fuel from pump to carb question

[sactoC10]

Checked the intake, clean as a whistle...tank clean as can be too. I did NOT plug the vent line, in fact I did not remove the canister. I plugged the two other inlets into the canister. Tried to BLOW through the vent line, was not easy to do...no WHOOSH sound...how do I clean that? Also pulled the cap off just to check, no whoosh sound...think that only happens when it has been operating for a while. Here are some pics, you can see how I tried to insulate the lines (hose against fuel line). NEXT???

[cableguy0]

it would have to run for a while to create the vaccum and more than likely only do it when the fuel level is lower. try this and dont do it with a full tank especially if you like your paint. dont screw the gas cap all the way tight and go for a long drive and see if it happens. if it doesnt happen most likely you need a different gas cap

[sactoC10]

Fuel is always low in my truck! This happened before, but I can try another gas cap at some point...any extras around Sacramento?

[neonlarry]

Next time it happens just reach out and loosen the gas cap and then put it back see if that helps.

[sactoC10]

Larry, great idea!
So let me see if I am understanding what I think may be the problem, some sort of venting matter related to the tank not venting correctly. And this could possibly be resolved with a different gas cap or clearing the vent line?

RON...
It should not be difficult to blow through that line, right? Should be easy like blowing through a straw or hose, correct?

[chickenwing]

Now wait a second.... You plugged the vent lines on the cannister.. Hmmm. Your tank is totally differant than mine. I see 2 vent lines goin from your tank in the top left corner of it. 1 of em a return from the fuel pump? and 1 of em going to the vapor cannister? I'll take a look see tomarrow. I can fer sure come over. Festivities start a 1:00. If even 1 of those 2 lines vents then your cap is definately NOT the problem.

[67ChevyRedneck]

Man, this kinda bothers me too. If it's not that vent line you may be in some trouble. I'm at a loss. I've never heard your truck run so I can't be sure what's going on....but if all else fails it's possible your intake manifold is plugged with carbon deposits. I really hate for you to tear into your motor though. You would have to remove it to see it. Now, if your motor is fairly new or rebuilt, there's no way that's possible, unless your timing is really advanced. All your fuel stuff is clean as a whistle....hmmm, what else? You could have a faulty distributor, if you have an HEI, when my control module went bad the truck sputtered for a couple weeks off and on before she cut off completely and wouldn't crank at all...At the same time it killed the coil. Maybe its electrical. Just thought of some more stuff...Check that your choke is working properly on the carb. Also, where did your carb come from? Maybe it's wrong for the intake or the carb itself is junk?? I hate for you to check all these things but I'm running out of ideas....sorry bro. Best of luck to ya!

[KEH]

I'm curious as to the source of your problem. I have a custom tank between the frame rails with the side marker fill and often have the vacum situation too. The tank is vented on top under the bedwood. Maybe not enough air there. Anyway, I never have fuel problem though.

[GMC AMI]

It is the locking gas cap that is the culprit. Same thing on my 68 when I put my new locking cap on. New cap not vented. Take off and you can drill a tiny hole through the cap and problem solved. Or buy a vented cap. On my truck after running it would colapse the tank sides in and you can hear the float scraping the sides when making a corner.

[RON WOODGEARD]

Originally Posted by sactoC10

Larry, great idea!
So let me see if I am understanding what I think may be the problem, some sort of venting matter related to the tank not venting correctly. And this could possibly be resolved with a different gas cap or clearing the vent line?

RON...
It should not be difficult to blow through that line, right? Should be easy like blowing through a straw or hose, correct?

Yes !!! It should be very easy...

Are you very sure, Your Fuel Pump is Good?????

Larry's got a good Idea.....


Let us know..

Ron

[Blue Beard]

Two things in the past might help....

1. Found a second fuel filter at the frame rail in front of the fuel pump (dirty as all get out)..

Replaced it and solved problem.

2. Once started at the carb...rebuilt carb, replaced fuel lines....but missed the one from the tank to the frame (turned out it had softened to the point of creating a valve inside the tube)

[sactoC10]

So far:

Can not blow through the fuel vent line to the cannister very easily at all, so obviously something is going on there.

Charcoal cannister had three lines going to it, one from the fuel tank which is still there, one from the smog pump (which had already been removed) and one from the carb if I remember correctly...all dumping into it.

No, I am not certain that the fuel pump is good...could be wanting to croak.

Back to the COIL matter. The negative line comes down from the coil very close to the intake manifold. If this is getting hot, could it be part of the problem? If so, then why would it resolve itself after setting for moment?

I still think it is a fuel (lack of) delivery problem as the truck has never smelled like gas or excessive fuel at any time. When I've wondered if I've flooded it, I think it was probably not getting much fuel at all if any.

Thanks for the help thus far.

[72MARIO]

Fule pumps are 13 dollars good thing to change for the price.. And gas caps are under $10.I would say that it's the gas cap. You most likely need a vented one. There are lots of people who put plastic bag under there lawnmower gas cap to stop gas from splashing out. Then it will run for a couple of minutes till it sucks all of the avaible air out and dies. the person who is running the machine takes the lid off and checks to seed if they ran out of gas, letting air into the tank. They see there is gas put the lid with the plastic bag under it back on and try starting it. It will now run. I hope the is a clear explaination.

[72MARIO]

Opps I forgot to explaing something that cap does not seal all of the air from coming in. It really slows the air down not allowing enough air in back in as the fuel is removed but in time the vaccume created will be leak back in aroung the cap. Allowing you to start the truck and drive it some more till the tank runs out of air. It's kinds like pouring a gas can into a tank there is a vent at the back if you try pouring it out with the vent closed the gas will flow out but not as fast as if you open the vent.

[sactoC10]

72Mario,

Chickenwing just left (thanks man) and we are both in agreement that this problem is related to the non-vented cap. Yet neither of us could make any sense of it. I did not understand the explanation.

This morning I took the truck for drive and tried to re-create the stall. Didn't take long driving it hard. Removed the gas cap (as NeonLarry suggested) and voila, instant HORSEPOWER...more than I remember...and no stall.

Drove it some more, put the cap back on close to home and wow...took it off while parked in the driveway and HUGE whoosh from the cap. With the cap on Chickenwing and I could not get air to flow through the venting system. With it off, the air traveled without any problem.

So, someone give me a 101 lesson in the tank venting system and why don't all of these trucks need vented caps? What's the difference in the set-up?

THANKS YA'ALL, have a GREAT day!

[neonlarry]

Iv'e been looking thru the service manual but I can't find a diagram of the evaporative system to back up my theory but I think your problem might be caused by blocking off the other two lines going to the canister.

[72MARIO]

Some of these truck had a vent tube that went up the rear cab post. That was the venting. Others used a vented gas cap I think that was the later style.

[RON WOODGEARD]

Originally Posted by 72MARIO

Some of these truck had a vent tube that went up the rear cab post. That was the venting. Others used a vented gas cap I think that was the later style.

Mine Vented up the rear of the cab as i said before..

Glad you are finally getting this thing solved !!! Things like this can drive a guy crazy...

[chickenwing]

Originally Posted by neonlarry

Iv'e been looking thru the service manual but I can't find a diagram of the evaporative system to back up my theory but I think your problem might be caused by blocking off the other two lines going to the canister.

Definately not. I thought the same thing and SactoC10 removed the canister and air traveled freely with the 2 ports plugged. I can't explain it. We disconnected every vent line he had and verified there were no obstructions anywhere. That darned tank should VENT without a vented cap. I just don't get it! I would like a venting primer lesson also.
Rich

[sactoC10]

really trying to think this through...

Guess the only simple thing I can compare it to is small red gasoline container. When you remove the yellow (usually) cap on the back side, the contents of the container will flow out more smoothly than when the cap is in place, right?

S-o-o-o-o-o-o

The vent tube opening is in the filler neck, extends across the tank, up into the cab on the passenger side, back down exiting the cab and then up to the charcoal cannister where it can exit freely. The (only?) place where air can enter/exit is the hole in the filler neck, unless the cap is vented?

Right? (and somewhere the air in the tank figures into this equation)

So what does a vented cap do in regard to moving or allow the movement of air in the tank? And HOW could suction take place when there is a vent tube there anyway? The tank is not technically sealed, so I don't get it. And what's going on with those tubes heading up into the top of cab anyway?

[Harold Shepard]

They are the in- cab vent system,EEC- evaporative emission control. If you look they are the supply line and return line into the tank,the return line is not for gas but for fumes. The fume line runs parallel to the fuel line and turns across the frame at the fuel pump and up. This line connects to the charcoal canister and then a line leaves the canister and goes to engine vaccumn(1/4) and the second line (3/8) runs to a tee at the PCV position on th carb. The fuel line runs exactly like any other fuel line on 67-72 truck. You may check the vent lines from the tank to the cab postand be sure they are not stopped up and also check the rubber lines that connect to the vapor canister.

[67ChevyRedneck]

I don't know either...my truck doesn't have a vented cap and it has NEVER created a vacuum in the tank....just glad your truck runs well!!

[sactoC10]

Harold,

Well, I have my smaller lines to/from the cannister capped, as the smog pump was removed before I got the truck (see picture of cannister). My confusion is high because as 67ChevyRedneck stated, he's never worried about whether or not the cap was vented, not had a problem...so why wouod this set-up create a problem?

Chickenwing and I tested all the lines for clogging and they were clear from the tank inlet, to the lines up the cab post. Tested those and they were clear too. Also tested the parallel line from the cannister to the cab, it too was OK...just weird.

So the question remains, why would the fume lines create a vacuum/suction problem within the tank?

OR MAYBE? The 1971's simply had a less refined version of the EEC system that still behaved more like the pre 70's vehicles and thus would still NEED the vented cap?

[Harold Shepard]

Do you have the line from the tank to the charcoal still hooked up? If so disconnect the hose going from the frame to the filter. This will open up things again... remeber you do have the other side caped let it vent to the atmosphere and all should be well with the cap you have.

[chickenwing]

That's just it Harold. The line that is currently fitted to the cannister DOES flow freely through the cannister. I got a healthy hit of gas fumes checking this.