GTA_doum

Hello,

I have this problem since a long time with my '72 442. I decided it was time to spend some time on this. The problem is that the carburator is out of gas if I let the car sit for few hours, or overnight. To avoid burning the starter by trying to get the gas back in (sometimes it would take more than 30 seconds of cranking to start), I was putting gas directly in the small tube of the carburator, and the car would start "in a quarter of a turn"

So I dismantled the carburator, put the body on a white towel, filled in the reservoir with some liquid (Not water, a mix of cleaner and lubricant), and waited overnight, thinking it would get out by some micro fractures... Not ! It was still at the level it was yesterday evening...

Anyone has a suggestion on why the carburator empties itself, and how to fix

Thanks,
Dominic

henryk8398

Very common issue. You've got a couple of things working against you:

1. Earlier quadrajets were known to leak fuel from the body in a place known as the "well plugs". Supposedly remedied by the early 70s, pretty much any quadrajet I've ever had runs dry in a few days, even a 1980s model. Overnight sounds like you have a serious leak in that area. There is a ton on information on how to fix the problem, but most "fixes" are temporary. This may help you:

http://www.highperformancepontiac.co...r/viewall.html

I recommend any quadrajet owner to buy this book. It's invaluable:

http://www.cliffshighperformance.com/buy_book_2.html

The Doug Roe book from the mid-1980s is great too, and is still highly recommended.

2. Today's fuel (or whatever it is pretending to be fuel) tends to evaporate very quickly. A lot of people running carbs add a small electric fuel pump to run for a few seconds. If you don't have a well plug issue, a "pusher" fuel pump may help you.

Lady72nRob71

You may want to repeat that test in a well ventilated area using gasoline. It is possible the liquid you used had anough thickness to not pass thought those tiny cracks.

GTA_doum

Bought the book and ask for a quote from Cliff's...
I will retry the test this week-end with gasoline !

Thanks!

MDchanic

Does your fuel filter have an anti-drainback valve?

Some do and some don't. It does make a difference.

Go to the auto parts store and check the fuel filters in the clear packs on the rack - look for the one with the rubber plug-like thing in the inlet, rather than an inlet with nothing in it.

- Eric

Highwayman

Another thing you can check is the filter in the carb inlet. The paper filter should have an anti-siphon valve build into it to prevent gas from being sucked back into the tank.


Edit: Eric, I think we are on the same wavelength or something. You're just faster with a post.

MDchanic

Hmmmmmmm...

AZ455

That sounds like a good idea...

Mine still has fuel after being parked a week, in the sun, with ethanol in the mix. So it's gotta be leaking somewhere..

Highwayman

Yeah, I type slow.

MDchanic

GTA_doum

The one that was there had the open end with a rubber... I taught anyway the the round thing at the end of the floater was to block the fuel from returning, and the hole is high enough so that the bowl could not get emptied even if it would...

BILL DEMMER

that is true, the bowl can not get siphoned dry. the check valve may help somewhat, but it's not a panacea. test your well plugs with alcohol, it's thin enough. you may have a fuel percolation issue. the primary/secondary well plugs do have issues, mainly in the '67 and earlier qj's, rarely do the later-type spun-in plugs(secondary well plugs) leak, but it is possible. the primary well plugs are just pounded-in throughout the years.


bill

liquidpower

I put some jb weld on the well plugs and that fixed it for me

GTA_doum

Old thread, but I am still living this problem with my Cutlass. I did many tests with the carburetor, and I cannot find where it leaks over the night!
I even did a test, carburetor out, heat it up to 90 degree Celsius, put gas in it and let it sit overnight with a white towel below. The morning after, it was still full of fuel! So only when in the car does it empty itself over hours of sitting down. My only taught is that the motor runs much higher than 90 and the fuel evaporates eventually... Does not make complete sense to me, but it is the only explanation that could explain it.
Any ideas?

MDchanic

Hmmmmm... The great enemy of internet rumors and truth decay: Empiric data.

So you actually did an experiment at operating temperature and found that the fuel did not, in fact, leak out.

You didn't say in your post, but have you experimentally established that the fuel does leak out under normal conditions?
Or have you just assumed it does?

Are you using a check valve, as mentioned above, in your fuel filter inlet (or elsewhere)?

All very interesting...

- Eric

oldcutlass

With the engine off, if you operate the accelerator in the morning while looking down into the carb are there 2 solid streams of fuel squirting in there? There is an old saying that most carb problems are ignition related. When was the last time you checked your tune up specs or gave the engine a tuneup?

joe_padavano

Proving once again that the well plugs are usually NOT the cause of the problem.

BillK

Yup, I am having the same problem with the one on my Wifes Riviera. When I went through the carb earlier this year I did the same thing as the op. I left it sitting on my bench at the shop with 87 octane unleaded Shell in it for almost three days and it did not lose a drop. I have the check valve filter on it. Cannot figure it out. This is the only Q-jet in 40 years that has kicked my butt Hers has been doing it for quite a while so its not a new problem.



I don't buy the ethanol fuel evaporating faster deal. My S-10 is carbureted and it starts every morning within a couple of revolutions. It can sit for a week and it still starts right up.



I actually just finished buying a used 84 carb on E-bay that looked fairly decent. I should have it in a few days and will see what happens after I go through it.

GTA_doum

It is difficult to see if it leaks in the intake manifold, I tried to look into it through the carburetor, but do not see any "wetness". Since it did not leak in my table test, I see two other options; it is aspirated or it has evaporated.

GTA_doum

In the morning, I can often see the squirts when depressing the pedal, sometimes it is enough to make the motor start for a spin or two, and then it turns with the starter until fuel reaches in.
Most of the time I pour fuel into the carburetor, to not stress the battery and starter. When I do this, it always starts on a quarter of a turn. I can run all day long without any issues, it is only when left for many many hours that it is out of fuel.

MDchanic

You have a small (possibly very small) air leak in the fuel lines between your tank and your fuel pump (including sender, hoses, and steel lines), which breaks the suction, or the "prime" of the fuel pump.

- Eric

GTA_doum

So an empty carburetor is normal, and the car starts because fuel is in the line waiting to be pushed by the pump? A long time ago, I did make a pressure test between the pump and the tank, at that time I found a hose that had a hole in it, but after fixing it, it just made it possible to start by letting the motor run by the starter a long time, with the hole, it was not possible to start the motor at all, since the fuel would never get to the pump.
My understanding is that the fuel tank does not stay under pressure, so I think it is not possible for the fuel to stay in the line. When the pump stops working, the fuel will return to the tank, by gravity.

OLDSter Ralph

Try this to understand why a hole in the fuel supply can let it drain back, and holeless fuel supply won't drain back.
1. Fill a glass with water.
2. Take a plastic soda straw and vertically stick it in the glass of water and put your finger over the top end.
3. Remove the straw from the glass (keeping your finger over the end).
4. Move the straw over an empty glass and remove your finger. You just let gravity overcome the vacuum.

Look for another hole or holes in the fuel supply lines. May also be a fuel pump diaphragm if you don't find holes ?

MDchanic

The carburetor shouldn't be empty, because it's essentially a bowl with a closed bottom (the jets in the bottom of the bowl lead upward again, like toddler's straw-cup), with a small air vent at the top.
As you observed, the fuel can't leak out downward, and it takes a while for it to evaporate upward.

The fuel supply enters at the top, through the airhorn, and then falls into the bowl, so the fuel can't be siphoned back through the fuel line.

However, a pinhole in the fuel line or pump diaphragm (or flaw in a pump check valve) can introduce enough air to break the relatively small amount of surface tension in the gasoline column in the line, allowing it to drain back into the tank over time, and requiring a whole bunch of cranking to get the fuel back up, after an initial second of running.

- Eric

BillK

Drove my Wife's car this morning so I could do a few things to it at work. Started right up......go figure

joe_padavano

I firmly believe that this is the source of most "well plug" complaints. People don't realize that the rubber check valves in the pump deteriorate from ethanol.

BillK

Possible but everything on mine is new. I will find out for sure when I get the different carburetor.

D. Yaros

Keep us posted. This problem is quite common and your resolution will be appreciated!

Sugar Bear

Isolate the carb and fuel pump by removing the steel fuel line between them, put a barbed fitting in the pump and the carb. Connect the pump and carb with a safely routed, quality, clamped rubber hose using the same routing as the steel one was routed. Put a shutoff valve in the hose or pinch it shut between shutoff/starts. If the carb is dry with the line pinched, the carb is leaking or the fuel is evaporating, if the carb is full then the carb is not the problem.

Reinstall the steel fuel line after the test.

Good luck!!!

OLDSter Ralph

Lots of great ideas. I have a similar problem with a 66 Toronado I just bought. The last thing I was suspecting was the Quadrajet. Now I have some very good ideas to isolate the problem.

dav45

I know this is an ongoing problem, so after reading all of your ideas, two things stand out as overlooked. First question is simple: remembering that a carburetor works by suction( air drawn into the cylinder ) How is one going to replicate that on a bench with a towel? Second question is a little more complex. comparing engine running temperature to bench temperature the expansion/contraction ratios are not quite the same as the bench is not as warm as an intake manifold, nor is does the pot-metal carb reach operating temp. THINK AIR BLEEDS, especially on the secondary side, Needles not seating, and about using a Straight Edge and flat file for warped surfaces. Remember that the check ball on the accelerator pump may not seat properly nor does it have an air bleed. Exhaust crossover in intake may be blocked which will increase baseplate temperature. over torqued screws and bolts will warp pot metal- more so at increased temps.

joe_padavano

What "suction" is taking place when the engine is sitting not running overnight? The problem is claimed to be caused by the well plugs leaking when the engine isn't running. There is no "suction" under those conditions - the intake manifold is at atmospheric pressure.

dav45

JOE, True! How ever the vacuum created by a running engine is what starts a siphon action. Engine off and, as the intake base is below the ports in the carb, this siphon will continue IF the air bleeds are blocked ,much the same as using an " Alabama Credit Card " to siphon gas from a tank. How are you going to create a siphon action on a normal work bench? Concerning the wells, Again TRUE engine heat causes the O-rings on the plugs to harden and crack. Yes there are O-Rings. Usual sign of this is residual in the well bowl on the base plate IF the base to bowl gasket seals that area. As this recess holds less than a teaspoon of fuel when assembled and the bowl holds from 1 to 11/2 oz, of fuel depending on float level , the recess cannot hold all UNLESS the well plug seals are leaking or the gasket is not designed to seal this area or is leaking. In either case on an assembled unit, a vacuum is required to start a siphon

joe_padavano

One more time. The test over a piece of paper has NOTHING to do with siphoning. It is ONLY intended to test if the well plugs are leaking or not. A leak out the bottom of the fuel bowl is NOT a "siphon".

dav45

So we agree, if the well plug seals are bad, you have a leak. Have a good week!.

MDchanic

Dave,
Ignoring for a moment the fine points of the Alabama banking system, I think you may misunderstand the principle of the siphon.

A siphon involves an uninterrupted column of a liquid in a closed container (such as a hose), wherein the molecular attraction of the molecules of the liquid creates enough adhesion that the portion of the liquid that is lower is able to "pull" liquid along behind it, thus pulling liquid from a tank that is slightly above it over a "hill" and back down again.

The key here is that the column of liquid must be uninterrupted. Any hole in the hose will admit air, which will break the column and interrupt the siphon.

Now, looking at the cross sections of QuadraJets below, you can see that there are no uninterrupted columns of liquid fuel.

All of the jets discharge (leave the closed container) above the level of the fuel in the float bowl.
Also, all columns are interrupted at the top by air bleeds, which help to add air to the fuel before it reaches the nozzle when the engine is running, and serve to break any potential siphon effect when it is not.

Finally, as a thought experiment, just think of the running problems a car would potentially have if a siphon effect were able to continue fuel flow after you let your foot off the throttle.

Three things that do not cause QuadraJet float bowl emptying are leaking down through the well plugs (on post-'66 or '67 units), siphoning back to the fuel pump, and siphoning forward through the nozzles.

- Eric

olds 307 and 403

I saw hard starting after sitting on my 78 chebby 1/2 ton which I put an Olds 307, then an Olds 350 in. It sometimes took a lot of cranking, got bad enough that it actually leaked fuel on the ground at the cracked hose to the tank. Go through every rubber hose and line, if it looks suspect, change it.

Nailheadcanuck

Used to see this sort of thing occurring on my older cars that still had a sediment bowl, they would drain back and would be empty by morning. Replaced the fuel pump and that was it.

The other issue could be with your heat riser. Older Oldsmobile engines (in my experience) seemed to have more issues with burned up heat risers. The heat riser may be burned through and the butterfly on the exhaust manifold could be either stuck or sticky. So in the end there's too much heat in the upper manifold. This will cause the contents of the carb to boil off which is a far more dangerous situation. (If your garage always smells like fresh gas, this could be the reason why) I've found a couple of Olds engines I've played with over the years to have this issue, but it's been mostly with the 4GC. The gaskets from the carb to the manifold were burned through and the heat riser was pretty much raw exhaust coming up. I've lost count of how many Olds engines I'd see with a stack of gaskets separating the carb from the manifold, seems at one point it must have been a common problem.
Something to consider, for those of you who still have a stock set up with stock manifolds.

sysmg

I vote fuel pump or lines from the tank. The gas in the carb get's the car to catch, but if no new fuel is pumped in the carburetor within a few seconds, the siphon in the carb will break, no fuel to the engine, and the car won't start. Jury rig an electric fuel pump, coming off a gas can. Start the car in your driveway, warm it up. Leave it overnight with the electric pump still connected and try to start it. I'll bet it starts right up.

GTA_doum

Hello,
Seems weather affects the carburetor emptying itself! I used the car three days in a row, three sunny and hot days, and the first night it was a chill night no rain, the second night it rained. During day time, I parked the car for hours (up to five) under the sun, and it started every time on a quarter of a turn. The morning after the night with no rain, it started without having to fill the carburetor with fuel, the morning following the rain, I had to put fuel in the carburetor.
Why would bad weather helped in emptying the carburetor?