Hi to everyone,

I'm a new member and recently purchased a mildly modified 69 convertible. The car has a 350 engine with aluminum heads, manifold, 600 CFM Edelbrock and an aftermarket camshaft. Unfortunately, the previous owner misplaced the invoive from the machine shop and does not have the specs.

Over the course of time the PO lost interset in the car and let it sit for months at a time without driving it and now I'm trying to resolve the gremlins.

I have removed the carberator and rebuilt it and installed fresh gas. While draining the old gas I noticed small black particles in the gas that appear to be silicone in nature. i have no idea what it is or where it's coming from. Rather than be frustrated by not knowing i'm considering replacing the tank and fuel lines.

I've learned there are two tanks; vented and unvented. If my understanding is correct, the vented tank was installed in cars that had Q-jets. Since I do not have a Q-jet on the car, what is the down side of installing a new unvented tank? The car will not be returned to original condition so that is not a concern. If I go with the unvented tank do I have to replace the fuel pump which is for a vented tank?

Sorry for the long post.

 

Original 69 tanks were not vented and relied on a vented cap for make-up air. 70 and later had a vent on the tank and used a non vented cap. 69 SB also had a return line nipple on the top passenger side of the tank. this was connected to the fuel filter return. The fuel pump does not care what the tank has, as long as air can get in there to replace the gas you are using.
If the sock is in place on the fuel pick-up in the tank, larger particles should not get thru it to cause problems. You can just clean out the tank and call it good if the inside of the tank is not rusting.

 

Being new to Corvettes, i must be mistaken. The car has a fuel pump with two lines coming out of the bottom. One feed gas to carb and the other attaches to a return line attached to front passenger side of the frame. I've attended several car shows and i've noticed some of the owners have capped that line and it is no longer in use. I thought that was the vent fro the tank. Thank you for the clarification.

 

The sock on the fuel pick up is in place and if as you suggest thise particles wont go through and cause problems not replacing the tank will sve me some time and bucks. I just dont know the source of the silicone like particles.

 

That extra line is not a vent, it's a return/bypass. To minimize vapour lock problems, part of the fuel pump output was routed back to the tank ensuring a continuous flow of fuel through the inlet line.

The vent, which allows air into the tank to replace the fuel as it is consumed, is either via the cap on '69 and earlier car or through the 'charcoal canister' on '70 and up cars.

Many vendors get their own terminology wrong.

 

Thanks for the in-depth explanation M.W.!

Now even I understands how the system works.

 

Hi,
My understanding the EEC process wasn't mainly about letting air into the tank, but rather to control the gasoline vapors being emitted from the tank.
A tank without the EEC (Evaporative Emissions Control) system allows the vapors to escape into the air through the vented cap.
A tank with an EEC system including a vapor canister uses a sealed cap; the gasoline vapors are stored in the canister until the engine is running at which time the vapors from the canister are sucked into the carburetor to be burned.
Everyone is correct, 69 can use the gas tank the is set up to use a vented cap.
Regards,
Alan

 

You are right Alan- I was clarifying that

1) the fuel return/bypass line from the pump is NOT a vent under any possible definition. Even the GM documentation is confusing as it mentions 'vapours'

2) there are two different methods GM used to replace the fuel in the tank with air. The EEC method was created to capture and deal with fuel vapours.

 

If you remove the filler neck to the 69's tank it is no problem to clean the tank of any debris if you have some length in your arms to reach the corners of the tank.
Check out getting a new filler neck gasket first. The torque on the retainer bolts is in inch pounds not foot pounds. I would have to look it up to give it now. 40 inch pounds comes to mind, but I don't know for sure.

 

Those fuel filler screws are very small, maybe 10-24. They are probably going to snap off, if it is an old tank. Don't forget that the gasoline is wetting their threads from the bottom. This is just a word of caution, happened to me. Lou.

 

Thanks for all the replies. The technical explanation of the "vent" line is above my comprehension level at this time. i think i'm going to drain the tank to remove as many particles as i can and replace the fuel pump with the more conventional one (one line into pump and one line going to carb). I'm going to cap off "vent" return line and see what happens.

 

Every fuel tank (or other enclosed container of liquid) must be vented so that dispensed liquid can be replaced with an equal volume of air. If that is not done, atmospheric pressure will tend to collapse the container (tank).

Either the tank must have a vent or the filler cap must have a vent. In the case of a vapor recovery system, the vapor canister serves as a vent.

You just need to install a vented cap on your tank.

The 'black particles' might be pieces of deteriorating rubber hoses connected to the tank. I would highly recommend that you replace ALL synthetic rubber hose pieces associated with the fuel tank and delivery system.

 

An original 69 system has the two line fuel pump and the pressure side would go up to a special three port metal AC fuel filter and then to the carb. The extra small port would go to the 1/4" fuel return line, back to the fuel tank. Lou.