Questions About A 72 With A 304

Hi Joltes - If it were mine, I'd try to put it back to original. Looking at the TSM, shouldn't your expansion tank have 5 nipples, one from each corner and one to the front? I thought the later tank had two nipples, so you'd need to plug two expansion tank nipples somehow, either by teeing the lines or with a cap of some sort.

The simplest/quickest thing to do is cap the line to the front and use a vented gas cap. This will let air/vapor exchange at the back corner, and may smell a bit if you park in a garage. Jeep was building vehicles with vented caps well past this time - my Maine-delivered '77 J10 has no vapor recovery and a vented cap from the factory. Vehicles above 6000 lbs GVWR (like my 49-states J10) had looser requirements for emissions well into the 80s, maybe beyond.

If you want to use the '73-style vapor recovery, you will need to remove expansion tank and replace it with a liquid check valve. Then your line to the front goes directly to a charcoal canister which both vents the tank and traps vapors. This would be like the canister shown in the '72 TSM. Purge for the canister is from a hose going to a nipple in the snorkel/snout of your air cleaner. When the engine is running, incoming air creates a small vacuum in the line, which draws air into the bottom of the canister, through the charcoal bed, and into the engine air to be burned. Pretty simple. Difficulty may be finding a suitable canister. MTS sells a check valve work-alike MTS COMPANY, L.C. - Jeep CJ/YJ Gas Tanks - see their part JLVC-1. The expansion tank may work in lieu of the liquid check valve - don't know, maybe. The original-style check valve is a 3198666, which was available recently but apparently NLA OMIX-ADA 17731.01 Fuel Check Valve for 84-86 Jeep CJ

Jeep used this style of check valve through the '70s, until the newer plastic tanks used a check valve inserted into a grommet in the tank (likely that's what the MTS part is, with an added housing to make it work outside the tank).

Two developments changed the canisters. First, they started venting the carb float bowl to the canister, to pick up most of the float bowl vapors when not running. See the '76 TSM:


This is from the '76 TSM at www.oljeep.com - there is also a '74 TSM there which will have a vapor recovery system like a '73, but not like a '72. You can see how the purge line goes to the snorkel here. That's just a tube cut with a bevel facing in that is positioned in the air flow. You could use one of these canisters for a 2-nipple style if you capped the bowl connection.

Then, they started actively purging the canisters using manifold vacuum. These canisters typically have 4 ports: tank, bowl, purge, manifold vacuum. When manifold vacuum is high, the purge valve opens and connects the purge line to the PCV line. This draws air in through the canister via the PCV vacuum. You could use one of these tanks if you install the extra plumbing. I'm running a canister from an XJ in my J20 because I could get a new one and it works the same as the original.



This is from the '82 TSM. To use a canister like this, you'd plug the bowl line, connect the secondary purge to manifold vacuum, connect primary purge to the PCV line, and connect to the tank through a liquid check valve.

I recall there is a Chevrolet design used for the 4.3L V6 vehicles that has 2 connections and uses an external purge valve. That might work in place of the '73ish 2-nipple canister. This is for an '88 S-10 More Information for STANDARD MOTOR PRODUCTS CP1022 - not cheap but maybe you could find one in a junkyard? They use an external solenoid-controlled purge valve (controlled by the EFI computer, I presume), so they should work ok with the snorkel purge.

 

Well, my plan thus far is to use the 72 canister. You are right with it having 5 nipples. One is specifically designated for the line to the front, and I believe it probably goes up into the canister all the way to the top, so it pulls vapor and not any liquid that may make it in there. Of the other 4 lines, I am hooking 2 up to the new tank and a third to the vent line on the new fuel sending unit. The 4th line from the canister will get the cap. This doesn't quite equal up to the 4 corners of the original tank, but it does give 3 separate places for it to pull vapors from. Does this sound kosher to you? I am going to post in the WTB sections for the valve, as you suggested and see if anything comes up.
I wish there were more aftermarket manufacturers of Jeep items, like there are for the old VW's....

 

I believe the new tank has internal lines that do the same as the nipples at each corner, so connecting the two lines on the side does equal the four vents at the corners. Personally I would cap the extra sender nipple, and loop hose between two of the nipples on the expansion tank, but the way you describe should be fine too. You could run a return line from the carburetor to the sender if you wanted - the return line is meant to help prevent vapor lock. Fuel flows constantly and stays cooler. You'd need the fuel filter with the extra nipple and a 1/4" line running to the rear.

Yeah, unfortunately Jeeps from this era are all oddballs today, though the CJ-5 is not the most extreme example. AMC started selling a lot of CJs in the early 70s, fueled by Baja racing and the off-road craze. Unfortunately '72 was a watershed/transition year, and the generation of intermediate Jeeps only lasted 4 years. There are quite a few more parts available for '76 up models, after the second great watershed produced the all-AMC design.