. The charcoal absorption of the fumes would still work.....so I wonder what the downside would be??
Actually earlier systems have an external fuel/vapor separator valve that works off of a vacuum signal (the switch) as apposed to the 75-77 fuel/vapor separator valve that's internal and part of the fuel sending unit. GM called it liquid separator.
Actually earlier systems have an external fuel/vapor separator valve that works off of a vacuum signal (the switch) as apposed to the 75-77 fuel/vapor separator valve that's internal and part of the fuel sending unit. GM called it liquid separator.
Having the purge valve on the EVAP cannister certainly is simpler than a separate valve hanging in the rat's nest of vacuum hoses.
I don't think any of us are on the same page here. I am not confused....but you are not describing the systems correctly. I know what my 77 has. There is NO vacuum "switch" of any kind on this car, as manufactured from Chevrolet. There is the canister in bottom picture of post #6. One hose from metal line coming from fuel tank, and one hose going up to carb....which I am pretty sure is going to ported vacuum fitting...not manifold vacuum fitting. SO....NO vacuum switch. A vacuum is applied to the canister ONLY when the throttle valve moves past idle and it really is nothing more than incoming air through the carb...pulling fumes out of the canister. There is no other component in the system.
The fuel sending unit on top oft he fuel tank does have a fitting. From that fitting is a rubber hose that goes to the driver side of the car, to a metal tube that runs full length up the side rail to just near the evap canister. Then a rubber hose from that hard line to the fitting on the evap canister labeled "tank" Fuel vapors in the top of the fuel tank just run freely....without any " switching" of any kind, i.e.....its just an open hole, venting vapors from the tank up to the canister. The activated charcoal apparently attempts to seperate or retain the vapors....and when the engine is running, above idle......the vapors are sucked into the engine. EPA stuff. IF you don't run the rubber line from canister to the carb......I suppose it just vents into the air. I really am not sure about that part. Some would be concerned about fuel smells.....other concerned about global warming and end of the world stuff. My 69 simply vents into the air, through the gas cap.....which I like best.
Last edited by Shovels and Vettes; Feb 6, 2023 at 04:40 PM.
Having the purge valve on the EVAP cannister certainly is simpler than a separate valve hanging in the rat's nest of vacuum hoses.
Purge is just a term, the vacuum signal is the purge.
75-77 Sending unit red arrow is the (switched) fuel/vapor separator, more like a check that closes under vacuum and opens when vacuum drops off.
Purge is just a term, the vacuum signal is the purge.
75-77 Sending unit red arrow is the (switched) fuel/vapor separator, more like a check that closes under vacuum and opens when vacuum drops off.
That picture is definitely what I have on my 77. I am not sure your description is correct. My understanding is that it sits there in the top of the tank.....I thought it was nothing more than on opening, so that vapors would travel to the fitting on the outside of this piece,...then through the rubber hose,...to the frame rail hard line, then to the TANK fitting on the evap canister. What the device you have pointed to in the picture may do is prevent LIQUID fuel from entering that line,....possibly like a float valve that closes when fuel pushes it up....liken when someone overfills the tank,..or heat expands a full tank of gas. It does appear to be MORE than just an opening. I don't follow your desription that when vacuum is applied (From the carb) this valve would close???? If it close with vacuum.....fuel vapors would be stopped, and NO venting would happen....while the car is running down the road, and fuel level lowering. IF that was the case, how is the tank vented?
I still don't undestand what you are saying. That desciption of the fuelcap is calling it HIGH pressure relief,...not negative pressure (vacuum) that occurs when a fuel level decreases in a sealed container (tank),....there has to be a vent. The cap may vent positive pressure. Metal aircraft fuel tanks will collapse the skin of a wing if the fuel vents are blocked. These terms get thrown around in these threads, and confusion results.
I honestly don't know how this system is supposed to work, nor am I sure what that fitting does on the tank sender. I do believe its a fuel / vapor seperator....and it does that by preventing raw fuel from getting into the fuel line and allowing ONLY fuel vapors to travel into the line. You suggested it somehow cuts off venting by the signal from the front? I don't understand that. If when vacuum is applied.....that valve closed....that would seal the gas tank when running down the road,....and would ONLY open when throttle is below idle....then allowing it to open, and vent to the canister.??
ON the other end.......people have disconnected the rubber hose from the evap canister,....and providing ZERO connection between the vacuum source (carb) and the fuel tank......and nothing changes.....except the fact that fuel vapor escape into the air rather than getting sucked into the engine.....which is why the government created the system......EPA attempt to save the world.
Maybe I am dense....could be.....but I still don't have a clear explanation of this entire system.
I will say I had it hooked up correctly for 20,000 plus miles on the car.....no smells, no fuel problems.
Last edited by Shovels and Vettes; Feb 6, 2023 at 07:19 PM.
I'm including an image from a 3-port cannister but used in 1978 and beyond. I suspect, with GM's way of SERVICE REPLACEMENT part system, labeled the three-port for previous 2-port systems and the 1978-82 ECS systems. Here's the write-up for both two and three hose connections...little bit more elaborative from what BM and others writes but should be the same end result;
ECS Hose Routing
A black hose exits the elbow PCV valve, runs to the carburetor base and then from another port, returns to the canister. In contrast to the smooth texture of other hoses, ECS hoses have alligator-skin texture. The PCV hose is marked in white 3/8 –GM 6107M KV. The ECS return hose from the carburetor to the canister is smaller diameter marked in white 1/4-GM 6107 M H-KV. Hose part numbers may not always be seen. A green spring clamp secures the larger hose to the PCV valve. A canister port labeled Fuel Tank has a yellow striped hose, held by a yellow spring clamp, connected to the steel tank return line.
This is a description of the 1978 ECS hose connections:
ECS Hose Routing
Five vacuum lines are attached to the three ports located on the canister top; three of which route through a clamp on the inner wheel well, and to a clamp at the rear of the alternator, toward the carburetor. One canister port labeled Fuel Tank has a yellow-striped hose, held by a yellow spring clamp, connects to the fuel tank return line that runs along the driver’s side frame rail.From the other port, a short-black hose connects to a Y fitting whose two hoses route to the carburetor; one hose connects to a second Y fitting and its hose connects to the PCV valve. A hose exits the elbow PCV valve, runs to the carburetor base and returns to the canister.In contrast to the smooth texture of other hoses, ECS hoses have alligator-skin texture. The PCV hose is marked in white 3/8 –GM 6107M KV. The ECS return hose from the carburetor to the canister is smaller diameter marked in white 1/4-GM 6107 M H-KV. Hose part numbers may not always be seen.
This supplements what others have written. Hope it helps give you some direction how to convert that three-port cannister to your two-port application,
Last edited by hunt4cleanair; Feb 7, 2023 at 03:53 AM.
Reason: edit quotes
ATR stands for “atmosphere” on the canisters. So it is not a third nipple or port for any hose connection. So, I guess since this is the only type the vendors are selling for 74-77 Vettes, the lower port gets the hose running back to the tank. On 74’s that hose goes back to the fuel separator “rollover” check valve located outside top left of tank. The valve changed to (inside tank) what is shown above for 75 and later. The smaller top port on the canister must then go up to the carb. If it was up to me I’d just rebuild my original canister, by opening it up from the bottom and replacing the activated charcoal and replace the filter.(A rather easy and fun job to do!) If you shake your canister and can hear the charcoal loosely moving around it’s still ok. Just replace the lower filter and clean up the canister. Re- install. If the charcoal is old fuel soaked and hard…will need replaced!
I’ve included another photo below from an original 76 showing its canister with the two hoses SIDE BY SIDE, not on top of each other. Also, the 76 engine photo showing the hose connections. The blue arrow is the hose coming off the evap canister to the carb. The other yellow arrow is PCV valve hose going to the carb.
Hope this all helps clarify things for the OP and his ‘76.
Tooch
ATR-“atmosphere” 76 evap canister yellow to tank, red arrow up to carb Yellow arrow pcv valve hose to carb, blue arrow evap canister to carb
