Surging in high gear

[Cliff Ruggles]

I've heard many times of folks curing fuel delivery issues by removing the stock fuel filter and installing an in-line filter.  The theory sounds great and it gets regurgitated frequently on many websites when the topic comes up related to using a Quadrajet in a "high performance" scenario.

So I took my car to a local test and tune session and did some testing.

When drag racing I used an 8AN line from the regulator (mounted at the front of the drivers side valve cover) to the carburetor.  The fuel filter is a high flow racing type unit mounted behind the tank next to the 140gph fuel pump.

Never one single issue with fuel delivery with this system.

So to test the capabilities of a stock paper filter put a 3/8NPT to 3/8" steel fuel line adapter in the regulator and ran a 3/8" rubber hose to the carbs stock fuel filter housing with a paper filter inside.

I had already rigged this up and made two passes before installing it to replace the 8AN set-up.

I made three more track runs, two are shown below, the first one is before the change and the second one after the change 23 minutes later:

R/T: .505        .514
60':  1.6230    1.6289
ET:   7.3169    7.3188
MPH: 94.35     94.36

These are 1/8th mile ET and MPH.  You'll notice that my daily driven street car is a little faster than most so fuel consumption on a run, and acceleration trying to put fuel back in the tank would be a little more taxing on the system than most set-ups.

I think this pretty much proves that fuel delivery to the nose of the carburetor is far more important than what filter you have in line on the pressure side of the system.  As long as it's not clogged up the factory paper filter used in a Q-jet certainly appears to be more than adequate to support big HP levels and pretty quick vehicle performance......FWIW.....Cliff

PS:  some folks may notice that it actually ran slightly quicker after the stock filter and smaller feed line from the regulator was installed.  This is going to be due to a very slight  improvement in weather conditions as it was done in the evening and things were cooling off pretty quickly that night.

[Hillbillyenginering]

I had always ran the stock filter. I have had multiple issues with my toro. Kept chasing my "carb" issue. In the process I found a bad distributor which next was the coil, replaced new MSD. Then wires. Rebuilt the carb with your kit. Kept having issues. On accel would  fall on its face. Installed vacuum gauge and vacuum would max at 22". Then flutters and drop all the way to zero and die, with WOT. Letting off would recover. Figured out that the turbo 425 needs vacuum to stay in gear.
Installed a vacuum can to trans. Same issue with vacuum. Installed a fuel pressure gauge between pump and carb. Good at 4.6 to 6 psi.  FINALLY figured out it was the filter. Yes it was new and in the correct way. Got so bad no fuel would go thorough. Torn it apart and could not see anything inside.
Of course I'm making the story shorter here.
I followed your directions and knew it couldn't be the carb. And after reading so many of these posts I just needed to keep looking.
It stumped me for so long because it was a new filter and never had one clogg up so quickly. Had to take a step back and ask myself. If this was somebody else's car what would you do.

[Cliff Ruggles]

I would say that fuel delivery is marginal right to start with if simply removing a stock filter and putting something else in line helped.

My post above simply shows that when fuel delivery is adequate for the power level the stock filter is NOT restrictive enough to cause any issues.

Even with that said just about every vehicle out there is different for fuel delivery as far as the configuration of parts.  Factory sumps are poorly located and easily uncovered on hard runs, and the "sock" on them can become quite restrictive over many years of laying in the bottom of the tank.  Stock lines have many bends and often get close to heat sources and are also subjected to a lot of heat as they snake their way up to the stock fuel pumps.

Mechanical pumps, even really good ones are still not nearly as good as electric pumps for fuel delivery.  They have valves in them that open/close so the fuel flow stops/starts with each pump stroke.  They still make really good high flow mechanical pumps, however my testing with them has shown that unless they are fed with HUGE fuel lines and unrestricted all the way back to the tank they can't reach their full potential.

It is also ALWAYS better to "push" all the fuel than to "pull" it thru nearly 14' of fuel lines for a "high performance" application......FWIW....

[73ss]

I had always ran the stock filter. I have had multiple issues with my toro. Kept chasing my "carb" issue. In the process I found a bad distributor which next was the coil, replaced new MSD. Then wires. Rebuilt the carb with your kit. Kept having issues. On accel would  fall on its face. Installed vacuum gauge and vacuum would max at 22". Then flutters and drop all the way to zero and die, with WOT. Letting off would recover. Figured out that the turbo 425 needs vacuum to stay in gear.
Installed a vacuum can to trans. Same issue with vacuum. Installed a fuel pressure gauge between pump and carb. Good at 4.6 to 6 psi.  FINALLY figured out it was the filter. Yes it was new and in the correct way. Got so bad no fuel would go thorough. Torn it apart and could not see anything inside.
Of course I'm making the story shorter here.
I followed your directions and knew it couldn't be the carb. And after reading so many of these posts I just needed to keep looking.
It stumped me for so long because it was a new filter and never had one clogg up so quickly. Had to take a step back and ask myself. If this was somebody else's car what would you do.

You probably had a filter with the check valve? That had me stumped a few years back

[Hillbillyenginering]

Yep. Had a check valve. But what confused me was how clogged it got. Would not pass any fuel through it!
And taking it apart did not show any evidence of contamination.

[73ss]

Was reading through this old post and I am having the exact issue that Cliff has described. I made some changes and finally got the car to start hooking, (1.8 60', not the greatest, but a small victory for me) only to have the ET and MPH slow down. Car noses over at 3500~4000 from 1st to second. The car has always been on the edge of running out of fuel. I can't get to excited about sumping the tank. I've been looking at some of the aeromotive in tank kits. Anyone have any experience with one of these? Any other ideas?