Fuel starvation issue - 68 1600 SU Carb Setup

[GaRoadsterRookie]

I need some wise counsel. I have been fighting this issue for several months and have been unable to resolve it. The main symptom is that my car will start losing power and cutting out while I am driving it (typically after at least 10-15 minutes). I stop the car and check the fuel filter. It has less than 1/4 of the fuel filter full of gas, sometimes less. Gasoline is flowing in to the filter at a trickle. If I rev, the engine, it does not increase the flow significantly. I am unable to duplicate cutting out symptom in the garage/driveway. My usual repair is to disconnect the line and blow it out back to the tank. Upon restarting the engine, fuel comes gushing into the fuel filter at a strong rate and I run okay for a while. I have noticed now that when I park the car and stop the engine, the fuel in the filter is very low but it slowly starts filling up (without the engine running) until it is almost completely full. The fuel is coming from the tank side. This is odd because the mechanical fuel pump is obviously no longer pumping. I suspect that I have some type of air in the fuel line issue.

Setup: Standard 1968 1600 SU setup, mechanical fuel pump (replaced 18 months ago), plastic clear fuel filter, original metal fuel lines from tank to engine compartment (cleaned during restoration), rubber fuel lines everywhere else. vented gas cap, vented gas filler tube.

Recently checked:
- Opened up fuel pump, everything seemed in order
- Replaced fuel filter
- checked screens in t-bolts at carbs
- confirmed that fuel lines are clear (from tank and return line)
- confirmed that fuel is using the return line past the 2nd carb
- no visible fuel leaks in the whole system

Recommendations on how to proceed?

How do I check for air in my fuel line problems?

Any help is appreciated. I have reached my limits and frustration level with this problem.

[GeoffM]

Hey Scott,
One thing that jumps out at me is that you mention a return line. My 1600 does not have a return line and I don't believe that it is required on the SU set-up.

[spl310]

Return line is stock, and unless the valve is bad, it is not the problem. You have something in the tank.

[keith0alan]

Try installing a T in the fuel line at the carbs and hook up a fuel pressure gauge. If the pressure is going away then try pinching off the return line. The return valve is thermal and to reduce vapor locking. When cool it passes a tiny amount of fuel. When warm it opens and circulates the warm fuel back to the tank.

My bet is a clogged pickup in the tank.

[GaRoadsterRookie]

I forgot to mention that I did drain the tank about 3 months ago. I blew it out and tried to rinse it out several times. Nothing substantial but a consistent fine grained particle that seems to be coming from my new tank liner. About 3 years ago I had this tank dipped and coated on the inside by a local professional. I do have my suspicions about my tank and the tank to fuel line connectors. It would be nice to have one of those optical inspector hose cameras.

Concerning the thermal valve, I tried to blow air through it this week when I was checking the T-bolts. I don't think that it was letting any air through when cold. Should I have been able to blow air through it when cold?

[keith0alan]

When closed you will not be able to blow much through it. There is a tiny pin hole and it really doesn't matter if it is clogged.

[GaRoadsterRookie]

Return line is stock, and unless the valve is bad, it is not the problem. You have something in the tank.

I am curious to your logic as it relates to my symptoms. If something is restricting my flow from the tank to the filter, why does the filter fill back up after I stop the engine. Is there some type of vacuum formed or is it just gravity after the restriction releases?

I have noticed that the plastic filter is occasionally slightly sucked in. I was unclear if that was caused by the metal clamp holder or another factor.

[spl310]

The mechanical fuel pump creates a vacuum in the fuel line as the restriction (piece of trash in the tank) is held up against the pickup. Once the engine stops, the vacuum remains, and fuel is slowly seeping by the restriction. Once it gets enough past it, the filter refills. The collapsing filter is your clue. I would empty the tank and look for trash.

[notoptoy]

^+1 - I agree it points to a restriction in the fuel flow, not a full blockage. If you did have an air leak, you would not get that vacuum.

[GaRoadsterRookie]

If one wanted to replace the original metal fuel line (from tank to filter), what would you recommend?

I can't seem to find any order-able fuel lines from the standard vendors.

[keith0alan]

It is amazingly similar to steel brake hard line. Do not replace it with rubber fuel line.

[msampsel]

If one wanted to replace the original metal fuel line (from tank to filter), what would you recommend?

I can't seem to find any order-able fuel lines from the standard vendors.

Here is some fuel line or brake line I found a while back. Sure there are others
http://search.eastwood.com/search?w=fuel+line

[spl310]

Why do you want to replace that line? I believe the problem is trash in the tank.

[GaRoadsterRookie]

Why do you want to replace that line? I believe the problem is trash in the tank.

When I bought the car, the fuel lines were clogged with hardened old gas. We cleaned it out as best as we could but I doubt we got all of it. So, in my mind, I think that something could possibly get wedged in that fuel line as well due to some old surface buildup. I have probably been pondering over this issue too long.

[msampsel]

Why do you want to replace that line? I believe the problem is trash in the tank.

When I bought the car, the fuel lines were clogged with hardened old gas. We cleaned it out as best as we could but I doubt we got all of it. So, in my mind, I think that something could possibly get wedged in that fuel line as well due to some old surface buildup. I have probably been pondering over this issue too long.

Did you remove the gas tank and have it "boiled out", I had some fuel and sputtering issues finally
took my fuel tank off to get it cleaned out good (took it to a radiator shop who does this). Blew out the lines good
with compressed air.
No more problem.