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I replaced the fuel filter on the original quadrajet carburetor inlet but after it was said and done, I realized that the "plug" I removed from the new fuel filter was not a plug at all but some sort of a flimsy check valve! I do own a 73 and 77 Cadillacs both with original quadrajets and their filters do not have this little check valve in the filter. Can someone shed some light on what the purpose of this is and whether it is important to have it installed?
Years ago when I was being taught how to rebuild a Q-jet by a GM mechanic I was told it was a check valve designed to stop fuel from spilling out of the carburetor in the event of a rollover. Don't know how true that is, but he would take them out and throw them away in his personal cars because they were considered problematic. They aren't really necessary and the car will run fine without it as long as you don't end up upside down.
It is an anti drain back fuel filter that is sold mostly by Napa and they are garbage. It was designed to prevent fuel draining back out of the carb to reduce hard starts due to fuel loss from sitting overnight or longer. They sort of work but also cause an obstruction in fuel flow entering the carb and since they are spring loaded, also stick in the closed position due to shrinkage from poor materials. The ball inside ends up bigger than that outside black shroud and then it sticks in the spring loaded closed position.
I had one in my stock 69 427 and couldn't figure out why it was running out of gas at the top of second gear under hard acceleration and that was it. The filter had less than 200 miles on it and was clean so wasn't worn or plugged. I tried another new one with the same results. Then Lars mentioned the same problem with a caution about using them.
They are complete junk and although the idea was a good one, the design and materials used can leave you very frustrated not to mention the really lean condition when it runs out and dies at high rpm. They were never used by GM.
Below is a pic of the one I removed and you can see how the ball inside is bigger or pushing out against the walls on the black sleeve. That (along with the spring) is enough to drastically reduce fuel flow under high flow conditions like full throttle.
Last edited by CanadaGrant; Dec 24, 2017 at 12:28 PM.
This is an old post but I can confirm it's garbage. I've been having issues with my 73 sputtering on the highway and today it completely cut out and had to get a tow home. Thought it might have been bad gas because I had just put some in I had sitting around. I got some heet treatment for the gas and figured I'd replace the fuel filter just in case it was clogged. The filter did look dirty and I noticed the new one I bought didn't have the check valve. Put the new filter in and it started almost immediately. Took it for a test drive and it feels like I gained 25 horsepower. No sputtering and it runs like a bat out of hell now.
Originally Posted by CanadaGrant
It is an anti drain back fuel filter that is sold mostly by Napa and they are garbage. It was designed to prevent fuel draining back out of the carb to reduce hard starts due to fuel loss from sitting overnight or longer. They sort of work but also cause an obstruction in fuel flow entering the carb and since they are spring loaded, also stick in the closed position due to shrinkage from poor materials. The ball inside ends up bigger than that outside black shroud and then it sticks in the spring loaded closed position.
I had one in my stock 69 427 and couldn't figure out why it was running out of gas at the top of second gear under hard acceleration and that was it. The filter had less than 200 miles on it and was clean so wasn't worn or plugged. I tried another new one with the same results. Then Lars mentioned the same problem with a caution about using them.
They are complete junk and although the idea was a good one, the design and materials used can leave you very frustrated not to mention the really lean condition when it runs out and dies at high rpm. They were never used by GM.
Below is a pic of the one I removed and you can see how the ball inside is bigger or pushing out against the walls on the black sleeve. That (along with the spring) is enough to drastically reduce fuel flow under high flow conditions like full throttle.
Which way did you have the check valve installed? Using a correct, check-valved fuel filter nearly eliminated the long crank, cold start issue in my 79.
Well folks I have a 78 and my Qjet was professionally overhauled by the QJet guru LARS...
With all that said regarding filters with or without that anti drain back plug he told me that there are several other facts about todays fuel problems and the way these carbs are built etc...
Pls take a few moments and read his papers that address that same issue..
The drain back works...but only if you have like a stock lo-po 350......the fuel demand of, say, a 350/350 is more than this can sustain.....
They were used to prevent drainback on the "windowed" needle seat. The non-window style holds the fuel in the bowl higher when shut off....and should bleed back if the float is set correctly. According to Cliff Ruggles...neither seat has an advantage in flow believe it or not.
79 - Fuel filter check valve?
