Pressure building in tank

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jrc4y4

jrc4y4

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72 Scamp, 318 bored .030, 470 lift cam, eddy 4 bbl intake and carb. Car bogs at WOT sometimes when it's hot (like 90* outside) but once cool outside (like 70*), runs fine. Made for a rough loss to a Monte Carlo at the cruise night LOL.

I'm going to run a 1/2 wood carb spacer to help as I think it is vapor locking. I'll start there as it's simple.

Next question is venting the gas tank. Every time I drive it, my dad taught me to open the gas cap to let out the pressure. Talking to my bro in law (he's a GM guy but knows more mechanical stuff in general) and he said pressure should not be building. Read another comment on here saying something similar.

There is what I believe is a charcoal canister on the passenger side front of the vehicle (will post pic of the can later) with a line that if memory serves me says fuel tank. The other ones are left open from when I rebuilt the engine. Do I need to remove this line to see if it's venting? Thanks for any help. These detailed things are beyond my past experiences!

Scamp1.jpg


Scamp2.jpg
 
If that's a metal fuel line I see in the pick you might be boiling the fuel in the engine compartment (fuel line) when it get too hot. You could wrap it with header wrap like I did or slide Silicone tubing over it?? I would think that pressure in the tank would help push gas to the engine?? Now vacuum in the tank might keep gas from getting out.

On the other hand, if it's building pressure it means that it doesn't have a vent which means that eventually you could pull a vacuum on the tank restricting fuel flow. Maybe your hearing the tank sucking air when you remove the gas cap???

treblig
 
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Maybe it is sucking air when I open it? But should it even do that? This is that canister I was talking about. None of the ports are hooked up except for the one that says fuel tank. And the retainer is on there, no idea how to get it off.

From left to right it says, Carb, PCV, tank, then carb bowl.

I will try the header wrap.

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canister1.jpg
 
jrc4y4 said:
Maybe it is sucking air when I open it? But should it even do that? This is that canister I was talking about. None of the ports are hooked up except for the one that says fuel tank. And the retainer is on there, no idea how to get it off.

From left to right it says, Carb, PCV, tank, then carb bowl.

I will try the header wrap.

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I own an older A body so mine car doesn't have a canister, someone else will probably be able to help with that issue. I would try the header wrap also, silicone tubing works real nice but if it ever catches on fire it's hard to put it out (don't ask how I know). Remember they sell the header wrap in wide and narrow strips, the narrow would be easier to apply.

PS - Don't forget to check the fuel line routing all the way back. If the line comes close to anything that gets "hot" you'll continue to have the same problem!!

treblig
 
Sounds good, thanks for the help! Wish you hadn't gotten out of the manifold business LOL. I keep having to do other crap before I can pull the trigger on them though.
 
jrc4y4 said:
Sounds good, thanks for the help! Wish you hadn't gotten out of the manifold business LOL. I keep having to do other crap before I can pull the trigger on them though.
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I can give all the scoop on what to buy and where. I still have a few sections of 120 degree mandrel bent pipe and flange adapter and cone washers........and a few (flat and bell) flared pipes!!

treblig
 
Building both pressure and vacuum is normal operation in a fuel system with a vented cap. The cap is not just "open" to the atmosphere all the time. It has a valve that must be overcome with either pressure or vacuum in order for either to be released.

If you don't have a vented cap, you should get one. Also, real vapor lock is pretty rare......although it does happen.

I recently had trouble with my truck and thought I was experiencing it, but it ended up being something else.

Any time you have fuel issues, you should first inspect the TANK, as it is the source. If it is dirty, it should be cleaned thoroughly, as any repairs without first cleaning the tank will be in vain.
 
There is only one vent to the tank in 72. That is the small 1/4" line coming up front. You need to determine if it's open, and there is a ball valve right near the engine end supposed to prevent liquid regurgitating up in there. If you are not going to use the carbon can, I would cut the line back to get rid of the ball, remove the cap and blow through it to clear it and make certain it is clear

Is all the plumbing properly connected back at the tank? You need a shop manual to see this, download one at MyMopar

You can run (hose if necessary) the 1/4 line "up high" in the engine bay and then turn downwards or you can plumb it into the outer bonnet of the air filter

Screenshot right out of the 72 manual. With the carbon can disconnected, this is what is important...........what is in the circle IS YOUR vent

evap.jpg
 
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RustyRatRod said:
Building both pressure and vacuum is normal operation in a fuel system with a vented cap. The cap is not just "open" to the atmosphere all the time. It has a valve that must be overcome with either pressure or vacuum in order for either to be released.

If you don't have a vented cap, you should get one. Also, real vapor lock is pretty rare......although it does happen.

I recently had trouble with my truck and thought I was experiencing it, but it ended up being something else.

Any time you have fuel issues, you should first inspect the TANK, as it is the source. If it is dirty, it should be cleaned thoroughly, as any repairs without first cleaning the tank will be in vain.
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Should my '69 Barracuda have a vented cap and how do I know if it has the correct cap on it now??
Jrc4y4 says that his engine runs fine in cooler weather and only acts up when it's hot so I wouldn't suspect the tank nearly as much as something else...unless it's related to the cap??

treblig
 
Your car is 72+ so you have a superior tank venting system. That's the good news.

The bad news is the fuel tank vent line which goes from the port on the top-front of the tank to the passenger side fender can often get crap in it over time. Remove that line from the charcoal can as well as the fuel tank and blow compressed air through it to clear it out.
 
67Dart273 said:
There is only one vent to the tank in 72. That is the small 1/4" line coming up front. You need to determine if it's open, and there is a ball valve right near the engine end supposed to prevent liquid regurgitating up in there. If you are not going to use the carbon can, I would cut the line back to get rid of the ball, remove the cap and blow through it to clear it and make certain it is clear

Is all the plumbing properly connected back at the tank? You need a shop manual to see this, download one at MyMopar

You can run (hose if necessary) the 1/4 line "up high" in the engine bay and then turn downwards or you can plumb it into the outer bonnet of the air filter

Screenshot right out of the 72 manual. With the carbon can disconnected, this is what is important...........what is in the circle IS YOUR vent

View attachment 1715067932
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Your picture is incorrect. He has a 72 venting system. That picture is for a 71 venting system which is very different.
 
kiss said:
Your car is 72+ so you have a superior tank venting system. That's the good news.

The bad news is the fuel tank vent line which goes from the port on the top-front of the tank to the passenger side fender can often get crap in it over time. Remove that line from the charcoal can as well as the fuel tank and blow compressed air through it to clear it out.
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I don't disagree that the vent line be cleared, that's always a good idea....but it won't explain why the engine only has problems in hotter type weather?? If the vent line was clogged/plugged it would effect the engine all the time (hot or cold)???


treblig
 
Treblig said:
I don't disagree that the vent line be cleared, that's always a good idea....but it won't explain why the engine only has problems in hotter type weather?? If the vent line was clogged/plugged it would effect the engine all the time (hot or cold)???


treblig
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I completely agree with you. Most of the time fuel tanks building pressure or vacuum won't affect vehicle operation. If you think it is, there's a super easy test to perform, take the gas cap off and see if the problems repeat itself. If they do it's not a tank pressure/vacuum issue. Doesn't sound like the OP has tried this yet.

If I was placing bets i'd bet that the OP, especially since running an Edelbrock carb, is having a fuel boiling in the carb issue, especially if the heat crossover on that performer intake wasn't blocked off.

@TrailBeast has mentioned this in other threads about Edelbrock carbs. I run an Edelbrock with a coolcarb spacer and heat shield to try and help this, along with heat crossover blocked off.
 
If I was placing bets i'd bet that the OP, especially since running an Edelbrock carb, is having a fuel boiling in the carb issue, especially if the heat crossover on that performer intake wasn't blocked off.

Now I'm going to have to agree with you because my Barracuda sometimes shuts down in hot weather. It starts up again but it's very annoying. I have a brand new air gap and was hoping that it would help!! Damn Edelbrock carbs!!!:BangHead:
Protecting the fuel lines from heat is never a bad idea!!

treblig
 
I'll look at the inside of my cap. I'm not sure how to pull off the line to the canister. The clip isn't adjustable or flexible. I've never seen a clip like it. I do not think these two issues are interrelated. Just brought them up at the same time. Sorry. I have not plugged the heat crossover in my performer intake. I've heard Eddy carbs are more prone to this issue. But swapping a carb right now isn't what I'm wanting to look into.

Kiss, how do you like that cool carb plate? I had been looking into them. Was gonna wrap my fuel line first with header wrap and go from there. Thanks for the conversations guys. I appreciate it.
 
Treblig said:
Should my '69 Barracuda have a vented cap and how do I know if it has the correct cap on it now??
Jrc4y4 says that his engine runs fine in cooler weather and only acts up when it's hot so I wouldn't suspect the tank nearly as much as something else...unless it's related to the cap??

treblig
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69 has a vent tube on the filler neck that vents thru a metal line thru the rubber trunk floor gasket right beside filler neck. I shouldn't have a vent gas cap
 
jrc4y4 said:
I'll look at the inside of my cap. I'm not sure how to pull off the line to the canister. The clip isn't adjustable or flexible. I've never seen a clip like it. I do not think these two issues are interrelated. Just brought them up at the same time. Sorry. I have not plugged the heat crossover in my performer intake. I've heard Eddy carbs are more prone to this issue. But swapping a carb right now isn't what I'm wanting to look into.

Kiss, how do you like that cool carb plate? I had been looking into them. Was gonna wrap my fuel line first with header wrap and go from there. Thanks for the conversations guys. I appreciate it.
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The CoolCarb heat shield and spacer are fantastic but I wouldn't run them if I still had heat crossover. I talked to the owner and he advised against it. There are other spacers and heat shields out there.

Don't get me wrong I like Edelbrock carbs, I run one, but the fuel sits directly over the middle of the intake and with the aluminum carb body and today's gasoline it boils, easily. Anything you can do will help, such as wrapping the fuel lines and running spacers. Ideally you'd have a fuel return system at the carb to constantly feed in cooler gas, and with an electric pump. I didn't do that personally as it wasn't worth it to me and my limited driving.
 
jrc4y4 said:
Car bogs at WOT sometimes when it's hot (like 90* outside) but once cool outside (like 70*), runs fine.
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Do all the other suggestions for fuel heat. More than likely you are running rich at 90* + temp. Check your fuel pressure and float level. The open air cleaner sucking in 200* plus air is also not helping the AFR.
 
mderoy340 said:
Do all the other suggestions for fuel heat. More than likely you are running rich at 90* + temp. Check your fuel pressure and float level. The open air cleaner sucking in 200* plus air is also not helping the AFR.
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Yeah I'd be interested in going a different route for an air cleaner. Although it dyno'd more with that air cleaner than even an open carb. But the hood was off, wasn't sitting in traffic, etc..
 
jrc4y4 said:
Thanks for this! I will verify that it'll fit with my 1405 carb and performer intake!
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you should have no problem with fit. For the $20 cost I would try this first before changing anythig else.
 
kiss said:
Your car is 72+ so you have a superior tank venting system. That's the good news.
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Yes, as long as it's working right!

Like the rest of your post says, inspect it and make sure!
 
kiss said:
Your picture is incorrect. He has a 72 venting system. That picture is for a 71 venting system which is very different.
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That photo is right out of the 72 shop manual. If you are talking about the trunk vapor separator, "at some point" might have been a mid year change, that function was put into the tank, getting it down to just one line up front besides fuel. So that part is the same..........fuell supply..........and vapor can vent line.

In fact look at page 14-76, 77, 78 of the 72 chassis manual. It actually contradicts itself. If you have different info, feel free to look it up and post it here
 
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