what is the cause and cure??

[BIG ORANGE]

If I let the 69 set for a week in the shop the fuel back feeds out of the fuel line and clear filter back to the fuel pump, not going into the crankcase or anything like that. To get it to crank I have to remove the gas cap and prime the carb. I have never seen this before. Have any of you?

[TBONE1964]

That is a little bit strange. What type of fuel filter are you using. Some of the inlines have a check valve that only lets fuel go in one direction.

The strange thing is that there should be enough fuel in the float bowl to get the engine to light even if the fuel line is empty. It may not stay running for long but it should start. What carb? It is possible that the float bowl is draining fuel through the base and into the intake.

You also need to have a vented cap if you don't already or some way to vent the fuel tank.

[GMtrucknut!!]

by chance does it have a quadrajet carb on it ? If it does the primary well could be leaking and causing your problem.

[towbar]

have the same problem with an edelbrock carb, usually about 2 week sit time, then have to prime to avoid running the battery down cranking so long. Have a carter fuel pump, some -6an lines, a Russell fuel filter with no check valve. Been wondering the same thing and how to fix it?

[Longhorn 70]

Originally Posted by GMtrucknut!!

by chance does it have a quadrajet carb on it ? If it does the primary well could be leaking and causing your problem.

Great question. One reason I got rid of the Q-Jet. They make welch plugs for the bottom of the well and rubber plugs to go in the throttle plate too. They are temporary fixes.

Originally Posted by towbar

have the same problem with an edelbrock carb, usually about 2 week sit time, then have to prime to avoid running the battery down cranking so long. Have a carter fuel pump, some -6an lines, a Russell fuel filter with no check valve. Been wondering the same thing and how to fix it?

Have never seen that with an Edelbrock or Carter 96xx as there are no gaskets or bores below the bowl assembly.

[Andy4639]

Mechanical pump has a diaphragm in it doesn't? Could be leaking back through it.

[low&slow]

I have the same problem with my truck. It has a 650 double pump holley, pressure gauge, and a new clear fuel filter. I have to pour a bit of gas in the carb about 2-3 times before gas actually gets pumped all the way too the carb. While doing all that, I leave my gas cap off, because for some reason the gas tank builds up a lot of pressure.

I never had this problem with my other truck and the longest it sat was 4 months without being started and then fired right up. Its pretty embarrassing having to do this every time with some neighbors that like to watch me cause the truck is really loud.

Anyways, hoping someone can find a cure to this ongoing problem.

[Ol Blue K20]

I think it's your fuel pump it has a valve to keep from bleeding back and its not doing its job
Posted via Mobile Device

[BIG ORANGE]

Well I see I'm not the only one with this issue, The complete fuel system is new and yes to the edelbrock carb also. I'm gonna change the fuel filter out and see if that helps. Ya let me know what you guy's do also, maybe together w can figure this out, check back w yall in a few days w a update!
Thanks
Donnie

[69GWC]

My 70 does this as well, it has a edelbrock on it. Its a pain if it sets more then 3/4 days.
Posted via Mobile Device

[MagmaJct]

To the OP and others...

My understanding is that the gas cap is designed to allow vacuum to pull air into the tank to allow for fuel consumption. If the tank is collapsing under vacuum with the engine running/driving, you may have to look into a new cap.

As far as the tank pressurizing, this is probably an issue with the evap system being disabled.

I have a Carter AFB (Edelbrock 1405 with the electric choke kit installed) that is tough to start if left sitting more than a week. I suspect the fuel in the float bowl is evaporating. These carbs were not designed to reduce emissions.

[luvbowties]

Originally Posted by MagmaJct

To the OP and others...

My understanding is that the gas cap is designed to allow vacuum to pull air into the tank to allow for fuel consumption. If the tank is collapsing under vacuum with the engine running/driving, you may have to look into a new cap.

As far as the tank pressurizing, this is probably an issue with the evap system being disabled.

I have a Carter AFB (Edelbrock 1405 with the electric choke kit installed) that is tough to start if left sitting more than a week. I suspect the fuel in the float bowl is evaporating. These carbs were not designed to reduce emissions.

With an electric choke, this problem can usually be minimized by tightening up on the choke-enrichment-adjustment.
Sam

[Andy4639]

Just another thought.

Donnie since you mentioned you had to take the cap off and vent the tank it maybe that the tank is sucking the gas back into the tank after sitting like the coolant in the recovery tank. Have u tried leaving the gas cap off when you park it to see what it does? I had a old Chevy Luv truck that I had to take the vent line loose to get to run if it was less than a 1/2 tank. It was pulling so much vacume it would kill the engine.

[GASoline71]

Get a vented gas cap... your '69 originally came with one, but someone could have replaced it with a non-vented cap over the years.

Gary

[mr48chev]

Originally Posted by Ol Blue K20

I think it's your fuel pump it has a valve to keep from bleeding back and its not doing its job
Posted via Mobile Device

Yep, the check valves in the fuel pump aren't holding a seal as well as they should and letting it bleed back. Not an issue if you drive it daily but it presents a problem if you let it set and it's universal across the board on carburetored engines with fuel pumps that have some miles on them. My Mom's 70 something Ford 300 acts the same way if it sits several days. Drive it every day and it fires right off and goes.

As far as Cap goes, if the cap isn't venting it will create a vacuum just as MagmaJct suggested. I had a lot of trouble getting a properly vented cap for one vehicle I had and went through three top brand caps before getting one that worked. The pressure you experience is from the sides of the tank popping back out in place after being sucked in by the vacuum of the fuel pump. The only way the tank in these trucks would build pressure in the tank is if you had the truck sitting out in the hot sun for a length of time and the heat vaporized the gas enough so that it built up vapor pressure in the tank, but then you wouldn't have a starting problem as the pressure would push fuel up the fuel line to the carb most likely.

[hatred]

Yep, fuel pump. Mine did it until I changed it, now it never does it.