Hi, I'm pretty new to this and was hoping someone might be able to help. Basically I've replaced the engine and carb on my '78 and am trying to reconnect the vapor canister.

There's 4 ports on the top of the canister excluding the fuel tank line. Two of them connect at a T and go to the PCV valve. I'm not sure on the other two as I was planning on getting rid of the EGR system.

For the thin EGR line (top right on the canister) can I just plug this off if I'm deleting the EGR valve and control switch? Or does it still need to connect to ported vacuum on the carb?

Also for the thicker "carb bowl" hose (bottom left on the canister) I can't see a carb bowl port on my holley carb diagrams to connect it to. Is there any other port this should connect to or is it best to plug it off? I've attached a diagram that came with the new carb.

​​​​​​Sorry if these are daft questions but any help would be appreciated!
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Many thanks.

 

I believe you can drop the egr nipple on the vapor cannister. Can't speak for the other issue you have. Have a great day

 

I have a 78 and the only port on the vapor cannister that is not capped off is the one going to the gas tank. I still have my original QJET.

Driver's side valve cover has the PCV with only one hose that goes directly from it to the port on the front of the carb . Hose from the dist to manifold vac on the front of the carb.. Other nipples on the carb I am not using have a rubber plug.

 

Have a look at this thread for some diagrams: https://www.corvetteforum.com/forums...e-routing.html

This diagram is from that thread:




The 'EGR' switch is what is called a Thermal Vacuum Switch (TVS). It is essentially a vacuum switch that will open when the engine temp reached a certain temp (operating temp). It is connected to a ported vacuum port on the carb, which is a port that only receives vacuum when the throttle blades are open (so not at idle). The valve on the evap canister that it connects to is normally closed, so it will open when a vacuum is applied. Since the TVS switch is connected to a ported vacuum port, two things have to be true for the evap canister to see vacuum; The engine must be at operating temp, and the throttle blades are open enough so there is ported vacuum available. When that valve on the evap canister is open, it allows vapors from the canister to get sucked into the carb and burned in the motor. Those conditions are important because when that canister is allowed to vent into the carb, it is essentially a vacuum leak that is AFTER the portion of the carb where it determines how much fuel to add. So if it were open at idle, the idle speed control would be rendered useless. Once the throttle blades are opened, the amount of air added is small compared to the volume the engine is using so it's not a big deal. Waiting until the engine is up to temp (TVS switch) also ensures that there is not a scenario where you are adding excess air when the choke is engaged.

Now the fun part come in when you look at the hose diagram for the other valve. The big hose gets connected to the carb bowl, which allows the fuel vapors from the carb bowl to vent into the evap canister (reducing gas smell). But the signal line for this valve is tied into the output of the other circuit, meaning a vacuum is applied to this valve when the other circuit is operational (off-idle and operating temp). This valve is also normally open, so it will be open when the TVS circuit valve is closed, effectively making this circuit work in opposite of the other. Which makes sense, you want to vent the fuel vapor from the carb bowl when you are at idle, but when you are under acceleration, you don't.

I hope that's clear as mud

In your application, since you do not have a port on the carb bowl, you can eliminate that line and any line that goes to that valve, it's useless.

For the other circuit, I guess you could route ported vacuum directly to the evap canister valve if you really wanted to eliminate the TVS switch, BUT all that would do is eliminate the valve not being opened until the engine is warmed up. That might actually give you some issue until the engine is warmed up.

I would recommend keeping the TVS switch as it is really helpful, and doesn't cost any power.

PS if you do decide to get rid of the TVS switch, either keep it or sell it on the forum, those switches are no longer made and go for a good penny on eBay

Hope this helps.

 

Thanks for taking the time to explain that, it is definitely a big help!