Pressure in the gas tank

[kully 560]

2 hours ago, kully 560 said:

I have a 74 and no check valve, and no pressure at all. it could be a clog return line, vent line

 

[Jeff Berk]

I found what I think is the charcoal vent line. It terminates at the front right fender, engine compartment. 

I blew air at about 30 psi into the tube while my son listened for air at the filler port. Nothing.

I'm assuming that the charcoal vent tube originates at the overflow tank inside the right rear fender so I'm going to start stripping panels. 

Update:

I used a hand vacuum pump on both the tube that terminates in the engine compartment and the hose that connects to the top of the overflow tank in the trunk. Both ends hold a vacuum.

Edited July 19, 2022 by Jeff Berk

[Captain Obvious]

5 hours ago, Jeff Berk said:

I used a hand vacuum pump on both the tube that terminates in the engine compartment and the hose that connects to the top of the overflow tank in the trunk. Both ends hold a vacuum.

Yeah... It's not supposed to do that. Guess you already knew that. 

So I looked at the FSM and the 74 does not have that two way check valve thing that the later years did, but there IS supposed to be an orifice restriction in the vent line between the rear of the car and the carbon can (FSM EF-33). If the line is plugged, I'd start there. It's probably green crusty brass at this point.

[Jeff Berk]

It seems that if all this tube does is route fumes from the overflow tank to the engine compartment and dumps it there (the carbon canister is missing), could I just cut the hose off beneath the car next to the gas tank and let it vent there? 

[Jeff Berk]

“O Captain, My Captain!” I see that orifice thing in the FSM but if it was located adjacent to the carbon canister, it is likely missing as well. 

Edited July 19, 2022 by Jeff Berk

[Captain Obvious]

I've seen that brass orifice in years gone past, but it's been so long that I don't remember where Datsun originally put it. I suspect it was stuffed into the rubber tube that connected between the carbon canister and the return hard line up in the engine compartment. I looked a little through old pics and didn't find any. I've got vague recollection about talking about it here on the forum a long time ago. Maybe Cliff can Rain Man a link for us.   

And yeah, If you've removed the carbon canister completely, then in theory it really doesn't make any difference where you vent that line. Front of car, back of car... No environmental difference. Just make sure it's vented high enough so you don't ever dump liquid fuel out onto the ground.

[siteunseen]

2 hours ago, Captain Obvious said:

I've seen that brass orifice in years gone past, but it's been so long that I don't remember where Datsun originally put it. I suspect it was stuffed into the rubber tube that connected between the carbon canister and the return hard line up in the engine compartment. I looked a little through old pics and didn't find any. I've got vague recollection about talking about it here on the forum a long time ago. Maybe Cliff can Rain Man a link for us.   

And yeah, If you've removed the carbon canister completely, then in theory it really doesn't make any difference where you vent that line. Front of car, back of car... No environmental difference. Just make sure it's vented high enough so you don't ever dump liquid fuel out onto the ground.

 

[siteunseen]

Post 8 maybe? 

[Captain Obvious]

Man Cliff, You are simply amazing with that.

I was heavily involved in that discussion (that wasn't even that long ago), and I couldn't pull up the details. Thanks for having my back!   

[kully 560]

I have a 1974 260z and I check the fsm-ef32 and ef33. on my car the vent line from the vapor tank above the fuel tank goes to the front canister on the passenger side. there is an orifice built into the hard steel line that when I removed the hose from the canister, I can't blow back to the vapor tank but can suck air from there. I believe that will stop pressure going back to the vapor tank/ fuel tank .

[kully 560]

I seem to remember some were way back then, that I also did want to get rid of the carbon canister. I was told never to do that because for a reason I can't remember that the car will never run right. as far as fuel pumps goes. I use a low- pressure carter at the back of the car with no pressure regulator on the return line and I have 3.42 lbs at the carbs. try to keep thing simple.

[kully 560]

if this helps, this morning I removed the vent line from the vapor tank, and I could not suck any air through the line. I guess because the car sat overnight, so today I drive the car and when I turned off the engine, I removed the vapor line at the canister and was able to suck air through the line.