My '85 MGM, "Maisa"

[Arquemann]

What do you guys think about fuel tank venting with the carb swap?
Should I run a vacuum line from the tank vent to the air cleaner or something? Do these have a charcoal canister?

[matth825]

My '85 MGM, "Maisa"

There is a charcoal canister in the passenger side of engine bay right up by the radiator. It’s mounted down low. I would keep it along with the purge valve and mimic what the carbed cars did for purge control. They used a ported vacuum signal that went through a thermal vacuum switch on the intake to open the purge valve. This would cause the canister to purge under cruise conditions when the engine was warm.

If you go with the Edelbrock performer 302 (part number 3121) it has a rear water crossover with a tapping to mount the switch. It also has provisions for an EGR valve. If you don’t want/need to run an EGR system they make a special carb spacer to block the port.

If you don’t want to do this then the easiest alternative would be to remove the canister, cap the vapor line from the tank and use a vented gas cap. You don’t want to run a vent line direct from the tank to the air cleaner.

Bottom line is if keeping the canister you need a control mechanism for purge. It can’t be connected to manifold vacuum continuously.


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[Arquemann]

Ehh, way too much work for not smelling like gas as much.
I am already an eco-terrorist in the eyes of the environmental authority by driving this de-smogged and my beemer having a DPF delete.

Vented gas cap it is, unless the vent line is better protected from filling with fuel while cornering. Then I'd just use that.

[sly]

The vent line is protected. You'd have to flip the car to get fuel to the vent and even then, there's a ball that would stop the fuel from flowing. Only vapors will go through the vent and those go to the charcoal canister. No need to change anything there. You could just vent the purge valve line if you don't want to put in a thermal vacuum valve. That's what the carbed B-Body Chevys did in the early 80s.

[Arquemann]

The vent line is protected. You'd have to flip the car to get fuel to the vent and even then, there's a ball that would stop the fuel from flowing. Only vapors will go through the vent and those go to the charcoal canister. No need to change anything there. You could just vent the purge valve line if you don't want to put in a thermal vacuum valve. That's what the carbed B-Body Chevys did in the early 80s.

This is pretty much what I was going for. Gonna figure out the vent line at the tank and remove/cut it and try to get the end as high as possible under the car.
Just a vent line to atmosphere, nothing complex and no gas pissing out through the gas cap.

[sly]

Probably a LOT easier to just vent the hose that goes from the canister to the intake/CFI unit. Then you'll still have the canister to help mitigate the fuel smell a little. It does make a large difference. Might even put a breather filter on the canister stub to keep pests from clogging the thing up.

[matth825]

Wouldn’t the canister eventually get saturated with fuel vapor, stop working and result in fuel vapor constantly coming out the disconnected purge line and accumulating under the hood? It wouldn’t be an issue when the car is driving but having all those vapors build up when the car is stationary might cause issues. Maybe I’m just overthinking things with that too…I tend to do that.


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[Arquemann]

I have no idea will the charcoal canister would function without vacuum, or how efficiently or for how long.
Mostly I wonder if the canister without vacuum will cause some pressure in the tank.

A very saturated canister without purging probably won't reduce the fuel smell too much. And having it vent after the canister would mean the gas smell would come from the front of the car, vs in the rear with a straight vent line.
And you can probably all agree that a fuel smell coming into the cabin is a no-go.

[sly]

While the vehicle is in use, the fuel going down will allow some draw from the canister to the tank. If it sits for some time, it will definitely get saturated. It will also "purge" some while fueling up. If the vehicle is used regularly, it's not a big deal. If it sits in storage, then it would be an issue, but a good dose of fuel stabilizer tends to fix that.

The 85 b-body I had didn't have the canister purge setup but did have a canister up front and I never had cabin fuel smell from it. YMMV.

[Arquemann]

Probably a LOT easier to just vent the hose that goes from the canister to the intake/CFI unit.

Seems this is going to be my first thing to try, further action only if needed. I'm not lazy but if the lazy way produces good results, I'm all for it.

[sly]

That's my line of thinking with fixing/modifying things generally speaking.

[gadget73]

you guys aren't putting nearly enough effort into over-complicating this. I are disappoint.

for what its worth the CFI to carb swap I was involved in, we left the charcoal box under the battery and just let it vent as it saw fit. If you want some sort of active venting, just run the vac line up to the air cleaner on the carb side of the filter. Anything in the box will get sucked down the carb.

[Brown_Muscle]

Arqueman- as far as the TV/Lokar fix... you can make the rod work, if you are willing to cut up your original CFI unit. I believe I'm the only one who has made an adapter to allow the use of a TV rod with aftermarket carb. However you need the basic bracket from your throttle body. there are some pictures here: http://www.grandmarq.net/vb/showthre...ice-Car/page29 beginning with post #571. I should have more somewhere if that route actually interests you

[Arquemann]

you guys aren't putting nearly enough effort into over-complicating this.

And that's exactly why I'm going with a carb, it just works. And after 3 summers of cursing at the CFI, I want something that just works. (And prefer not to spend a grand to get to that point) Considering a fuel tank vent can literally just be a hole, I am 100% NOT interested in (re-)creating some vacuum-solenoid-mess after getting all of that crap removed in the first place.

Arqueman- as far as the TV/Lokar fix... you can make the rod work, if you are willing to cut up your original CFI unit. I believe I'm the only one who has made an adapter to allow the use of a TV rod with aftermarket carb. However you need the basic bracket from your throttle body. there are some pictures here: http://www.grandmarq.net/vb/showthre...ice-Car/page29 beginning with post #571. I should have more somewhere if that route actually interests you

You certainly piqued my interest, always interested in simplicity and saving money! You are the second person I know to use the TV rod with a 4bbl, the other being the Finn I mentioned earlier in this thread.
Unlike his modifications, yours seems a bit more elegant and reproduceable, particularly his had the movement flipped, which was the biggest turnoff for me to try it. That and the rod being manhandled like no tomorrow.

I'll have to check the CFI units linkages and the linkage of the Holley today after work. The CFI's linkages might prove a bit harder to mount onto the Holley vs your Edelbrock, since the Holleys linkage is way more complicated and multi-leveled.
I'm not very handy with sheetmetal, but I see the basic idea is just to match the pivot point of the CFI's linkage piece to the carbs pivot point.
Is it crucial to have the TV rod on a separately moving linkage like it is on the CFI? I.e. you can move the TV rod by hand without opening the throttle.


Also, my 1000th post!

[Brown_Muscle]

It may require a bit more bending, but nothing crazy, if you have a vice and a little patience, you can do it. The only part that might be prohibitive is having to weld the linkage to that sheetmetal piece, but if you can find a welder they can do it in seconds.

You technically can have the TV rod linkage fixed, but you will lose the ability to make small pressure adjustments, you'd have to do it from below which might be tricky to get it just right. I have a Holley Sniper unit that I'll need to re-adapt my linkage for, I think it is the same setup as your carb