Hi.
I rebuilt the q jet. Car fires up and runs great/ better and seems faster.
Except.
Floored it from stop to 1/4 mile on the highway.
Near the end it stumbled alot, like running out of fuel.
the car has work done to it and was always surprised the factory carb and fuel pump are big enough.

 

Each A/F mixture screw set to the highest vacuum possible with a vacuum gauge (recommended)? At a minimum, set each A/F mixture screw to the highest RPM possible (without bogging or hesitation). Without specific knowledge regarding changes outside of the OEM cam, you should be able to achieve 17" Hg - 20" Hg. The float might need adjustment. I'd check vacuum first (easiest).

 

Was the stumble there before the rebuild?

 

Could be worn or broken air valve cam for secondary metering rods.

 

i've had similar symptoms from a q-jet . when i rebuild them i drill the holes that fill the secondary wells to (iirc) 1/16 .the factory holes are tiny and the secondary wells aren't exactly huge

 

Are you running the paper filter in the carb? I had starvation at the end of a 1/8 mile track. I went to a metal inline filter and I replaced the Ebay special fuel pump with a Carter fuel pump and never had the issue again.

 

More importantly, is the filter the kind with anti drain back valve? If so, get rid of it, replace it with a regular filter. Those valves are a huge restriction.

 

Check integrity of hoses between fuel tank & steel fuel line, plus the short pieces of hose up front. If they’re spongy or porous they can cause problems under high demand. Filter & pump can’t be ignored (post #6). Fuel sock in tank clear? Sounds like a delivery issue under prolonged, high demand.

If engine is pushing some power, you may need to upgrade fuel delivery system, ie Robb Mc pump, fuel pickup, maybe go electric, in combo with EFI tank. The “fuel wells” ie Robb Mc ‘Power Surge” are the quickest, easiest solution imo, but not for everyone. I chased delivery problems for a while and finally licked it. Licked it with money from the tank right to the fuel inlet! Good news it that it solved my problem.

Tough to keep a Qjet’s tummy full.
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Mine filter had the valve, it makes sense it would even be more restrictive. I had cracked rubber hoses on my 78 Olds 350 powered 1/2 ton truck. After sitting a couple of days, it really took a lot of cranking to start. It would also sputter under hard throttle, it is worth a look.

 

FYI that check valve can be removed from the filter very easily. Did that a lot when I worked at auto parts stores in college.

 

Yup, this bad boy can mess with the flow:

 

And, if you happen to have the filter w/ the check valve and it was installed backwards, you'll end up trying to push fuel in the opposite direction for which the check valve was designed to operate (which would mean you're trying to push fuel past a closed check valve).

 

The part numbers on this site demonstrate the two fuel filters. Part # 2253CV (w/ check valve; Rochester OEM # GF-471), Part # 2253 w/o check valve (Rochester OEM # GF-441).

https://quadrajetparts.com/long-pape...k1mf16iqpd4no4

 

Besides previous suggestions have someone sit in the car with engine off & check full opening of throttles, you may need a flashlight to look down the secondary side. Also check float level, 1/4" is standard on the one's I do.

 

If your filter is backwards, you may not even get gas to the bowl. Or you'd have to rely on the fuel pump to push its way past the filter end and filter spring pressure to bypass the filter, again causing a super-huge restriction. On cars with the drain back check valve in the filter, the spring isn't designed as a relief valve. I've never installed a filter backwards in a Q-jet, but they filter from the inside out. The check valve wouldn't even enter in the equation on a backwards filter.

I've had stock 455s pull all day long through the stock GM 441 filter, and unless there's something wrong with the filter, you shouldn't have a flow problem there. Even with a higher HP mod. I do agree that you don't need the check valve filter as that was a rollover safety measure and drainback band-aid.

I do agree on the fuel starvation part of it being the most likely cause. If the engine is running great up until the stumble, it's either the pump not up to snuff, the suction line not getting free flow such as dirty tank socks or crud buildup in the line, or something to do with the float level. This is assuming what others posted about the condition of the other parts of the carb are in good shape, like proper secondary rods, parts wear, etc.

Another carb-related item is the inlet needle valve and seat. Some of the aftermarket carb valves and seats are smaller than others, and may restrict fuel flow at higher rpm. They make higher flow versions of the valve and seat up to 50% larger than stock. Not sure how much that would even help and likely not needed, but every ounce matters to a Q-jet. The larger the inlet hole, the more risk of flooding, though. That's kinda reaching here, but you never said if it had this stumble before or this is something new.

Don't rule out ignition issues at high rpm, though. Timing ok? Point bounce used to be a "thing". Carbon tracking, misfires, weak coil, voltage issues, etc. Again, just reaching for some straws here. Fixing cars via the internet isn't the same thing as being there.

 

I know for a fact the anti drainback style filter is a huge restriction, and doesn’t solve anything.

During my pre Drag Week 2016 maintenance, I installed new plugs, replaced the Q-Jet filter, plus a few other odds and ends. Imagine my shock and horror during the first pass the car fell flat on its face a little over the 1/8 mile point. I was panicking, I had already spent a grand on registration, U haul rentals, and hotels and couldn’t make it down the track once.

After checking some things, I decided to roll the dice on removing the drain back valve. Voila! Problem solved.

That was my only headache that week. A friend of mine torched 3 LS engines in his Malibu that week. Granted, he had a 9 second car. We got pretty good at swapping engines!! He did finish the event.

 

What was the application/usage of these? I’ve had my hands in a fair number of Qjets from all GM divisions and I have never encountered one of them.

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They were used in the later years. I think it was started in Q-jets some time in the mid-70s.

GF471 was the check valve version of the GF441 filter. Same filter, just had a check valve in it. It was sorta not a drain back stop, although it would do that if you had a windowed needle seat tower. Stock factory cars they put in high-wall needle seats that would accomplish some drain back mitigation, but the check valve was really supposed to be a rollover safety device, however I'm not sure how effective it was. Wouldn't stop your fuel bowl from emptying, but was supposed to keep the gas tank from draining if you were upside down in a ditch with the tank higher than the carb. Not sure how much pressure it takes to unseat the spring loaded check valve, but even if you had say, the gas tank 9 feet above the carb level with the car standing on its front bumper, it's probably 4 psi or so of pressure at the valve. Fuel pumps are usually 5-7, so in theory, it should work to hold back the fuel from just flowing out the carb vent. I just don't know what the breakaway pressure is for the valve. At least it should slow it down.

In fact, if you drive your car daily or regularly, you technically don't even need the needle valve hanger. Then it should act like a check valve if you're worried about drain back and it doesn't hurt flow. Only reason that's needed is to ensure the needle doesn't stick closed after sitting a month and your gas bowl dries out. Use the pull-out method if you don't want the filter check valve.

 

Thank you. The latest Qjet I remember having apart was iirc a ‘76 and that still had “the same old filter” though I can’t remember if it was a paper or sintered element. Guess I don’t have any worries about encountering one of these.

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Thanks for the replies.
sorry I've been gone for a bit. I need to read all this.

I should have said.

The carb was "hot rodded" but didn't stumble when I ran it before.
I didn't need to rebuild it...but did just to do it.

No filter / check.
3/8 fiber intake seal
Choke completly removed, with caps over holes.
all metering tubes and holes drilled out. I didn't do that stuff,