fuel filter flow quantity

[Cahhh68]

Fuel filter had not been replaced for years and was becoming dark. Last ride the 68 1600 had problems staying running at idle. Choked it and restarted but on way back to garage it was running rough. Checked both carbs and found no float bowl overflow. Venturi cylinders operating freely. No fuel coming from perimeter of pump flange. So I decided to renew the fuel filter first. Once installed and cranked, which was immediate, the filter filled except for a small air space at top. Then a few minutes after driving it, I checked the filter and the delivery to the filter was very small although it was running smoothly and correctly at speed and idling well. Revving the engine increased the flow slightly but the filter did not refill like when first cranked. Anyone know if the filter is supposed to be almost filled with fuel while running or is it just dribbling into the filter? Thanks for any insight. Cahhh68

[rwmann]

If you run a fuel return/bypass line, the filter will flow more.
If you don't, it only flows what is metered and jetted into the carbs.
A bubble in the filter is not an issue and most typically pass through the filter and carbs or bypass with jostling under acceleration/bumps/cornering/deceleration.

[Cahhh68]

FuelInFilter.JPG

I hope you can visualize the attached photo in proper perspective. It shows the fuel level in the filter while engine is running at idle. The bottom of the photo is actually to the right. As I see it in the preview screen, the level of fuel is showing as a vertical line as opposed to horizontal. For whatever reason the photo turned 90 degrees counter clockwise when attaching the file. I expected the filter to be almost full while engine was running but it is actually 1/8 or less full while running. This car has the original fuel recycle line from carbs back to the tank. Is that the "fuel return/bypass line" you are referring to? Cahhh68

[rwmann]

What does it look like after a drive?

Yes, your recycle line is the return line.

[redroadster]

A good test is a HF hand vacuum pump with cup on. It should fill that cup quick 2 pumps
if not you likely have calcium in the line
It will pull air past the gas if it's too restricted
Another way is a riding mower gas tank hooked to the line fill it to see if it drains to the tank quick

[Cahhh68]

After a drive it has just a little more than in the photo bringing it to maybe 1/5 full.

[rwmann]

Is the engine experiencing fuel starvation? Is the input line full of fuel?

[redroadster]

My 70 has a S bend in the line that's smaller than the rest of the line and was. calcium ed up impossible to clean . So I cut it out and it flowed good . But I realized the bend was likely there for a reason , it would slow or stop fuel flow in a wreck , I seen cars that had a long U line in the gas line on a vertical part of the body on carbed cars ,it's an improved gas safety device. I'm going to put one on my firewall I recommend it
If you had a pin hole in a line or hose you should have a gas leak be able to smell gas.
Another way to test. By pass the mech pump and pump with the cheap elect pump to see where it stopping

[david premo]

So the problem could be with any number of things. The first thing that comes to mind is to check if the fuel line upstream of the fuel pump is in good condition, [no cracks or leaks or anything that could cause a vacuum leak] the pump is a vacuum pump. Also check that all the mechanical connections on the steel line upstream are tight and no drawing air into the line. Air is obviously much easier to draw than liquid and when not properly sealed it can be a problem. Also look for kinks in the hose and metal lines upstream of the pump. Also try removing the gas cap and running the engine, I once had a 57 Chevy that had the wrong type of gas cap on it and it was literally sucking all the air out of the tank. When I took the cap off the car you could hear air rushing into the tank. I replaced the cap problem was gone. The fuel pump also should be tested, the method is simple you remove the fuel line on the downstream side of the pump and crank the engine without starting it. While doing this time and measure the fuel quantity you get while cranking the engine over. The pump should deliver better than half a quart in 30 seconds to be considered good. Be sure to completely remove the power to the ignition by unplugging the coil so there is no spark to ignite fuel. You also verify your fuel pressure downstream from the pump which should be between 2.5 to 4.5 PSI. Good luck with your testing.
Dave

[Cahhh68]

Thank you, gentlemen, for the good pointers. To rwmann's question - the filter is now almost 1/2 full before cranking and driving this morning. It ran perfectly in all speeds, idle, acceleration. No fuel flowed from the input line before cranking this morning. And no fuel flowed from the line that comes into the top of the front carb before cranking. So I expect there is an air leak somewhere and with the original stock fuel system on the '68, that could be in many places but of course some of it is metal tubing though I've seen where some have had corrosion issues and breaking with metal. I'll follow david premo's advice and check all hoses, tubes, clamps and test the pump. Based on what I'm seeing flowing into the filter, it won't come near delivering 1/2 a quart in 30 seconds. I expected pump failure from the start of this but started with changing the filter and it has run correctly since I changed the filter. I tested the old filter to see if it was clogged and it had good flow so that presents a mystery. To redroadster's point - no telling what crud has deposited in the fuel lines as old as the Cahhh is. I've replaced the rubber fuel lines but not the metal ones. I have a rebuild kit for the fuel pump so that I'll hold until I check all the lines. Thanks and smooth roads to you all.

[rwmann]

“Making fuel” while parked suggests it was drawing a vacuum through an obstruction.

[Cahhh68]

A crack was found in the rubber fuel line a few inches before the front carb in the 90 degree bend. Found another long crack where the same line comes up from the fuel pump before it turns in front of the thermostat housing. That crack wasn't through and through but was several inches long. These are genuine fuel lines but obviously over a few years they must be developing dry rot from the outside.

[JackAuffeltrades]

I was following this thread because I was having similar issues. I had some tank and fuel line contamination problems that I was working on since I got the car last year (cleaning a severely nasty tank etc.) I found some more crud in the lines and decided to just plumb one single new line from the tank to the filter. I had already replaced the lines from the filter to the pump and then to the carbs but I did note that after I shut off the engine it appeared that there was reverse flow backwards from the carbs to the tank. I had issues with the filter filling with fuel and had scary low levels in the filter for a week. But it ran. I then replaced the soft line from the carbs to the return, thinking there might be some cracks. Haven't had an issue since and the new filter filled to the top, except for one small bubble, in one day. Must have been a crack in the return. Don't skimp. It's cheaper than your labor. Spend the extra few minutes and just replace it if there's a question. It frees up time to sort out other simple problems, like my unreliably reliable electrical grounding issues!

[fj20spl311]

There is ONE VERY IMPORTANT FACT you are missing from this discussion,,,,,sorry if it was presented, only scanned the responses.

The full filter is not supposed to be FULL.

As the filter gets dirty, the level of fuel in the filter increases to a "clean" area.
When the air is gone, time to replace the filter.

[JackAuffeltrades]

There is ONE VERY IMPORTANT FACT you are missing from this discussion,,,,,sorry if it was presented, only scanned the responses.

The full filter is not supposed to be FULL.

As the filter gets dirty, the level of fuel in the filter increases to a "clean" area.
When the air is gone, time to replace the filter.

Thanks for this. I need to amend my previous post to reflect your point. I did indeed need to change the filter. I apparently still had some remnants of the fuel system's history in the return line and it contaminated my gas tank. Had to drop it and clean it out again. I ran multiple solvents and compressed air through the lines and put it back together. Now I'm back to having low levels in the fuel filter. The car seems to run ok but after having to pull over due to fuel starvation a number of times while sorting out this issue over the months, It's just not a comfortable feeling seeing the fuel level that low. One other note, when I park it, the fuel level is low. After it sits for a while, the filter is about a third full. I guess I still have more to learn about the flow dynamics in this system.