Stalling/Sputtering after 20 minutes

[blkseetin]

Need some help on what to start looking at.

I have a 70 C10 and it was running fine for a while. It's got a new carb (quad) and new HEI ignition. This problem seems to only happen after running fine for about 20 minutes. At about half throttle, it falls on it's face and dies. Sometimes I can pump it out of it and keep it running. The think I don't understand, is it runs great on my way to work for instance (20 minute drive) but on the way home after about 20 minutes it starts. Next morning it's no problems. The truck is pretty much factory and it has 50k miles on it. Oh, I was also smelling gas, and noticed it was leaking out of the gas cap so I got a new one. thats when this started happening. I was thinking vapor lock, so I started reaching back and opening up the cap on my way home, and that seemed to work at first, but the longer I drove, the more is start hesitating on acceleration and then even with the cap completely off, it was still doing it. My grandfather, also added a second and third gas tank (he liked to travel) years ago, and he has an inline filter in the fuel line. I've heard that unnecessary and was planning to try and remove that. If that doesn't fix it, I was going to replace the fuel pump. I've read there may be a vent line in the fuel tank that could be clogged, any advice on where I can find that?

Is this a familiar problem to anyone that might point me in the right direction?

[geezer#99]

Are your extra tanks all tied togethor into one line or does it have a selector valve?
If it's all in one you could be vapor locking from the other tanks.
Also if you have a switching valve it could be worn out and sucking in air.

[Mr Chevorlet]

Put your other cap back on, if you bought another cap, it is not vented and if your gas tank is not vented your are building pressure in your tank and pushing high pressure fuel to your carb and flooding it, I do not know how it gets past the fuel pump but it was happening to me. I now do not ever tighten my cap and never fill my tank. And it runs fine. But hey it could be something else just thought I would suggest to start there.

[blkseetin]

Originally Posted by geezer#99

Are your extra tanks all tied togethor into one line or does it have a selector valve?
If it's all in one you could be vapor locking from the other tanks.
Also if you have a switching valve it could be worn out and sucking in air.

There is a selector valve.

[hamjet]

Did you change out the resister wire to the HEI distributor when you switched also?

[blkseetin]

Originally Posted by hamjet

Did you change out the resister wire to the HEI distributor when you switched also?

No, I did not.

Originally Posted by Mr Chevorlet

Put your other cap back on, if you bought another cap, it is not vented and if your gas tank is not vented your are building pressure in your tank and pushing high pressure fuel to your carb and flooding it, I do not know how it gets past the fuel pump but it was happening to me. I now do not ever tighten my cap and never fill my tank. And it runs fine. But hey it could be something else just thought I would suggest to start there.

This sounds very much like whats happening, as when I do remove the cap, it's always pressurized. Problem is that the old cap was leaking fuel, so every time I'd go around the corner, and bunch of gas pours out. Any other suggestions on a different gas cap that will vent? I just purchased one from the closest autoparts store.

[Mr Chevorlet]

Originally Posted by blkseetin

No, I did not.



This sounds very much like whats happening, as when I do remove the cap, it's always pressurized. Problem is that the old cap was leaking fuel, so every time I'd go around the corner, and bunch of gas pours out. Any other suggestions on a different gas cap that will vent? I just purchased one from the closest autoparts store.

I far i as i can tell even if you buy a cap and says its vented they are not, I have been searching for quite some time for a vented cap. I just do not fill my tank more than half full, of course i do not drive mine daily.

[blkseetin]

There has to be a way to resolve this pressure problem and allows me to use a gas cap that keeps the gas inside the tank.

[geezer#99]

Process of elimination.
Eliminate the other two tanks and the selector valve by running a temporary fuel line from your main tank to your fuel pump. Then put your vented cap back on.
If it cures it then you know the problem is somewhere in the system of the other two tanks and valve.

[MagmaJct]

What kind of evaporative emissions control did they install on these trucks for the '70 model years?

Regards,
Mike

[blkseetin]

Originally Posted by MagmaJct

What kind of evaporative emissions control did they install on these trucks for the '70 model years?

Regards,
Mike

none that I'm aware of. I don't recall seeing anything in the engine bay, but I could be mistaken.

[MagmaJct]

The reason I ask, is because the evaporative system is part of the tank's ability to "breathe". Different manufacturers used different methods. These methods also changed over the years.

Take my '74 aircooled VW for instance. It uses a sealed gas cap, it's not vented. All breathing is done through the charcoal canister. But it uses pressurized air from the fan housing to purge the canister of vapors.

My '71 C-10 uses a vacuum pressure cap. Unfortunately, the text I have does NOT explain at all what that means, leaving me to speculate! I believe the cap is not vented per se, but instead had a check valve in it. That is, under vacuum, air is admitted into the tank. Under pressure, the cap maintains a certain pressure level. The excess pressure "moves" into the charcoal canister. It's common for owners of vintage vehicles to delete the charcoal canister and "seal" the lines.

This often causes vapor lock, because as the fuel level is drawn down from driving, a vacuum occurs in the tank lowering the relative pressure of fuel ahead of the pump. Because of gasoline's volatility, when in a sealed container and heated, it gives of vapor which increases pressure. This typically happens when parked, but I supposed it can happen on a hot highway. The pressure increases the relative pressure ahead of the pump. This additional pressure could potentially cause flooding, as others have recommended.

If out if your truck had evaporative emissions control. If it did, try to get it back to how it should be set up. If not, find out how the vapors were vented and correct that. Once that's been eliminated, we can move further down the trouble tree.

Regards,
Mike