Question for Quadrajet Users - Disapearing Gas from Fuel Bowl

[RWBeringer4x4]

Hi all,

Hopefully there's still a few SBC guys out there who haven't converted to EFI yet! I'm finally back to working out kinks in carburetors after 3 years with no engine!

I have been chasing a "disapearing fuel" demon for years now, across two different quadrajets on two different V8's and two different fuel pumps.

Things I've tried:

1. I know quadrajets have a notoriety for leaking from the welch plugs, but both of these carbs have had their plugs epoxied. The first, (a very early Qjet) I drilled, tapped, and epoxied myself. The second was a much newer version of a quadrajet (ModQuad) from a reputable builder (Sean Murphy Inductions) who epoxies them as part of the build.

In both cases - with epoxied plugs on a 283 SBC, and later on my current 350, the bowl is still empty in 24-48 hours.

2. Next I thought that MAYBE I had a syphon issue, where gas was being pulled back out of the quadrajet and draining back to the tank. I installed an anti-drainback check valve in the fuel line right by the carb to ensure this doesn't happen. It still happens...

My QUESTION:

Where is my gas going? My only thought is that my under-hood temperatures may be getting high enough that I'm boiling/vaporizing the gas in the carb, causing it to leak out the jets when the engine is shut down. This would purge a lot of gas before it cooled, and maybe leave a small enough amount in the carb that it could evaporate normally afterward...

That said, my IR thermometer only shows the carb getting to about 120-130 degrees. The carb is already sitting on a thick (1/4") base-plate gasket. Looking down the throat of the carb a few minutes after shut-down, I don't see a trickle of gas or anything that looks like leaking. I'm not running into any vapor lock or hard start issues - but it might have a BIT of a rich kick when I first start it after it sat in a parking lot for a while (small puff of black smoke).

Could 130-140 degrees be enough to boil/vaporize 10% ethanol gas? The usual "go-to" to resolve this a phenolic spacer under the carb, but given the lack of insulation on the hood, I feel like radiant heat would get the carb to 140 degrees pretty quickly even if it isn't sitting on the manifold.

Is there anywhere else in a Quadrajet gas could be escaping that I'm not thinking of?

As always, thoughts, opinions, and advice are much appreciated.

 

[pbgbottle]

I’m still running the carb also. But have no advice.
How do you know it’s draining., just curious. Mine drains after sitting 6 months or evaporates. I know no help. Lol

 

[Farmall Dave]

Gremlins with straws that like to party on gas?
I have a quadrajet but I use an electric fuel pump. I can tell by the sound that the pump makes that the float bowl is empty, and wait a second or two longer before cranking the starter.
How about an experiment - actually several. Next time it is warmed up and about to be parked for the night you could consider placing the old carb with gas in the float bowl under the hood and see if fuel disappears via evaporation. You could also disconnect your carb and block the fuel inlet to eliminate the possibility that the check valve is not preventing drain back - then see if fuel disappears. This is assuming you still have the old carb.

 

[RWBeringer4x4]

I’m still running the carb also. But have no advice.
How do you know it’s draining., just curious. Mine drains after sitting 6 months or evaporates. I know no help. Lol

I don’t know that it “drains,” per se - I just know that in 24-48 hours I have to crank the engine over on the starter for 10 seconds to pump fuel back into the carb. The in runs perfectly.

I could completely understand if it evaporated over the span of 6 months, or even a week or two. They are not closed systems and gas is fairly volatile. It definitely doesn’t seem normal that it would be empty after a day or two.

 

[RWBeringer4x4]

Gremlins with straws that like to party on gas?
I have a quadrajet but I use an electric fuel pump. I can tell by the sound that the pump makes that the float bowl is empty, and wait a second or two longer before cranking the starter.
How about an experiment - actually several. Next time it is warmed up and about to be parked for the night you could consider placing the old carb with gas in the float bowl under the hood and see if fuel disappears via evaporation. You could also disconnect your carb and block the fuel inlet to eliminate the possibility that the check valve is not preventing drain back - then see if fuel disappears. This is assuming you still have the old carb.

The check valve definitely works just by blowing on it. It lets air through one way, blocks it the other. Interesting thought, throwing the second carb under the hood. Not sure it would be fully “apples to apples” unless I could KEEP it under the hood and get it hot with the other carb. I suppose I could just as easily simulate it by cooking it with a heat gun until the whole carb was 130 degrees - although gas and heat guns are probably not the best idea!

 

[pbgbottle]

Ok , I understand. My rig is sitting right now I’ll do a test I’ll start mine up every two - three days and see how it starts and see how long it cranks over ,I too epoxied my carb. And it is also a carb I took off of my 1977 gmc van. So it is fairly old.
I”ll see how it goes.

I don’t know that it “drains,” per se - I just know that in 24-48 hours I have to crank the engine over on the starter for 10 seconds to pump fuel back into the carb. The in runs perfectly.

I could completely understand if it evaporated over the span of 6 months, or even a week or two. They are not closed systems and gas is fairly volatile. It definitely doesn’t seem normal that it would be empty after a day or two.

 

[RWBeringer4x4]

So I went out for a long drive tonight (half hour) in about 80 degree weather and made sure things got nice and hot. Then I shut the truck down and let it sit for about 10 minutes to let the temps come up after shut down.

The carb was about 150 degrees. I poked around with a stethoscope and think I heard some bubbling or dripping down toward the fuel bowl. Nothing like a “rolling boil” but more like a single air bubble popping every few seconds. I still didn’t see anything leaking from the jets though.

 

[RWBeringer4x4]

Ok , I understand. My rig is sitting right now I’ll do a test I’ll start mine up every two - three days and see how it starts and see how long it cranks over ,I too epoxied my carb. And it is also a carb I took off of my 1977 gmc van. So it is fairly old.
I”ll see how it goes.

Cool - keep me posted! I’m going to try and fire mine up tomorrow night again and see if there is still fuel. Oftentimes it really happens that fast.

 

[65swb45]

Rick

I got tired of this decades ago and installed a ‘booster’ pump in line on my Q-Jet K5. I run a SW fuel pressure guage as well.

Hit the toggle in the morning. Watch pressure go from 0-4psi. Toggle off. Truck starts on first crank, every time.

 

[toyotaspeed90]

If you believe it's a fuel issue - I would let the vehicle sit for a day or 2, pull the air filter, spray some starter fluid in and see if it fires right up, or still takes a few seconds of cranking before starting.

I had a quadrajet, it was epoxied, I'm pretty sure it still leaked.

You could, actually, be having the opposite problem. The carb is well sealed and doesn't leak fuel - so it might take more than a few seconds for fuel to make it from the carb to the cylinders (when cold and only cranking rpm). Meaning - a carb that has a leak might actually fire up faster because the fuel is sitting in the intake manifold.

 

[RWBeringer4x4]

Rick

I got tired of this decades ago and installed a ‘booster’ pump in line on my Q-Jet K5. I run a SW fuel pressure guage as well.

Hit the toggle in the morning. Watch pressure go from 0-4psi. Toggle off. Truck starts on first crank, every time.

Hey Mark, thanks for chiming in! Isn't installing a booster (or an electric pump) etc. just a band aid? The gas has to be going somewhere. Were you concerned it's boiling off into the manifold thereby making its way to the pistons and washing the oil off the rings? Cranking the engine over the with starter isn't really a big deal to me - my bigger concern is where the gas is disappearing to...

 

[RWBeringer4x4]

If you believe it's a fuel issue - I would let the vehicle sit for a day or 2, pull the air filter, spray some starter fluid in and see if it fires right up, or still takes a few seconds of cranking before starting.

I had a quadrajet, it was epoxied, I'm pretty sure it still leaked.

You could, actually, be having the opposite problem. The carb is well sealed and doesn't leak fuel - so it might take more than a few seconds for fuel to make it from the carb to the cylinders (when cold and only cranking rpm). Meaning - a carb that has a leak might actually fire up faster because the fuel is sitting in the intake manifold.

It's definitely a lack of fuel (empty bowl). After two days I can look down the barrel of the carb and crank the throttle to actuate the accelerator pump - no gas sprays out. Occasionally I'll still have a shot of fuel in the accelerator pump, so the engine will start, but it will die almost immediately. Likewise, it will start on a shot of starter fluid, then immediately die if the pump doesn't get gas up to the bowl in time.

 

[shelfboy1]

that was normal for us qj guys back in the 60s never thought about it until now

 

[Pin_Head]

I think it is due to reformulated / oxygenated gas and it happens to all carbs. The fuel is creeping out of the idle port due to surface tension (at least that is where it was dripping in my carb). You will find it at the bottom of your intake manifold. Try some non-oxygenated gas and see if the problem goes away.

 

[RWBeringer4x4]

@Pin_Head as usual, your post has proven just how much I still have to learn.

1. What is oxygenated (vs. nonoxygenated) gas - and where do I find the non-ox variety?

2. I’ve always been curious about this - if the gasoline leaks into the inlet in liquid form - it it really THAT likely it will make it into the cylinders and wash the rings? It seems like a leak might generate a puddle in the bottom of the inlet, but unless it was a SERIOUS leak: it seems like it would never puddle big enough to make it down the runners to the cylinders.

 

[RWBeringer4x4]

Also, @pbgbottle - I just fired up the truck. It started on the first crank, no hesitation, and didn’t stall out. So there was definitely fuel still in the bowl after 24 hours. It was 96 degrees today, all day. I let it run until the hood got hot to heat simulate behind driven. I’ll see if it starts again in 48 hours...

 

[Pin_Head]

1. Google “real gas near me”

2. Fill a glass with water and then put a strip of paper towel in the glass with the end drooping to the table and see what happens. That is where your gas is going slowly.

 

[RWBeringer4x4]

@Pin_Head - I know it’s hard to judge, but aside from annoying hard starts, does this cause any problems long term? Do I need to worry gas getting in the oil or is it probably slow enough/minimal enough that it won’t do any harm?

 

[Pin_Head]

It depends on your intake and the amount of fuel that leaks. If it is enough to get in to the cylinders, it can wash the oil off the walls and dilute the oil.

 

[RWBeringer4x4]

Well, 48 hours later, my truck is calling me a liar - After running it up to operating temperature and getting things nice and hot, I shut it down and let it sit for two days. It fired right up and ran again tonight, so there was still gas in the bowl... It has been 96+ degrees around here lately so this is a good time to be testing this stuff...so far, so good. I guess I'll push it out to 3 days next and see if it still starts.